# Is it possible to tell if a certain extrasolar planet produces its own magnetic field?

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I know by observing the dimness of a star it is possible to calculate an exoplanet's distance from the star and its mass by how much the star wobbles. However, is there any way to accurately determine if said planet has any magnetic field through observation alone directly?

There are three approaches with which people have looked for this, and not yet been too successful:

• A transiting planet planet with a strong magnetic dipole and/or very strong host star winds might produce a visible signal when the magnetospheric bow-shock passes in front of the star as well. The idea is that at the bow-shock the streaming hydrogen would be heated up, ionize and produce a detectable ultraviolet photometry signal upon recombination at a characteristic distance (the stand-off distance) from the planet.
HD189733b was for a time the first planet to have measured its magnetic field in that way, however the discovery was retracted because the ultraviolet opacities that the group used to determine the bow-shock position were wrong.
It turns out there is no signal from the bow-shock.

• The second idea is to look at a hot Jupiter that has an evaporating atmosphere. The shape of the ionized hydrogen tail in the magnetosphere will be influenced by the strength of the planetary dipole. But then, the amount of hydrogen that is in the planet's shadow is also a function of the magnetic field strength.
Hydrogen that comes into the planet's shadow will recombine with magnetospheric electrons due to the lack of direct irradiation from the star and produce a signal in the Lyman-$alpha$ recombination line.
Going further, only the particles that stream away from the planet, and towards us will be part of this process. Thus the blue-shifted side of the Lyman-$alpha$ line can inform us about the strength of the magnetic field.
So far the theory. The practice is that our last remaining UV-spectrometer in space that could perform such a thing (mounted on the Hubble) is even only barely capable of observing this process. But it has been done for the planet HD209458b The data quality is lousy, and the interpretation that has been done in Kislyakova et al. 2015 has more parameter than datapoints and is therefore questionable.
But if you choose to believe the authors, then this planet has a magnetic moment of $0.1$ Jupiter magnetic moments.

• The third possibility that could yield real results are radio observations with the LOFAR array. LOFAR observes at the edge of observable frequencies from Earth (at very low frequencies the ionosphere interferes with radio observations) at the for the moment highest achievable spatial resolution. The idea here is to look for analogues of Jupiters decametric radiation, which is simply synchrotron radiation produced when ionized particles start gyrating around a magnetic field line.
This already has been tried at higher frequencies / different observatories with the planets around $epsilon$ Andromedae and $au$ Bootes, as yet to no avail.

## The Search for Exoplanets: A Habitable Suitor

Since humanity first turned its eyes towards the night sky and gazed in awe at the infinite wonder of the universe, we have been captivated by space. Exploration and discovery is as much a part of our human nature, as our desire to observe the depths of our origins, and understand the unknown. The search for exoplanets is one of the foremost, growing fields in the exploration of space. Exoplanets are planets located in distant extrasolar systems, orbiting stars other than our own, and vary in size from larger than Jupiter, to smaller than Earth.¹⁶ In light of the challenges of overpopulation, extreme seasonality and climate change, as well as the depletion of the world’s natural resources, something must be done. There is a new impetus to find and potentially colonize a habitable planet similar to our own. It may not be possible today, but theoretically, in the near future this may become reality. The first step is to look and learn. In this paper, we will explore some of the methods of how we have come to learn what we know today about exoplanets. Several criteria and conditions are necessary for a planet to be considered habitable. Habitable planets have a very specific range of conditions that can support life, and when searching for exoplanets, we look for similar conditions that we find on Earth. Evolution is theoretically capable of producing endless variations of life, which is another interesting aspect related to the study of exoplanets. Very few planets discovered to date have the appropriate conditions to support life, as we know it. A further consideration of the methods of discovery, and conditions of habitable features that pertain to extrasolar planets is necessary. We aim to look at which exoplanets may have the potential to support and sustain Earth-like, carbon based lifeforms. We need to be able to detect Earth size planets, orbiting Sun like stars, at distances that would support liquid water. How do we find such Exoplanets, that have the environmental conditions to support life?

Methods for Detecting Exoplanets

Figure 1: This is a figure that gives a brief history of the Kepler Space Telescope, as well as its function. Source: http://www.space.com/32850-nasa-kepler-telescope-finds-1284-alien-planets.html

Astronomers have used many different methods to discover planets beyond the solar system. It is important to note that the majority of exoplanets are of too great a distance to be viewed using traditional imaging methods from observatories. Many of the earliest exoplanet discoveries were Jupiter-sized, or larger gas giants, that were orbiting close to their parent stars. That’s because astronomers had to rely on the radial velocity technique, which measures how much the star wobbles when a planet orbits it. These large planets, being relatively close, produce a correspondingly significant effect on their parent star, causing a wobble that is comparatively easy to detect. Certain techniques of detection have recently reached their technical limit, restricting the amount of data that can be collected. The knowledge learned from the Kepler missions, combined with the application of next generation space telescopes, will allow for more detailed studies of other solar systems and the planets orbiting their host stars. Future missions will emphasize these efforts to observe more specific characteristics within individual solar systems, unveiling ringed worlds similar to Saturn, planetary moons, as well as large collections of asteroids. This would be complementary to the pre-existing models that have been created based on data (phase curves) from multiple previous missions. Observing stars, and their orbital transits for longer periods of time, is one such current objective among the astrophysics community. This will aid in improving the models based off the data from planets and stars which have already been discovered.¹

In order to find and observe these planets, scientists use various methods of observation, as exoplanets are incredibly difficult to see directly from Earth. Directly observing exoplanets, especially distant ones is extremely difficult even with advanced technology. It is therefore necessary to indirectly determine their presence. This is performed not by measuring the planets themselves, but their affect on objects nearby. The development of indirectly detecting exoplanets has vastly increased our capabilities for finding alien worlds.¹

Using data from multiple space telescopes that are fixated on certain exoplanets could tell us more about the particular features of the planet, such as atmosphere, elemental composition, and geography. As space telescopes observe stars and planets, they stream this information to Earth where computers run a series of algorithms on the data, and astronomers can then analyze the results. The culmination of multiple indirect methods being used, while detecting and observing exoplanets of interest around stars, may give greater insight into which could be a possible suitor and potentially habitable. A summary of the methods used by astronomers to observe exoplanets and stars is necessary in understanding how we know, and what we know about them today.

When it comes to finding extrasolar planets outside our own solar system, NASA’s space mission Kepler has found more than 1000 confirmed exoplanets in the last 5 years. One technique is called the Time Series Transit Photometry, which is most effective for detecting large planets in close range orbits relative to a star. The Transit Method works by detecting a routine drop in the apparent brightness of a star caused by a planet passing between the star and the Earth. This data is collected over extended periods, collecting information on all planets orbiting the star. This data is examined, and drops with routine intervals are indicative of planets. The length and intensity of the intervals can be used to determine multiple characteristics of the planet including size and distance from the star.¹ One of the main challenges with this method is that the exoplanet and the host star have to align, in a line of sight sequence when being viewed from the focal point of Earth. A majority of planets that orbit other stars will not pass in front of their star in this manner, and therefore cannot be detected using this method when relatively viewed from Earth. Another difficulty when taking into account the limits of this planetary detection method is the star’s total luminosity. During a transit, the signature ‘drop’ in apparent brightness detected is less than 0.01%, and is a result of the planets being smaller than their host stars. Considering these difficulties and technical constraints, this primarily remains one of the best methods for detecting and further studying exoplanets.This method is even capable of detecting exoplanets that are orbiting at an Earth like distance from its star but it is very difficult to detect such planets because these planets must be orbiting in a detectable plane which has a small chance of occurring, specifically, the chance is 0.43%.⁴ This observational method of detection may also be able to observe ring structures, such as the ones around Saturn or Jupiter’s moons, by measuring and using an understanding of orbital mechanics. The Kepler space telescope is one that finds exoplanets by carefully watching the star light exhibited by a solar system’s host star. Future missions such as NASA’s TESS and PLATO will also use this method.⁴

Figure 2: This figure explains clearly how the radial velocity method works. It also gives a visual representation of how this method of observation works. Source: http://lcogt.net/spacebook/radial-velocity-method

Another primary method that scientists use to detect extrasolar planets is the Radial Velocity method. Both the Transit Method and the Radial Velocity Method are the two big indirect imaging methods, which means that scientists are not directly taking pictures or images of objects that orbit other stars. With the Radial Velocity Method, we can see that as a planet orbits its host star, it initially appears that there is only a gravitational pull being exerted on the orbiting planet, while the star remains stationary. However, considering what we have come to learn about Newtonian physics and orbital mechanics, we know that the planet also produces a significant gravitational field that acts as a force pulling on the star as well, essentially causing the star to wobble slightly as the planet complete an orbit because it changes the center of mass away from the center of the star. This small yet significant wobble can be detected by telescopes and observatories on Earth as the star’s spectrum of colour shifts colour and is a result of the Doppler Effect. The Doppler effect is what causes light waves to stretch and contract as the light source moves towards or away from the observer. This change in wavelength causes the colour of stars to shift red when they are moving away from us, and shift blue when moving closer to us, as displayed in figure 2. Due to the wobbling of the star caused by the orbiting planet, the star can be pulled towards or away from Earth. When the star wobbles towards Earth, the visible light waves that it emits get stacked closer together causing the light to appear blue. As it wobbles away from Earth the light waves become more spread apart, making the visible light look red. Astronomers are able to detect exoplanets with this method by measuring the spectral fluctuations of stars. This method is currently the best for detecting big planets that closely orbit their stars.¹

Figure 3: This is an animation that shows the first extrasolar planets that were discovered using direct imaging. We can see the extrasolar planets orbiting their sun in this animation. Source: https://commons.wikimedia.org/w/index.php?curid=55463078

With the right conditions, an exoplanet can be directly imaged around the star it orbits. The stars visible light will be millions of times brighter than the planets that orbit it but when looking at the infrared light of a star system, a star is only thousands of times brighter than its surrounding planets. These differences are greater in younger star systems because these systems are still cooling which emits more infrared light than that of older star systems. When imaging a planet directly, the planet must be must at a considerable distance from its parent star so that it can be defined in the the glare of of the light from its parent star.¹

There are many other methods of detecting exoplanets, but they are used less frequently because they are much harder to detect exoplanets with. However, we will go through a couple of the less common methods of detection. One very successful way to detect exoplanets is through the timing method. This method, most of the time, requires the exoplanet to be orbiting around a pulsar in order for detection and therefore cannot be used regularly.¹ A pulsar is a star that emits a highly magnetic field and has an axial rotation that makes it appear like it is pulsing, hence pulsar. These pulses are highly regular and therefore can be timed very precisely. When an exoplanet is orbiting a pulsar, the timing of these pulses is altered because the gravity of the exoplanet causes the pulsar to wobble. Even though this wobble is very small, it has an effect on the timing and pulses emitted by the pulsar and the presence of an exoplanet can be inferred because of this change.¹ This method can also be used with regular stars, but it also also much less accurate. With regular stars, their luminosity fluctuates and these fluctuations in luminosity are also predictable. When the predictability pattern of these stars is altered, an exoplanet is inferred to be orbiting the star.¹

Figure 4: This figure shows how a star an wobble because of a change in center of mass with the presence of an orbiting planet. This shift causes both the planet and the star to orbit the center of mass. The large white circle represents the star, while the small circle represents the exoplanet. Source: http://www.skymarvels.com/infopages/exoplanets.htm

The astrometry method involves looking at the wobble of a star which is caused by the gravity of an orbiting exoplanet. This involves tracking the changing position of a star over time and imaging the star directly which is much more difficult than using the Doppler shift, as explained earlier in the radial velocity section, effect from the radial velocity method because it requires very precise measurement of the very slight wobble of the star, in contrast to the radial velocity method where you only have to measure a change in colour of the star. There are also many difficulties in using this method with ground based telescopes because atmospheric changes can make observing the changing position of the stars very difficult.¹ In 2013, the European Space Agency (ESA) launched a spacecraft called GAIA that is currently searching for exoplanets using this method because it avoids the atmospheric disturbances that we would observe in ground based telescopes.¹ GAIA began its work in July 2014 and has since cataloged the position and brightness of over a billion stars which gives us a tremendous amount of data to search for exoplanets. ¹⁵

Gravitational Microlensing

Figure 5: This figure shows how light is bent around a lens star to produce the lensing effect. Source: http://www.teara.govt.nz/en/diagram/8008/gravitational-microlensing

Gravity is something that we have seen to affect everything in our universe, this includes light. Most planets in our galaxy are too small to significantly influence light, but stars are much more massive and can have massive effects on light. We use this in order to observe gravitational microlensing. This method of detecting exoplanets consists of observing gravity bending light from behind a massive object, such as another star, and measuring this bent light.¹ When Albert Einstein published his theory of general relativity, one of the consequences of this was the prediction that the path of light could be deflected by gravity. This means that light passing a massive body, such as a star, can be bent.This method works because this bending of light is similar as to what happens with light when it passes through a magnifying glass and scientists can predict, thanks to Einstein’s theory of general relativity, how this light will bend around the star. When there is a disruption in the predicted bend of light, then scientists can infer an exoplanet is orbiting the star the lens star because the gravity of this orbiting planet contributes to the lensing effect of the lens star (figure 5). This method only works when the lens star and the observed star are perfectly lined up and these cases of microlensing are relatively rare and difficult to predict, and for this reason, this method is not used nearly as much as other methods.¹

Conditions to Support and Sustain Life (As Prescribed by Earth)

One might see the conditions to support life as being fairly straightforward. We would obviously need to be looking for a planet with a suitable atmosphere similar to Earth’s and liquid water, but in reality there are numerous other considerations that need to go into finding a planet that has all the conditions to support life. Astronomers like to refer to these ideal conditions as the ‘Goldilocks principle’, after the classic fairytale of ‘Goldilocks and the Three Bears’. The first thing we should look for when looking for an exoplanet with the ideal conditions for life, is a planet that contains the right elements and compounds that life would need to survive. These include water, oxygen, nitrogen, carbon, and hydrogen just to name a few. 5 These are ingredients that all life, as we know, needs to survive.

The next thing we would look for is a planet with the right crust. The Earth has a solid crust and a molten center that creates a magnetic field that protects it from much of the sun’s harmful radiation. 5 This means that we cannot live on world that is molten because the crust would be too hot, nor could we live on a gas giant, such as Jupiter, because there is no stable crust to live on.

Another very important condition we would need on a habitable planet is the right temperature. This is because, as mentioned earlier, one of the conditions for life is liquid water and if the temperature is too hot or too cold the water will either be in gaseous or solid states. Earth’s position in relation to the Sun is vital in dictating its temperature and so are the presence of proper atmospheric compounds, such as carbon dioxide, methane, and water, since they work together to create a greenhouse effect which maintains the perfect temperature where liquid water can thrive on Earth’s surface. 5 Currently, Earth is situated comfortably in the Sun’s habitable zone and the atmosphere is effectively trapping the Sun’s heat, creating a situation where liquid water, and subsequently life, can exist.

Figure 6: This figure explains the Earth Similarity Index and gives a visual representation on how the planets in our solar system compare to Earth using this index. Source: http://phl.upr.edu/projects/habitable-exoplanets-catalog/methods

The presence of our Solar System’s largest planet, the gas giant Jupiter, also plays a role in maintaining habitability. Due to its massive size and gravitational pull, it has helped determine Earth’s orbital path around the Sun. 5 Jupiter also acts as a shield for Earth since it absorbs many meteorite strikes. However, the massive gravity from the planet could alter the paths of other small objects, possibly sending them towards Earth. 5 Overall, the effects of Jupiter have played a role in the placement of Earth in the habitable zone of our Solar System as well as the general protection of Earth from stellar bombardment, contributing to Earth’s habitability and longevity as a planet.

As we can see, there are many conditions to take into account when looking for a habitable exoplanet and it may be very rare to find one with all the right conditions. According to Earth’s standards, the habitability of an exoplanet is generally reliant on “plate tectonics, a global magnetic field, a hydrosphere, and the distance of the habitable zone from its host star”. 5 Although the conditions for a perfectly, Earth-like planet are quite extensive and specific, the search for such a planet continues. And in the grand scope of the known universe, it’s only a matter of time until more Earth-like planets, and possibly other signs of life, are discovered.

Possibly Habitable Exoplanets

Using the described technology and criteria several planets have been discovered meeting the habitable criteria. The closest potentially habitable planet is Proxima Centauri B located around the star Proxima Centauri 4.2 light years away. The planet is likely to be tidally locked to the star and larger than Earth.⁷ While this does cast doubt on its habitability, it is still possible for the planet to be habitable. Since its discovery in 2014 using the Kepler Spacecraft, Kepler-186f has held the title of most likely to be habitable. Its orbit places it nicely it its habitable zone (unlike many other potentially habitable planets that are nearer to the edge). More importantly though is Kepler-186f’s mass. Weighing within 10% of Earth’s mass, it is the closest exoplanet in comparison with the Earth, excluding the most recent discoveries (that we still don’t know much about). ⁸ Besides knowing the planet is rocky, the chemical composition of Kepler-186f has not yet been determined and therefore it is unknown if the planet is truly habitable. There has also been a recent discovery earlier this year regarding habitable planets. In a press conference on February 22 NASA announced the discovery of a seven-planet system dubbed TRAPPIST-1. The system, a mere 40 light years away was discovered by the Spitzer Space Telescope. Three of the planets of Earth size have been determined to be within their star’s habitable zone, and are believed to be rocky planets. Due to the recentness of the discovery not much else is known about the planets. ⁹ There are multiple other exoplanets out there that have been discovered that could be considered habitable, and there are likely many more yet to be discovered.

Figure 7: This figure shows all the discovered exoplanets as of 2014. Only the the planets that are less than 10 Earth masses are labeled. The green shaded areas are the potential habitable zones and the size of the circles in the figure corresponds to the radius of the planets. i.e Larger circles have a larger radius. Source: http://phl.upr.edu/projects/habitable-exoplanets-catalog

The discovery of exoplanets is significant through the unveiling of the specific details that are vital in determining their habitability. The nature of these details, such as atmosphere, geographical features of the planet, and seasonality, along with other significant findings, could potentially be produced if a number of factors were to be considered and implemented. The first of these is the study and development of new observational methods and techniques. Additionally, the number of astronomers and scientists dedicated to using these techniques and analyzing the light spectrum data from these exoplanets would be critically important. Secondly, while considering the distance to other planets in other solar systems, we would need to utilize the capacity of multiple space telescopes and observatories aimed simultaneously at planets of interest. Significant resources would have to be allocated to the development of software and hardware that we would need to use to process the vast amounts of data incoming from these telescopes over prolonged periods of time. Astronomers and scientists have recently begun applying the use of artificial intelligence to help process all of the of data streaming in from space telescopes. Another factor that will greatly help the study and detection of exoplanets is the deployment of additional new-age deep space telescopes and satellites. All these tools play a major role in what we may find. Prolonged exposure by observation and the cumulative efforts of detection and techniques could potentially bare answers to some of our many questions and mysteries of the universe.

## How The Extrasolar Planets Are Detected

We no longer harbour any doubt that we are not alone even in our own galaxy Milky Way, leave aside the whole universe, which, incidentally, is just one of an infinite number of universes according to many cosmologists. The number of planets discovered outside our solar system stood at about one thousand at the end of 2013. Over three thousand five hundred more were awaiting confirmation.Yet these numbers, unthinkable twenty years ago, sink to insignificance with what we are hearing from astronomers nowadays. Backed by a torrent of data which an array of ever more sensitive instruments provide, they estimate the number of Earth-like planets in Milky Way with orbits within the &ldquohabitable zones&rdquo suitable for life, in tens of billions, not to mention as numerous &ldquoHot Jupiters&rdquo getting roasted in orbits almost skimming the surfaces of their stars, or ice worlds orbiting way, way out. Astronomers make these exciting discoveries, by using imaginative methods. Let&rsquos have a look at the main ones.

Two gravitationally bound bodies in space orbit each other around a common center of gravity. That is, a planet does not revolve around a star the star and its planet revolve around each other. But because the star is far more massive than the planet or planets orbiting it, this common center of gravity lies somewhere within the star&rsquos radius. That means, star, too, follows an orbit ─ be it very small ─ around that common center of gravity within itself. The tangible effect of this emerges as a periodic &ldquowobble&rdquo in the movement of star.

Hence, if the wobble is in our line of sight, the star slightly approaches and moves away from the observer with regular intervals. This movement causes tiny fluctuations in the spectrum of the light coming fom the star, due to a process called &ldquoDoppler shift&rdquo. When the star is coming toward us, the spectrum of its light shifts to shorter wavelengths, towards blue light. And as it moves away, the shift is towards the longer, red wavelengths. Astronomers monitoring the movement of likely planet-harboring stars with extremely sensitive devices, confirm the presence of the orbiting planet from these miniscule periodic shifts in the starlight&rsquos spectrum

The most advanced of these devices dubbed &ldquospectrometers&rdquo, are sensitive enough to detect velocity changes of one meter per second. The method, also known as &ldquoDoppler spectrometry&rdquo, is one of the most successful in the ongoing planet hunt. Before the Kepler space telescope began smashing records with the &ldquotransit method&rdquo it employed, most of the extrasolar planets were discovered with this method.

But although the method is independent of distance, it allows the monitoring of stars with a maximum distance of 160 light years from the sun (one light year roughly equals 10 trillion kilometers) since it requires a far stronger signal than the background noise for necessary precision. This method is particularly suitable for the detection of Jupiter-size giants in close orbits to their stars (so called &ldquohot Jupiters&rdquo) but the detection of planets orbiting at great distances require years of monitoring. Planets on orbits with wider angles to our line of sight produce smaller wobbles and hence are harder to detect.

The mass of a star can be inferred from the spectrum of the light it emits from its surface. For, the color of its light is a function of its surface temperature (see, &ldquoHow stars are classified&rdquo in Wide Angle section) Theoretical models for stellar formation and evolution permit the calculation of star&rsquos mass, age and chemical composition from its temperature. And once the star mass is known, the magnitude of the wobble enables the determination of the planet&rsquos mass. A problem with this method is that it can only give a minimum mass for the planet. The true mass can be 20 percent above that limit. If the orbit plane is tilted close to right angle to our line of sight, the inferred mass is closer to the true value. When the method is used together with the transit method to confirm detections, the mass of the confirmed planet can be more precisely calculated.

Yes, but what if the star&rsquos wobble is not radial, but lateral? If, in other words, the planet&rsquos orbit plane is perpendicular to our line of sight? Or, in even more simple terms, if we are viewing the putative planet&rsquos orbital plane from above? Well, the circular or elliptical orbital motion of the star around the common center of gravity can give a clue about the presence or absence of a planet. But since the orbital radius will be very small (center of the gravity being within the star,) it will be hard to detect. In fact, planet discoveries reportedly made in 1950s and 60s with this method, were later proven false.

But the method can be used in a different way. What the astronomers have to do is finding a nearby fixed &ldquoreference star&rdquo to the rear of star with the suspected planet. It is important that this reference star should be relatively stationary, for some stars have high &ldquoproper motions&rdquo and their positions in the sky may change during long years of planet search.

The wobble which gives away the presence of a star can be detected if the target star is seen periodically moving toward and away from the fixed star. Yet changes in the positions of the monitored stars are so tiny that adequately precise measurements cannot be made even with the most advanced telescopes on Earth. But in 2002, astronomers using the Hubble space telescope employed the astrometric method to determine the parameters of a planet discovered earlier around the star Gliese 876.

Despite its shortcomings, the upside of the astrometric method is its particular suitability for the detection of planets on distant orbits. This feature makes it a complementary tool for other methods normally more sensitive to closer orbits. Downside, however, is years or even decades of monitoring required for the detection of planets on orbits distant enough to allow the use of astrometric methods, since they take very long times to complete their orbits.

A planet passing in front of a star under observation causes a tiny dip in the intensity of its light. Analysis of such periodic dips, recorded with sensitive measurements, can reveal the presence of a planet or planets orbiting it. The advantage of this method compared to radial velocity and astrometry is that reveals the size (radius) of the planet. This is a key parameter, for when taken together with the mass determined with the radial velocity method, the density of the planet can be calculated, which, in turn, yields information about its physical makeup (whether it&rsquos a rocky planet, a gas-giant, or a water world with a global ocean). The method also provides data on gases in the atmosphere of the planet as well as their ratios. As the planet traverses its star, gases in its atmosphere absorb some of the spectral lines in the starlight. And the locations and thicknesses of these absorption lines identify the gases and their ratios in the planetary atmosphere. The presence of a planetary atmosphere (and thereby that of the planet itself) can be gleaned from the measured polarization of the starlight passing through or being reflected off the planet&rsquos atmosphere.

Another advantage the method provides is the ability to measure the radiation emitted by the planet. If during the secondary eclipse (when the planet moves behind its star) , the photometric intensity (luminosity) is subtracted from the value for the period before or after the secondary eclipse, the remainder will be the planet&rsquos share in the total. And once that parameter is known, the surface temperature of the planet and even the possible signs of cloud formation can be figured out. In fact, surface temperatures of two planets were determined this way by two separate groups using the Spitzer space telescope in 2005. Planet TrES-1 was found to have a surface temperature of 790 ⁰C, while that of HD 209458b was an even more scorching 860 ⁰C. The COROT spacecraft lofted by the French Space Agency to make sensitive observations with the aim of finding a few earth-mass planets discovered two such worlds.

Yet the undisputed king of the transit method was the Kepler spacecraft NASA had launched in 2009. Before stuck gyroscopes ended its main mission of planet hunting, it monitored 150.000 stars at the same time every 30 minutes for four years to flag rocky Earth alikes. The perused data for the first three years yielded 3538 planet candidates, 167 of which were confirmed. Kepler data showed that most of the planets in the Milky Way were small planets with masses similar to Earth&rsquos and among them, the number of those orbiting their stars in &ldquohabitable zones&rdquo at right distances where temperatures permitted the existence of liquid water essential for life as we know it, could add up to tens of billions.

The method, however, has two serious drawbacks: First of all, for the discovery of a planet with this method, the orbital path of the planet has to be on the same plane with the observer&rsquos line of sight. In other words, the observer has to view the orbital plane from edge on, an alignment with an extremely low probability. The probability of observing a planet as it transits its star on its equatorial plane is mathematically expressed as the ratio of the star&rsquos radius to the radius of the planet&rsquos orbit. Of the planets with close orbits, only 10 percent are observed as transiting their stars on their equatorial planes. This ratio goes further down for the planets with distant orbits. The probability of a sun-like star being observed as it is transited by an Earth-like planet at a distance of one astronomic unit (AU = Sun-Earth distance, or 150 million kilometers) is 0.47 percent. Still, the number of planets discovered in transit surveys monitoring thousands or even hundreds of thousands of stars at the same time may exceed the number of those bagged with radial velocity method. But there is another problem here: Buradaysa bir başka sorun var: It is not possible to identify the host star of the discovered planet. Still another problem is the unreliability of the method, necessitating subsequent analysis with the radial velocity method for confirmation of discoveries.

PULSAR CHRONOMETER METHOD

Pulsar is a special kind of neutron stars. The latter are products of supernovas which put a spectacular end to the short lives of giant stars. When no longer able to generate the energy to counter the weight of outer layers, the core of the massive star collapses on itself and the ensuing shock wave tears apart the star and catapults the outer layers to space with an explosion visible from billions of light years away. The core of about 1.5 solar mass is squeezed so tight that it becomes a sphere of 12-20 kilometers diameter ─ about that of a medium-sized town. The collapse so much whips up the original spin of the collapsing star that the neutron star completes a revolution in the order of milliseconds. The spin has a regularity exceeding those of most precise chronometers. Another feature of the neutron stars is the immense power of their magnetic fields, which can be trillions, or even quadrillions of times more powerful than that of the Earth. They emit powerful beams of radio waves from their magnetic poles.

Such radio-emitting neutron stars are called pulsars. Pulsing behavior is a result of the often-misaligned magnetic and spin axes of the star (just as the case with geographic and magnetic poles of the Earth). The misalignment causes the magnatic poles to draw circles around the spin axis as the neutron star rotates. And when a point on that circle crosses the line of sight of a powerful radio telescope on Earth, radio pulses are received from that point in exceedingly regular intervals (in the order of seconds for most pulsars or even thousandths of a second for some which are called millisecond pulsars). Since the intervals between the pulses are extremely regular, anomalies in these enable the observers to trace the pulsar&rsquos movements. If they have planets circling them, as is the case with normal stars and their planets, pulsars and their planets, too, orbit a common center of gravity. And variations in the times of pulses give away the presence and masses of the planets.

The method is so sensitive that it permits the detection of planets even with masses a tenth of the Earth&rsquos. It can also capture the gravitational interactions within a planetary system. In 1992, astronomers Aleksandr Wolszczan and Dale Frailbir gezegen sistemi içindeki karşılıklı kütleçekim etkileşmelerini de belirleyebiliyor. 1992 yılında Aleksander Wolszczan ve Dale Frail made use of this method to lure out the planets around pulsar PSR 1257+12.

However, since pulsars are relatively rare objects, It is doubtful that high numbers of planets can be detected with this method. Even supposing that they could, the emergence of &ldquolife as we know it&rdquo is impossible because of the extremely high-energy particles and radiation spewed out by their parent pulsars.

Let&rsquos suppose we are observing a star to detect a putative planet: One of the stars in the background of the target star is also in our line of sight. Suddenly we see the background star brighten and after a while return to its former luminosity. Now we can begin to search for the planet in earnest, for what we have seen is a microlensing event. The path of the light coming from the background star was warped by the gravity of the star in our line of sight. According to Einstein&rsquos theory of general relativity, what we sense as gravity is actually an effect of the curvature of space-time. Any object which has a mass bends the space-time. The photons of light coming from the star behind follow the curvature of this warped space and change direction. That is, a greater number of photons start coming in our direction, or, in other words, they are focused. Thus, we see an increase in the brightness of the star behind.

But things are not all that simple. Microlensing is a variant of the gravitational lensing phenomenon, which has emerged as the outcome of one of Einstein&rsquos famous thought experiments and has been verified by astronomical observations numerous times. When the &ldquogravitational lens&rdquo in between is as massive as a galaxy or even a cluster of galaxies, the &ldquosource&rdquo which we cannot directly observe as it is hidden behind, is naturally a source as big as another galaxy. And since the intervening &ldquolens&rdquo bends the light coming from the source, multiple images of the source in the form of elongated (and brightened) segments of a loop, differing according to small imperfections in the alignment of observer, lens and source, appaear around the lens. Because the alignment of the observer on the Earth, lensing galaxy or cluster and the source galaxy does not change much over thousands or even millions of years, these multiple images of the source galaxy remain in place, and detailed analyses of the images enable precise calculations of the distance, mass and shape of the galaxy lying behind.

But difficulties arise when the lens is a small astronomical object such as a star or even a planet. For one, in the observer-lens-source alignment, the source has to be behind and slightly above the lens. The lensing effect in such an alignment produces only two images of the source focused as arches and the distance between these two arches are so small that even the most powerful telescopes on earth fail to resolve them into distinct shapes. In the end, two separate shapes are perceived as a single, superimposed image. And the name &ldquomicrolensing&rdquo derives from the fact that separation between the two arches is too small to be imaged.

Another problem is the brevity of the microlensing event, lasting a few days or weeks because the Earth and the source and the lensing star are in motion relative to each other.

If the foreground (lensing) star has a planet too, the gravitational field of the planet makes a detectable contribution to the lensing effect, thereby giving away its presence. But since the probability of such an alignment is extremely low, to catch a meaningful number of planets using this method requires the simultaneous and continuous observation of a large number of distant stars . Therefore, forming collaborations among themselves like OGLE (Optical Gravitational Lensing Experiment), MOA (Microlensing Observations in Astrophysics) and PLANET (Probing Lensing Anomalies NETwork), astronomers have turned their observing instruments to the dense central bulge of the Milky Way and its satellites, the Small and Large Magellanic Clouds in the southern celestial hemisphere. The surveys have yielded at least two yet-unconfirmed and two confirmed planetary candidates.

Planet OGLE-003-BLG-235/MOA discovered by the OGLE collaboration in 2003 with the microlensing method, and its star which could be discovered two years later.

An obvious problem of the method is that since the fortituous alignment cannot be repeated, the microlensing is a one-off event, not leaving adequate time for extensive studies. And since the detected planets are kiloparsecs away, the find cannot be confirmed with other methods. (A parsec is a unit used for long distances in astronomy corresponding to 3.26 light years. A kiloparsec is 1000 parsecs, or 3260 light years).

Because the light a star emits is thousands or even millions of times brighter than the light reflected off a planet, normally the light planet reflects cannot be seen. But if the starlight is blocked by an opaque mask installed in telescopes, called a coronograph, the feeble light from nearby planets may come into view. Especially if the planet is large (Its radius has to be much bigger than that of Jupiter), is far fromits star and if it is young. Youth causes the planet to be hot and radiate strongly in infrared.

One of the most dramatic discoveries made with the coronography method was the detection of the planet orbiting Fomalhaut, one of the brightest stars of southern skies, 25 light years away. Despite the masking, the light of the A-class star, more massive and hotter than the sun, is seen as leaking out from the perimeter of the coronograph as spokes.

A triple planet solar system announced on November 13, 2008, one of the discoveries mentioned in the caption was detected through observations made with Keck and Gemini telescopes, among the largest on Earth. Hubble&rsquos discovery of Fomalhaut&rsquos planet of three Jupiter masses was made public the same day. Both systems are surropnded by disks reminiscent of the Kuiper belt in our solar system. Finally, with the detection of the planet orbiting Beta Pictoris, this method, too, took its place among the more promising of the planet hunting instruments.

Many stars have disks of space dust surrounding them. These are also called debris disks. What makes them visible is their absorption of the starlight, which then they re-emit in the infrared. Although the total mass of these dust particles is less than Earth&rsquos mass, they outshine the stars they orbit in infrared wavelengths because of the vastness of their combined surface area. Those disks, which the Hubble and Spitzer space telescopes can pick up, have been found around 15 percent of the stars lying relatively close to Sun and having similar masses. The dust in these disks is thought is believed to be relics of collisions between comets and asteroids. Since the radiation pressure from the star should have blown the dust to space in a relatively short time, the continuing existence of dust disks leads to the conclusion that the dust is continuously re-created through collisions and attests to the existence of such star formation leftovers surrounding the central star. For instance, the dust disk around the star tau Ceti, is seen as the sign of existence of belt of rocky and icy bodies and comets, reminiscent of the Kuiper belt outside Neptun&rsquos orbit in our Solar system, only ten times thicker.

Signs were seen pointing to the presence of comets around Beta Pictoris, a young star 20 million years old at most. These dust disks are thought to be relics of collisons between leftovers of star formation like asteroids and comets.

Meanwhile, certain features observed in the structures of dust disks, could signify the persence of planet-size objects. Holes observed at centers of some disks, show that the disks are circular. The empty region is thought to have been created by a planet which has swept away the dust lying between itself and the star. Some other disks display bulges which could have been formed by gravitational pull of a planet. Both these features are seen around the star epsilon Eridani, indicating that besides an inner planet discovered earlier with the radial velocity method, another is orbiting at a distance of 40 astronomical units.

Measurements made in space yield more precise results both because the atmosphere&rsquos distorting effects are avoided, and the observation equipment can make use of infrared wavelengths blocked by the atmosphere. Beyond the detection of rocky Earth-alikes, astronomers aim to study the makeup of the atmospheres of such worlds and search for signs of life with observations from the space. The Kepler space telescope which NASA launched in March 2009 simultaneously monitored 150.000 stars at constellation Cygnus before losing its planet hunting ability due to malfunctioning gyroscopes. NASA is now seeking ways of assigning the spacecraft to alternative tasks which do not require a fixed orientation.

NASA is planning to launch Kepler&rsquos successor, the Transiting Exoplanet Survey Satellite (TESS) in 2017.

The new spacecraft, which will be set on an elliptical orbit around the the Earth and the moon, will survey the whole sky, unlike Kepler which could only track an area the size of one four-hundredths of the space.

To be able to do that, TESS will not point continuously to the same spot, it will change its orientation every month. The interesting candidates it finds will be inspected from the Earth by the existing and next generation telescopes with gigantic light harvesting mirrors of 20-to-30 meters.

Satellite clusters that would jointly seek planets using interferometry such as the Darwin, proected by the European Space Agency (ESA) and NASA&rsquos Terrestrial Planet Finder were later shelved due to technological hurdles and prohibitive costs entailed.

ECLIPSING BINARY PHOTOMETRY

If the stars of a binary system orbiting a common center of gravity are positioned as eclipsing each other in our line of sight, the system is called an &ldquoeclipsing binary&rdquo. When the star with the brighter surface is eclipsed, even partly, by the disk of the companion, the phase with the lowest total luminosity is dubbed the &ldquoprimary eclipse&rdquo. And when the brighter star masks a part of its companion half an orbital tour later, a &ldquosecondary eclipse&rdquo is observed.

These phases of minimum luminosity, occur with a regularity rivaling the precision of a pulsar, the only difference being periodic dips in the intensity of light instead of bright light pulses. In case a planet is orbiting the binary, the stars of the system will also orbit the common center of gravity with the planet and there will be a periodic displacement in the times of lowest luminosity (the minimum will be delayed, will be on time, will be ahead of time, and then will lag again etc.) Following these periodic time shifts are seen as the most reliable method of detecting planets orbiting binary stars.

Kepler was to shoulder the task in this method but now this, too, will seemingly be assumed by TESS. Besides its main target of terrestrial planets, the spacecraft will also keep an eye on the ligh reflecting off the giant planets in very close orbits around their stars. Since such a planet will have phases ranging from total blackness to total reflectivity just like the phases of the Moon. Periodic variations, however miniscule, in the reflected starlight will herald the presence of a planet. Because the phases of the reflective light will be independent of the inclination of the orbital plane. Astronomers believe the method could provide information as to the makeup of the planet&rsquos atmosphere.

The light emitted by the star is not polarized. That is, it oscillates in random directions. But when the light is reflected off the atmosphere of a planet, light waves interact with molecules in the atmosphere and get polarized. Measurements can be made with extreme precision on the light given off together by the star and its planet ─ the latter&rsquos share being one part in a million. Devices used for measuring the polarization, called polarimeters, have the ability of selecting the polarized light and rejecting the unpolarized light (of the star). Although such collaborations as ZIMPOL/CHEOPS and PLANETPOL are searching for exoplanets with polarimeters, no planet has been detected with this method so far.

## 'Winged Cryptids: Humanoids, Monsters & Anomalous Creatures Casebook' is now available on Amazon.com and from our new publisher Singular Fortean Publishing

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## Are planets setting the sun's pace?

The Sun's activity is determined by the Sun's magnetic field. Two combined effects are responsible for the latter: The omega and the alpha effect. Exactly where and how the alpha effect originates is currently unknown. Researchers at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) are putting forward a new theory for this in the journal Solar Physics. Their calculations suggest that tidal forces from Venus, the Earth and Jupiter can directly influence the Sun's activity.

Many questions regarding the Sun's magnetic field are still unanswered. "As with the Earth, we are dealing with a dynamo. Through self-excitation, a magnetic field is created from virtually nothing, whereby the complex movement of the conductive plasma serves as an energy source," says the physicist Dr. Frank Stefani from HZDR. The Sun's so-called alpha-omega dynamo is subject to a regular cycle. Approximately every eleven years the polarity of the Sun's magnetic field is reversed, with solar activity peaking with the same frequency. This manifests itself in an increase in sunspots -- dark patches on the Sun's surface which originate from strongly concentrated magnetic fields.

"Interestingly, every 11.07 years, the Sun and the planets Venus, the Earth and Jupiter are aligned. We asked ourselves: Is it a coincidence that the solar cycle corresponds with the cycle of the conjunction or the opposition of the three planets?" ponders Stefani. Although this question is by no means new, up to now scientists could not identify a plausible physical mechanism for how the very weak tidal effects of Venus, the Earth and Jupiter could influence the Sun's dynamo.

Strengthening through resonance

"If you only just give a swing small pushes, it will swing higher with time," as Frank Stefani explains the principle of resonance. He and his team discovered in recent calculations that the alpha effect is prone to oscillations under certain conditions. "The impulse for this alpha-oscillation requires almost no energy. The planetary tides could act as sufficient pace setters for this." The so-called Tayler instability plays a crucial role for the resonance of the Sun's dynamo. It always arises when a strong enough current flows through a conductive liquid or a plasma. Above a certain strength, the interaction of the current with its own magnetic field generates a flow -- in the case of the colossal Sun, a turbulent one.

It is generally understood that the solar dynamo relies on the interaction of two induction mechanisms. Largely undisputed is the omega effect, which originates in the tachocline. This is the name of a narrow band between the Sun's inner radiative zone and the outer areas in which convection takes place, where heat is transported using the movement of the hot plasma. In the tachocline, various, differentially rotating areas converge. This differential rotation generates the so-called toroidal magnetic field in the form of two "life belts" situated north and south of the solar equator.

A new recipe for the solar Dynamo

There is significant lack of clarity regarding the position and cause of the alpha effect, which uses the toroidal field to create a poloidal field -- the latter running along the Sun's lines of longitude. According to a prevalent theory, the alpha effect's place of origin is near the sunspots, on the Sun's surface. The Dresden researchers have chosen an alternative approach which links the alpha effect to the right- or left-handedness of the Tayler instability. In turn, the Tayler instability arises due to strongly developed toroidal fields in the tachocline. "That way we can essentially also locate the alpha effect in the tachocline," says Frank Stefani.

Now the HZDR scientists have discovered the first evidence for the Tayler instability also oscillating back and forth between right- and left-handedness. What is special about this is that the reversal happens with virtually no change to the flow energy. This means that very small forces are enough to initiate an oscillation in the alpha effect. "Our calculations show that planetary tidal forces act here as minute external pace setters. The oscillation in the alpha effect, which is triggered approximately every eleven years, could cause the polarity reversal of the solar magnetic field and, ultimately, dictate the 22-year cycle of the solar dynamo," according to Stefani.

The scientists surrounding Frank Stefani have been researching magnetic fields in the cosmos and on Earth for many years. They were also the first group in the world to successfully prove both the Tayler instability and the magnetorotational instability in laboratory experiments. In 1999, the specialists in magnetohydrodynamics were also involved in the first demonstration of the homogeneous dynamo effect in Riga.

The Tayler instability restricts new liquid-metal batteries

"Interestingly, we stumbled upon the Tayler instability in the context of our research into new liquid-metal batteries, which are currently being investigated as possible inexpensive storage containers for the strongly fluctuating solar energy," explains Frank Stefani. The fundamental principle of liquid-metal batteries is extremely simple. It consists of two liquid metals of differing densities -- the electrodes -- which are only separated by a thin layer of salt. The benefits are an extremely quick charging time, an (at least theoretically) infinite number of charging cycles and low costs, if a battery which is one square meter in size can successfully be produced. "For these batteries, the Tayler instability poses a serious danger because it inevitably arises when the cells get bigger and bigger. Without certain technological tricks, which we have already patented, the Tayler instability would destroy the battery's stratification," adds Stefani.

## Turbulence in molten core helps amplify Earth's magnetic field

Researchers at the Institut des Sciences de la Terre (CNRS/Université Joseph Fourier Grenoble 1/IRD/Université de Savoie/IFSTTAR) have shown that turbulence, random motion that takes place in the molten metal in Earth's core, makes a contribution to our planet's magnetic field. To obtain this result, they modeled Earth's outer core using liquid sodium enclosed between two rotating concentric metal spheres, a set-up they dubbed the Derviche Tourneur Sodium (DTS) experiment. 1

Their findings have just been published in the journal Physical Review Letters.

Like many planets and most stars, Earth produces its own magnetic field by dynamo action, i.e. because of the motion of an electrically conducting fluid-in this case, a mixture of molten iron and nickel. This ocean of liquid metal, the outer core, surrounds the inner core, which is made of solid metal. It is set in motion by the convection caused by the cooling of the core. The resulting flow is particularly complex: in addition to movement of fluid over long distances, which is well understood and generates a magnetic field, there are also turbulent fluctuations, involving erratic, random motion over short distances. Although turbulence also exists in the atmosphere and the oceans, the turbulence in Earth's core is different, since it is under the combined influence of Earth's rotation and of a strong magnetic field. It is not currently possible to reproduce this distinctive turbulence either by laboratory experiments or by computer simulations . 2 Until now, it was therefore impossible for geophysicists to determine its role with regard to the magnetic field.

In order to better understand the interactions between turbulence and the magnetic field, researchers at the Institut des Sciences de la Terre, in Grenoble, used the Derviche Tourneur Sodium experiment, begun in 2005. In this miniature model of Earth's core, 40 litres of liquid sodium (an electrically conducting fluid) is enclosed in the space between two concentric spheres. What makes this model unique is that a magnet in the center of the inner sphere provides a strong magnetic field, while the rotation of this core drives the flow of the conducting liquid very effectively. Under these conditions, the liquid sodium is subjected to a strong magnetic field and to fast rotation, as would be expected in Earth's core, and undergoes both large-scale motion and random fluctuations.

Sensors placed around the outer sphere and inside the sodium were used to map the magnetic field, while ultrasound beams measured the rate of flow of the fluid using the Doppler effect. The data enabled the researchers to show that turbulent motion increases the fluid's ability to conduct electricity and therefore amplifies the magnetic field, rather than reducing it as earlier experiments had appeared to show. This phenomenon, observed for the first time in the laboratory, was confirmed by numerical simulations.

The findings also apply to planets with a magnetic field and to stars. The discovery of this new component of the magnetic field may explain why in the case of Venus, Earth's 'twin' planet, the liquid metal core does not produce a magnetic field. Closer to home, a better understanding of these turbulent fluctuations could help us to understand magnetic field reversals.

(1) Refers to the members of the Sufi order, the whirling dervishes ('derviches tourneurs' in French), who perform a whirling dance.

(2) A complete numerical simulation of the motions taking place in the outer core would make it necessary to cover a wide range of scales with a very small time step, which is out of reach with current capabilities.

## Early Earth's Magnetic Field Was a Weakling

The protective magnetic field shrouding the early Earth waslikely only half as strong as it is today, a new study suggests.

The research also found that the Earth's magnetic field is 200million years older than previously thought, which has implications for theamount of water that was originally present on the early Earth, and perhapseven on the development of life. Such a weak field in the Earth?s early days mayhave also made for some spectacularauroras, or Northern Lights, at latitudes as low as what is now New YorkCity, researchers said.

Earth'smagnetic field is generated by the turbulent, convective motions of theplanet's molten core. The field extends around the Earth for quite somedistance into space until it meets the sun's incoming solar wind (the stream ofcharged solar particles constantly flowing away from the sun). The boundarywhere the two meet is called the magnetopause.

It is the magnetic field that protects the Earth's surface,and all of its inhabitants, from this energeticsolar radiation, which would harm living organisms and strip away much ofEarth's atmosphere (Mars has no significant magnetic field, which is thought tobe the reason it has such a miniscule atmosphere).

But little is known about the magnetic field as it existedjust after the Earth formed, around 4.5 billion years ago. To learn more aboutthis early magnetic field, John Tarduno of the University of Rochester and hiscolleagues from the University of KwaZulu-Natal in South Africa, turned to thecrystals in ancient rocks that preserve magnetic signatures.

Certain igneous rocks called dacites contain smallmillimeter-sized quartz crystals, which in turn have tiny nanometer-sizedmagnetic inclusions that act as mini compasses, locking in a record of theEarth's magnetic field at the time that the dacites cooled from molten magmainto hard rock.

To look for preserved records of the early magnetic field,Tarduno and his colleagues used the best preserved grains from 3.5billion-year-old dacite outcroppins in South Africa, some of the oldest rocksknown to still exist on the Earth's surface.

Using a specialized magnetic detector, the team found thatthe 3.5 billion-year-old crystals in the rocks recorded a field that is about30 to 50 percent weaker than the field that exists today. The finding isdetailed in the March 5 issue of the journal Science.

Some scientists have suggested that there was no magneticfield on the early Earth, so this result "demonstrates that there was afield at that time," Tarduno said.

This weaker magnetic field also has implications forconditions on the early Earth.

Because "the magnetic field stands off the solar wind,"Tarduno says, it keeps solar particles from eating away at the molecules in Earth'satmosphere.

But in the past, not only was this field weaker, the sun waslikely rotating more rapidly and therefore spinning off a stronger solar windand a magnetopause that was likely much closer to Earth ? today it is at adistance of about 10.7 Earth radii, but then it would likely have been around 5Earth radii out (Earth's average radius is about 4,960 miles, or 6,370 km).

The solar wind situation on the Earth at the time may havebeen something like the Halloween solar storm of 2003, which affectedsatellites, communication, air traffic and power generating systems.

"That means that the particles streaming out of the sunwere much more likely to reach Earth," Tarduno said. The implication ofthat situation is that "it's very likely the solar wind was removingvolatile molecules, like hydrogen, from the atmosphere at a much greater ratethan we're losing them today," he said. And the loss of hydrogen implies aloss of water as well.

In turn, if a lot of water was stripped away early in Earth'shistory, to get the amount of water that we have now, the planet must havestarted "with a fairly robust inventory of water," Tarduno toldSPACE.com.

Both water and a protective magnetic field are essential tothe development of life as we know it, so the finding also has implications forunderstanding how life arose on our own planet, as well as potential lifebeyond on our solar system.

Tarduno and his team's study "suggests that themagnetic field may predate the establishmentof life" on Earth, wrote Moira Jardine, an astronomer at theUniversity of St. Andrews in Scotland who was not involved with the study, inan essay accompanying the new study in Science.

The development of magnetic fields around other planetsoutside the solar system and how well they guard any potential life is alsosomething to consider when looking for planets around young stars, which seemto have stronger solar winds and more frequent solar storms than the sun doesnow, Jardine wrote.

It also means that to evolve an extrasolar planet that isEarth-like, "we need to start with a fairly healthy inventory ofwater," Tarduno said.

The weak magnetic field on the early Earth may have also ledto much more spectacular, possibly extending over an area three times the sizeas they currently do and extending to lower latitudes, possibly as far as thecurrent position of New York City. Auroras are the light shows generated whensolar particles are funneled down the polar axes of the magnetic field andinteract with atoms in the Earth's atmosphere, exciting them and causing themto give off photons of light.

## How are neutron stars magnetic?

If spinning-and-moving charges make magnetic fields, why does a giant neutral thing have one?

“By allowing the positive ions to pass through an electric field and thus giving them a certain velocity, it is possible to distinguish them from the neutral, stationary atoms.” -Johannes Stark

A little bit of physics goes a long way, and that’s especially true in astrophysics, where the tiniest of forces and the smallest of effects become the only things that matter. It is, of course, due to the extreme concentrations and amounts of material that we’re dealing with! Take something as innocuous as our little, insignificant planet.

The fact that we have a molten, rotating and changing core with an active magnetic dynamo inside of it does much more than make compass needles point towards the pole. The magnetic field generated at the Earth’s core extends well out into space, protecting us from cosmic dangers and diverting fast-moving charged particles away from us.

The Sun gets in on the action to an even greater extent its magnetic field is huge, and the plasma often traces out the path of those field lines. We can often see the hot, ionized plasma of the Sun extending upwards and outwards many times the diameter of the Earth, even (on occasion) forming a complete loop and “raining down” like a fiery waterfall.

It’s not so hard to imagine why the Sun or the Earth does this. Think about the following facts:

• These objects are made up of atoms, which in turn are made up of positively charged atomic nuclei and negatively charged electrons.
• There’s a gravitational gradient and a temperature gradient, meaning that objects of different sizes, masses and cross-sections will be affected differently.
• If these phenomena can produce even a small separation of charge, since the Sun and Earth are spinning, these charges that move differently will generate magnetic fields.

But what about neutron stars? Instead of being made out of atomic nuclei and electrons, aren’t they made out of… well, neutrons?

You know, those neutral things — found in atomic nuclei — that aren’t charged?

So how, then, would they make a magnetic field, which themselves are generated by moving electric charges?

This wouldn’t be such an interesting question if we hadn’t made observations like this one.

These are X-rays emitted from the Crab Nebula, as observed with NASA’s Chandra X-ray telescope. We know there’s a pulsing neutron star at its core, and that these X-rays are emitted as a result of a centrally located intense magnetic source affecting the ionized plasma around it.

It’s more than just in the X-ray, mind you Hubble sees these effects in visible light, too!

And as far as scale goes, the Crab Nebula — created in a 1054 supernova explosion — is about 3 light-years in diameter by this point, nearly a millennium after its birth. But what might surprise you is the tremendous size of this magnetic feature it’s more than a light-year in size on its own!

The key is that a neutron star isn’t just a simple ball of neutrons it’s actually layered. As we progress from the outside-in, we find layers of:

• electrons, followed by
• the nuclei of atoms (like iron), followed by
• a layer where nuclei are layered (like impurities) inside an ocean of neutrons, followed by
• a transition zone to the core,
• where the core is a neutron superfluid (a liquid-like phase with absolutely zero friction) along with charged-particle impurities of various masses inside of it.

It’s not like having one single, neutral entity at all! And don’t forget that neutrons themselves are not fundamental, neutral particles, they themselves are made up of charged particles that have different charges and masses from one another!

The neutrons themselves have intrinsic magnetic moments (since they’re made up of these charged quarks), and the incredibly high energies inside the neutron star can not only create particle/antiparticle pairs, but can create exotic particles as well. The charged particles that exist inside the neutron star are highly conductive, plus there are still gravitational, density, temperature and conductivity gradients inside of the neutron star.

And at approximately 10 km in radius — with all the angular momentum of a typical Sun-like star — these things rotate at speeds of between 10-and-70% the speed of light!

In short, that’s a recipe for a magnetic field on the order of 100 million Tesla, or about a trillion times what we find at the Earth’s surface.

No wonder that’s exactly what we see! Even without being absolutely certain as to what’s happening in the innermost core of a neutron star — whether we have high-energy quarks, muons and taus, or any other types of particles rarely found in nature — conservative, conventional physics in these extreme environments makes an ultra-strong magnetic field all but inevitable.

And that’s how a neutron star generates a super-strong magnetic field!

Now the big next question is: can we have a super-strong magnetic field coming from the inside of a black hole? (We see black hole magnetic fields, but are they generated inside the event horizon or outside, such as in the accretion disk?) And if they do come from the inside, what’s the physics behind that? Until we know the answer, the question provides us with more than enough food-for-thought to sate even the hungriest appetite!

## Why Would Aliens Even Bother with Earth?

As an astrobiologist I spend a lot of my time working in the lab with samples from some of the most extreme places on Earth, investigating how life might survive on other worlds in our solar system and what signs of their existence we could detect. If there is biology beyond the Earth, the vast majority of life in the Galaxy will be microbial—hardy single-celled life forms that tolerate a much greater range of conditions than more complex organisms can. To be honest, my own point of view is pretty pessimistic. Don’t get me wrong—if the Earth received an alien tweet tomorrow, or some other text message beamed at us by radio or laser pulse, then I’d be absolutely thrilled. So far, though, we’ve seen no convincing evidence of other civilizations among the stars in our skies.

But let’s say, just for the sake of argument, that there are one or more star-faring alien civilizations in the Milky Way. We’re all familiar with Hollywood’s darker depictions of what aliens might do when they come to the Earth: zapping the White House, harvesting humanity for food like a herd of cattle, or sucking our oceans dry. These scenarios make great films, but don’t really stand up to rational scrutiny. So let’s run through a thought experiment on what reasons aliens might possibly have to visit the Earth, not because I reckon we need to ready our defenses or assemble a welcoming party, but because I think considering these possibilities is a great way of exploring many of the core themes of the science of astrobiology.

Aliens come to Earth to enslave humanity or for breeding partners

Alien races enslaving each other is a common trope of many science fiction universes. While enslavement of defeated enemies or other vulnerable populations has regrettably been a common feature of our history on Earth, it’s hard to see why a species with the capability of voyaging between the stars, and therefore having already demonstrated the mastery of a highly advanced level of machinery and of marshaling energy resources, would have any need for slaves. Constructing robots, or other forms of automation or mechanization, would be a far more effective solution for labor—people are feeble in comparison, harder to fix, and need to be fed.

Likewise, the idea of an alien species needing humans for breeding doesn’t really stand up to scrutiny. The act of sexual reproduction, on a genetic level, involves the combination of DNA from two individuals. So on the most fundamental level, for an alien race to be compatible with us, they would need not only to use the same polymer, deoxyribonucleic acid, as the storage molecule for their genetic information, but also to use the same four “letters” for their genetic alphabet (and not other purine and pyrimidine bases that exist in chemistry), and the same coding system for translating those sequences of genetic letters into proteins, and the same organizational structure of the DNA strands into chromosomes, and so on. There is a lot of ongoing research on whether extraterrestrial life is likely to use DNA, or what molecular alternatives there might be, but it is a huge stretch to expect alien life to be that similar to human genetics. Humans can’t even interbreed with our closest evolutionary relatives on Earth, the chimpanzees (indeed, this is the basis of the definition for different species—two organisms which are not able to reproduce fertile offspring), and so it is overwhelmingly improbable that an alien life form from a completely different evolutionary lineage would be compatible.

Aliens come to Earth to harvest us for food

If aliens wouldn’t be bothered about enslaving or breeding with us, might they simply be coming to Earth for a drive-by meal? The question of whether an alien biochemistry would be able to digest us as food actually comes down to some very fundamental features of the molecules of life. Our cells are made up of various organic molecules: proteins (polymers of amino acids), nucleic acids DNA and RNA (polymers of bases and sugars), and membranes of phospholipids. And so for making more cells for reproduction, growth and repair of our bodies we need a source of these simple building blocks. We eat other animals or plants and our digestive system breaks them down into their component amino acids, sugars, and fatty acids, which we then use as the building blocks for ourselves. So in order to derive any useful nutrition from eating a human, an alien monster would need to be based on very similar biochemistry, and thus have the enzymes needed for processing the molecules we are built from.

A whole variety of amino acids, sugars and fatty molecules are actually found in certain meteorites, having been produced by astrochemistry in outer space, and so maybe extraterrestrial life would be based on the same basic building blocks as us. But there’s another, very interesting subtlety here. Simple organic molecules like amino acids and sugars can exist in two different forms, mirror images of each other (in the same way your two hands are similar shapes but can’t be placed exactly one on top of the other). These two versions are known as enantiomers, and it turns out that all life on Earth uses only left-handed amino acids and right-handed sugars, whereas non-living chemistry produces even mixtures of both kinds.

So if we do find traces of amino acids on Mars, one very good way of telling whether these organic molecules are the relics of ancient Martian life or are just the product of astrochemistry would be to check if they are mostly left- or right-handed forms, or just an even mixture. The most exciting discovery would be to detect traces of ancient bacteria on Mars and to find that they employ the opposite forms of organic molecules to us: right-handed amino acids or left-handed sugars, because then we would know for sure that this life was definitely extraterrestrial and not merely contamination from Earth. So here’s a fascinating thought: alien invaders could be based on exactly the same organic molecules (amino acids, sugars, etc.), but they still wouldn’t gain any nutrition from eating us as the origins of life on their own planet settled on the opposite enantiomers. We’d be mirror images of each other, on a molecular level.

Aliens come to Earth to suck our oceans dry

If alien marauders would need to have an essentially identical biochemistry to bother culling us for food, maybe they come to Earth to harvest some other vital substance. All life on Earth is water-based H2O is astonishingly versatile as a solvent and participant in biochemistry and so it seems likely that extraterrestrial life would also be based on this compound. Perhaps, then, aliens may be drawn to the Earth for our wonderfully wet oceans and seas and rivers and lakes—to siphon off our hydrological cycle.

The problem with this supposition is that there are loads of far better sources of water in space. In fact, we think that when Earth first formed from the swirling disc of gas and dust around the proto-Sun it was actually a pretty dry planet the water to fill our oceans was delivered later by a barrage of comets and asteroids from the colder, outer regions of the solar system. In fact, Europa, one of the moons orbiting Jupiter, contains more liquid water in the global ocean beneath its frozen surface than our entire planet—Europa, and not Earth, is the Waterworld of our solar system. So if you were an alien voyaging between star systems in need of a drink, you’d have access to a far greater amount of water in the icy moons and cometary halo of the outer solar system. You’d also find it much more practical to operate in deep space, rather than trying to suck up the oceans against the gravitational pull of the planet Earth.

Aliens come to Earth for some other raw material

If not water, then maybe there’s some other natural resource that aliens might invade the Earth to exploit. Perhaps they intend to wipe away our cities and begin strip-mining the crust of the planet for ores to extract metals and build more vast spaceships. But in fact, because the Earth formed from a molten state with iron sinking down to the core, our planet’s crust is actually pretty depleted of useful metals like iron, nickel, platinum, tungsten and gold. And as with the water, it’s hard to see why aliens would bother extracting material against the gravity of the Earth when the asteroids are composed of the same basic rocky stuff. In fact, some asteroids are believed to be essentially pure lumps of metal—they were once the cores of protoplanets that were smashed apart again by the colossal collisions in the early history of the solar system. Several companies are already proposing to launch asteroid mining operations to exploit these exceedingly valuable resources.

Perhaps, though, there might be a reason that our hypothetical aliens would come to mine the Earth. While it’s true that the asteroids and Earth, and other terrestrial planets, are made up of essentially the same rocky material, the Earth isn’t simply an inert lump it’s a very active, dynamic place. In particular, the thin crust of the Earth is fractured into separate shards that are continually sliding around on top of the hot gooey mantle, rubbing alongside each other, crunching head-on, subducting one beneath another, or pulling apart to create fresh crust. This is the churning process of plate tectonics.

So far, astronomers have already found over four and a half thousand extrasolar planets—worlds orbiting other stars—and the expectation now is that there are billions of rocky planets in our Galaxy. But here’s a thought right on the forefront of current planetary science and astrobiology. Perhaps terrestrial planets are common, but terrestrial planets with plate tectonics are rare. Plate tectonics is thought to be vital for keeping the Earth’s climate stable over billions of years to allow complex life like ours to evolve, and it also acts to concentrate certain metals into rich ores. It seems likely that only a small proportion of terrestrial planets undergo plate tectonics (neither Mars nor Venus does). So perhaps an alien civilization would come to the Earth for our exceptional plate tectonics and concentration of particular metals, and the fact that the same tectonics had also enabled a rich biosphere to develop would be merely an inconvenience.

Aliens come to Earth looking for a new home

There is a considerable amount of rocky real estate in the galaxy for aliens to consider moving home to, but a terrestrial planet might need to offer more than just a habitable zone locale to be able to support complex life. Communities of hardy microbial cells thriving off inorganic energy deep underground might be able to survive pretty much anywhere, but complex life requires much narrower environmental conditions on the surface. Various features of the Earth beyond our warm oceans are thought to be crucial to maintaining a stable surface environment for geological time periods. These include plate tectonics acting to regulate the climate, a large moon preventing the spin axis of the planet from wobbling too much, and a global magnetic field for deflecting aside the solar wind and preventing the atmosphere being blown away into space. For these reasons, maybe planets like the Earth are something of a rarity, and so present particularly desirable targets for alien colonization.

But while it’s true that such worlds may well be required for complex life to evolve in the first place, once an intelligent species becomes technologically advanced enough to travel between the stars it’s also likely to be able to artificially manage a planet’s environment. For example, many people are already starting to talk seriously about “mega-engineering” or “geoengineering” projects to avoid the worst effects of global warming on Earth, and we’ve worked out, at least in broad terms so far, how further in the future we could “terraform” Mars to create a habitable environment for humans to live on the surface without needing spacesuits. Indeed, the very fact that Earth is already teeming with its own life (most of which is tenacious microbes that affect the chemistry of the atmosphere and oceans) may well be a hindrance to an alien species, with its own quirky biochemistry, looking for somewhere to colonize. It may well be easier to find a terrestrial world that hasn’t already developed life of its own, and install its own biosphere on an empty planet.

Aliens come to the Earth for the Earthlings

To my mind, then, the enormous amounts of time and energy that are likely to be necessary for traveling between the stars in a galaxy, and the fact that raw materials can be sought elsewhere more practically, would rule out aliens coming to the Earth simply to take something we have. I think we can safely rest assured that even if intelligent alien species do exist in our galaxy, they are not about to appear in our skies with an invasion fleet to subjugate humanity and begin stripping our world. Perhaps the thing that may attract only extraterrestrials to Earth is us. I suspect that if aliens did come to Earth, it would be as researchers: biologists, anthropologists, linguists, keen to understand the peculiar workings of life on Earth, to meet humanity and learn of our art, music, culture, languages, philosophies and religions.

If aliens do come to pay us a visit, there’s one final way that the movies have probably got it all wrong. The laws of physics (at least as we currently understand them—after all, in 100 years we may have worked out how to build a practical warp drive or stretch stable wormholes through the fabric of space-time) strongly constrain movement across the vast gulfs between stars. To make the journey time from one star system to the next anything less than scores of millennia, you need to accelerate your spaceship to a fair fraction of the speed of light. The greater the mass you need to accelerate, the greater the energy required, so you really want to keep your starships as small and light as possible.

Intelligent life forms like humans are inherently bulky things, particularly when you want to send a team of them along with all the life support machinery and regeneration systems for keeping them alive in space. But a much more plausible alternative presents itself. Perhaps it’s unrealistic to expect ET to go through all the discomfort and bother of actually voyaging in person across the oceans of interstellar space to far-flung worlds, but instead to travel by proxy. To cross the galaxy not by encasing wet, vulnerable biological organisms within complex life-support technology, but as the hardened, durable technology itself. With a more complete understanding of how the human brain works—the neuronal wiring diagram and other interactions that give rise to intelligence and consciousness—it stands to reason that we could not only simulate this perfectly within hardware to construct an AI (artificial intelligence), but also potentially upload the consciousness of a living person into a computer.

Contained within a capsule of miniaturized electronics and systems for self-repair you’d not only be essentially immortal, but also incredibly compact and light and much better suited for inter- stellar travel. In this sense, perhaps most life in the galaxy isn’t carbon-based (organic), but silicon-based. I don’t mean this in the sense of silicoid monsters imagined living inside volcanoes in The X-Files or Star Trek, but as the hardware supporting complex sentient computer programs. Silicon life would be second generation, existing only because it has been designed and created by a precursor organic species, which itself evolved naturally on a habitable world.

For these reasons, it strikes me that if there is intelligent alien life out there in our galaxy, they almost certainly wouldn’t pay us a visit in person in huge city-sized motherships, but by sending their sentient robots as emissaries. But how would they know we’re here in the first place? Humanity has been leaking (or deliberately transmitting) radio waves out into space for roughly a century,

So an alien civilization running a SETI program with sensitive radio telescopes could detect us. But this radio bubble announcing our technological emergence, centered on the Earth and expanding out into space at light speed, is only around 200 light years across. That is a minuscule region of space in the galaxy as a whole, a disc 100,000 light years in diameter, and so even if the galaxy does contain other intelligent life forms, they would likely still be oblivious to our recent appearance. But although humanity has only been detectably civilized for a century, the Earth itself has been conspicuously alive for many hundreds of millions of years, and this links to one of the hottest topics in current astrobiology. Life on Earth, and specifically photosynthetic life such as plants and cyanobacteria that grow by absorbing the energy of sunlight and splitting water, has been releasing oxygen as a waste gas at such a high rate that it has built up in the atmosphere, first to just a few per cent, and today constituting a fifth of the Earth’s air. Oxygen is a very reactive gas, and the only reason it has been able to accumulate in the atmosphere is that it is constantly being replenished by living organisms. In fact, the presence of oxygen in the atmosphere is thought to be so unusual to the geochemistry of a planet that astrobiologists consider it to be a biosignature of life (specifically if oxygen and a reduced gas like methane are both present).

We are currently on the verge of building space-based telescopes that use spectroscopy to read the composition of the atmospheres of terrestrial exoplanets, and so survey the night sky for signs of life. And we’re only relative newcomers on the galactic scene. There’s nothing special about this exact moment in galactic history, and life on another planet could have evolved intelligence millions of years ago and used their own telescopes to look out for planets displaying the telltale sign of an oxygen-rich atmosphere. But apparently, as far as we can tell the fact that the Earth is obviously sporting biology has not prompted anyone to say hello.

This is a very curious observation, and to my mind could be down to two equally intriguing possibilities. The fact that Earth’s oxygen-rich atmosphere has apparently attracted no one’s attention may simply be because life is so rare that there is not a single other civilization in the galaxy with us to have their attention drawn. Or perhaps planets with an oxygen-rich atmosphere are so staggeringly common that the Earth just doesn’t stand out among the masses. In the first possibility we are solitary and lonely intelligent beings in the galaxy in the second, life is absolutely rife in the cosmos. Both, to me, are equally profound realizations. And the most exciting aspect is that within your and my lifetime we will have launched our atmosphere-reading space telescopes and the science of astrobiology will have been able to tell which one is right.

## Is it possible to tell if a certain extrasolar planet produces its own magnetic field? - Astronomy

The sample tests are intended to acquaint you with the types of questions I will ask, along with the manner that I will ask them. Although some questions on the actual test may be similar, they will not be identical. Of course, if you still have trouble, stop by my office and we'll go over the questions that are giving you a problem.

Each Question is Worth 4 Points (25 questions = 100 points)

Choose the best answer to each question and mark that answer on the answer sheet.

1) Of A, B, and C below, which is NOT a true statement about comets? (If A, B, and C are all true, then D is the answer.)

A) Comets always orbit the sun in the same direction as the planets orbit.

B) A comet's tail always points away from the sun.

C) Kepler's laws describe the orbits of comets.

D) All of the above are true.

2) The fact that the moons of Uranus lie in the planet's equatorial plane are an indication that

A) they formed after the planet was tilted.

B) the are captured asteroids.

C) they were tilted at the same time that the planet was tilted.

D) they are held by a strong magnetic field.

3) The rings of Jupiter were first discovered

A) by a passing spacecraft (Voyager).

B) by our intrepid hero, Spaceman Spiff (from Calvin and Hobbs).

C) by direct telescopic observation (they were seen with a telescope) by Galileo.

D) by observation from the Earth of a stellar occultation.

4) The rings of Saturn were shown not to be solid by astronomers at the Allegheny Observatory (University of Pittsburgh) in about 1885. Spectra were taken of the inner and outer edges of the rings. These spectra showed, by analysis of the redshift and blueshift,

A) that the inner edges of the rings orbited faster than the outer edges.

B) that the outer edges of the rings orbited faster than the inner edges.

C) that the outer edges and the inner edges orbited at speeds consistent with the rings being solid, but that the edges were made of completely different materials.

5) Why don't the rings of those planets that have rings like Saturn condense to form one or two large moons?

A) The material in the rings lies within the Roche limit, and so is prevented from existing as a large body by gravitational forces.

B) The material is orbiting too fast to condense into one or two large bodies.

C) There is not enough material in the rings to form a large body.

6) Which of the following statements concerning Jupiter and Saturn is false?

A) They both have a ring or rings.

B) They are both gas giants.

C) Jupiter was known in antiquity, but Saturn wasn't discovered until the late 19th century.

D) They both have "years" that are longer than that of Mars.

7) The icy surface of Europa is largely free of craters compared to many other moons. This fact likely points to what?

A) Europa's gravity is considerably weaker than that of the other Galilean moons.

B) A strong electrical current between Europa and Jupiter.

C) Europa having an internal source of heat.

D) There is no known mechanism to account of the lack of craters on Europa.

8) Which of the following was the first planet predicted to exist before it was found, based on perturbations of another planet's orbit ?

A) The planet orbiting 51 Pegasi

9) Periodic meteor showers (ones which reoccur each year) such as the Leonid or the Draconid meteor showers are the result of

A) the Earth passing through the Kuiper belt each year.

B) small asteroids that have their normal orbital paths altered by the gravity of Jupiter at the same point each year.

C) bits of material that hit the Earth as it passes through the orbital path of a periodic comet.

D) debris that was expelled into interstellar space by a star that exploded millions of years ago in whatever constellation the showers are named for.

10) Mercury and Titan are close in size, but Mercury has no atmosphere, and Titan has a substantial atmosphere. Why is Titan able to hold a more substantial atmosphere than Mercury?

A) Because Titan has much higher gravity than Mercury.

B) Because Titan is farther from the sun than Mercury, and thus receives far less energy from the sun. This results in lower speed for the molecules in the atmosphere of Titan than for Mercury, so more of an atmosphere can stay with Titan.

C) Because Titan's atmosphere is composed of very thick, viscous semi-liquid gasses that actually stick to Titan, rather like molasses, where Mercury's ancient atmosphere was only light gasses that escaped quickly.

D) Because Titan orbits such a large planet (Saturn), and thus the gravity of the planet makes it easier for Titan to keep an atmosphere, but Mercury is a planet all by itself.

11) Our search for planets around other stars (extrasolar planets) has so far yielded relatively large, Jupiter-like planets, which orbit mostly fairly close to their stars. Does this mean that Earth-sized and smaller planets are rare?

A) No, although we've been unable to actually see these smaller planets, we've seen their shadows pass across the disk of their "sun", so we know that they're out there, waiting to be discovered.

B) No, the methods we use to search for extrasolar planets are more likely to detect large planets which orbit close to their star, so its not surprising that this is what we have found.

C) Yes, if there were Earth-sized planets orbiting these stars we would easily be able to detect them.

D) Yes, we've been able to detect traces of elements in the atmospheres of the large extrasolar planets that indicate that these large planets have absorbed the smaller planets. We are just lucky that a similar thing didn't happen here.

12) The "Great Red Spot" on Jupiter is similar to other spots on several of the Jovian planets. These spots can be partly explained by the coriolis force. Applying the principle of the coriolis force, we would expect that as an object in the atmosphere

A) moves away from the equator, either north or south, that it would appear to move faster than the ground under it.

B) moves toward the equator, from either the north or the south, that it would appear to move faster than the ground under it.

C) moves away from the equator, going north , that it would appear to move faster than the ground under it, and going south, it would appear to move slower than the ground under it.

D) moves away from the equator, going north, that it would appear to move slower than the ground under it, and going south, it would appear to move faster than the ground under it.

13) Consider an object in space that hits the Earth's atmosphere and continues down to strike the surface. What names do we assign to the object in each of its phases, in order (in space, in the atmosphere, on the ground)?

A) asteroid in space meteor in the atmosphere meteoroid on the ground

B) meteoroid in space meteorite in the atmosphere meteor on the ground

C) meteor in space meteoroid in the atmosphere meteor on the ground

D) meteoroid in space meteor in the atmosphere meteorite on the ground

14) Which of the Jovian planets has retrograde rotation?

15) There are no asteroids at the Jupiter's L1, L2, and L3 points, even though the Trojan asteroids orbit at the L4 and L5 points. Why is this?

A) Most likely no asteroids have ever wandered into the L1, L2, and L3 points, because if some did, they would be "stuck" in orbit there.

B) The L1, L2, and L3 points are unstable. Any asteroids that were once in them would have drifted out over a short period of time due to perturbations from other bodies.

C) The L1, L2, and L3 points lie within Jupiter's Roche limit, L4 and L5 do not.

D) The L1, L2, and L3 points are all contained in Jupiter's ring.

16) The Kirkwood gaps in the asteroid belt

A) are at random orbital distances, and their cause is not well understood.

B) are caused by the gravitation of Jupiter, and all at integer resonances with Jupiter's orbit (like 1:2, 1:3 2:3, 3:5 and so on)

C) are the result of collisions between asteroids, which are more likely in certain orbits, and which sweep out certain gaps in the asteroid belt.

D) are the result of comets passing through the asteroid belt.

17) The volume of Jupiter is about 1400 times that of the Earth, yet we determine the mass of Jupiter to be only about 318 times the mass of the Earth. What does this tell us about Jupiter?

A) Jupiter is made of materials that are less dense than those making up the Earth.

B) We have miscalculated either the volume or the mass of Jupiter.

C) Jupiter is most likely made up of materials like the Earth, since we would expect mass to increase more slowly than volume for any material.

D) Jupiter is made of materials that are more dense than those making up the Earth.

18) One method that we use to date the surface of the Galilean moons is:

A) radioactive dating of samples of the surfaces.

B) counting the number of craters per square kilometer combined with an estimate of the rate of impacts on the surfaces.

C) comparing the distance at which the moons orbit Jupiter and using Kepler's Laws.

19) The most common element in the solar system is:

20) We find that the smaller moons of the Jovian planets tend to be irregular shapes, not spherical, while the larger moons are spherical. Is there a reason why we might expect this to be true?

A) No, there is no particular reason to expect small bodies to be irregularly shaped.

B) Yes, we would expect them to be irregularly shaped due to their extreme distance from the sun.

C) Yes, we would expect them to be irregularly shaped because they are too small for their own gravity to "mush" them into spheres.

D) Yes, we would expect them to be irregular due to the greater tidal effects of the large Jovian planets on the smaller moons.

21) The layer of the sun that produces most of the visible light is:

22) The sun gets its energy main from

A) the fission of helium to hydrogen.

B) the fusion of uranium into plutonium.

C) the fusion of hydrogen into helium.

23) The sun's magnetic field reverses polarity on a fairly regular basis. What is the average time between magnetic pole reversals? (Flipping from N-S to S-N)

24) The average period of Halley's comet is about 76 years, which makes it a short period comet. Short period comets come primarily from which region of the solar system?

25) We can precisely measure the positions of stars with possible planetary systems over time to search for extrasolar planetary systems because:

A) Large planets will cause the star to wobble slightly.

B) Stars with planets remain at precisely the same location as the planets orbit, due to the constant slight pulling in different directions caused by the planets' gravity

C) None of the above. Measuring the positions of stars cannot possibly give us information whether they have planets.