# Can a planet in our system eclipse the sun as seen from another one?

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When the Sun, Earth and Moon get properly aligned, we get eclipses, where the Sun is partially or totally hidden by the Moon when seen from Earth.

Is it possible for one planet of the solar system to produce an eclipse on another one?

There are two underlying questions in this:

• are alignments of the Sun and two planets actually possible (and if so, how frequent)? Given the slightly different orbital planes of the planets, this seems difficult?

• even though planets are much smaller than the Sun, with appropriate distances between the three bodies considered, is it actually possible for one planet to partially or completely hide the Sun like we see in the Sun-Moon-Earth case, or does it only happen for planet-satellite combinations given the relative distances (i.e. in all other cases if would just be a tiny point transiting across the Sun)?

I understand that the Earth would not be able to project a shadow far enough to get such a phenomenon on other planets, but are there other combinations where it's possible/did happen?

In our solar system, it is possible for one planet to partially eclipse the sun, but it is not possible for any planet to cause a full solar eclipse as seen from another planet.

The sun is too big and the planets are too small and too far apart. Transits occur, and can occur for any pair of planets, but they are very rare. As seen from earth, Mercury only transits a dozen or so times per century, and before 2004, the last time Venus had transited the sun was 1882. http://www.eclipsewise.com/transit/transit.html Orbital periods increase for the planets further away from the sun, so transits across the sun by the outer planets (while possible since no planets are in orbital resonance since Pluto got kicked out) are increasingly rare. Michael Seifert dug up these treasures, documenting transits of the outer planets: Jupiter, Saturn, Uranus. As expected, the transit of Uranus from Neptune is the rarest, occurring next in October, 38172 CE.

Proof that no planets could fully eclipse the sun from another planet: To rule out the possibility that any eclipses are possible, We only have to verify orbits pairwise in order of increasing distance from the sun. Since, if the $$nth$$ planet can't fully eclipse the sun from the $$n+1_{th}$$ planet than it certainly won't be able to do so from any other planet. For example, if Jupiter can't eclipse the sun from Saturn, it certainly won't be able to from Neptune.

As seen from a planet, if the apparent size of the sun in the sky is smaller than the apparent size of the next inner planet, then a full eclipse is possible. The largest an inner planet can appear from an outer planet is when the inner planet is at aphelion and the outer planet is at perihelion and the pass is directly overhead of the observer.

The formula for angular radius of a sphere is $$delta = arcsin (d/2D)$$ where d is the diameter of the sphere and D is the distance between the observer and the center of the sphere.

I ran these calculations using matlab and data from NASA. https://nssdc.gsfc.nasa.gov/planetary/factsheet/ For my input data, the rows are planets, and the columns are diameter (km), perihelion (e6km), and aphelion (e6km).

Here are my results. Since the values in the second column are smaller than the first for each row, no inter-planetary solar eclipse is possible. As an example of how to read this table, an observer on Venus will see an angular radius of 6.48e-3 radians for the sun in the sky, but an angular radius of only 6.47e-5 radians for the next inner planet, which is Mercury.

Here is my matlab code for posterity:

sunDiameter = 1.3927e6; planetVals = [4879,12104,12756,6792,142984,120536,51118,49528;… 46.0,107.5,147.1,206.6,740.5,1352.6,2741.3,4444.5;… 69.8,108.9,152.1,249.2,816.6,1514.5,3003.6,4545.7]; results = zeros(2,7); for i=1:7 innerPlanetDiam = planetVals(1,i); innerPlanetApogee = planetVals(3,i)*1e6; outerPlanetPerigee = planetVals(2,i+1)*1e6; outerPlanetDiam = planetVals(1,i+1); sunApparentSize = asin(sunDiameter/(2*(outerPlanetPerigee-outerPlanetDiam/2))); innerPlanetApparentSize = asin(innerPlanetDiam/(2*(outerPlanetPerigee-outerPlanetDiam/2-innerPlanetApogee))); results(1,i) = sunApparentSize; results(2,i) = innerPlanetApparentSize; end results'

The short answer is no. The alignment occurs, but the eclipse does not, due to the distances and size differences involved.

There are a few things to consider here. You already mentioned a lot of them, I'll address those considerations separately.

## Alignment

are alignements of the Sun and two planets actually possible (and if so, how frequent)?

We can intuitively gauge this, as we've seen this happen during the transit of Venus:

This situation describes your scenario. The sun, Venus and the Earth are aligned (within some reasonable range).

But, in fairness, you're asking about any of the planets, not just Earth and Venus. So is this possible for any other (or possible even all) combinations of two planets?

This is two questions in one: Do two planets' orbits overlap, and do these planets ever find themselves at this overlap point at the same time?

Do two planets' orbits overlap?

Quite interestingly, not only can they overlap, they must do so. The only way for two orbits to never overlap would be if they were parallel to each other (e.g. one orbit around the equator and one orbit around the tropic of cancer), but that second orbit is impossible.

All orbital planes must intersect with the parent body's center of mass. It's not possible to have an orbit that does not. To put it into visual terms:

There is no way to position these two orbits in a way that they (a) are physically valid and (b) not have any overlap.

Do these planets ever find themselves at this overlap point at the same time?

The only way for them to not find themselves at this overlap point would be if their orbits were periodic. I would argue that perfect periodicity is unlikely to occur in nature, but that's an intuitive argument.

Looking at the planets' orbital periods, none stand out as being particularly periodic. I'm unsure if we can ever prove that our measurements or orbital period at any given time have perfect precision to conclude that perfect periodicity exists.

Other answers have already indicated the frequency with which such overlaps could occur. I'm mostly interested in whether they can occur or not, to see if we can conclude that your idea is possible or not.

So far, we haven't actually disproven the theory. Orbital planes will overlap, and our planetal orbits are not periodic so they will eventually overlap. But now we get to the other part: the shadow. Will the shadow of the nearest planest reach the furthest planet?

Let's look at the expected size of the shadow. Do you think it will be larger or smaller than the planet who casts it?

The simple rule of thumb here is that if a shadow is larger than the object itself, then the object itself must be larger than the light source. If instead the light source is significantly bigger, then the object's shadow will shrink, and effectively disappear past its focal point.

Note: I call it a "focal point" because of what it looks like on a diagram, and for lack of a better name. If anyone has a more official term, I'd be happy to hear it.

To put it in visual terms:

I don't think we need to explain how much bigger the Sun is compared to any of the planets.

The relative size of the Sun vs any planet renders the penumbra negligible. It could possible be perceived by lightsensitive equipment (edit: I don't know if it can be perceived, I just can't prove that it can't be perceived), but not by a human who observes this while standing on the further planet. It won't look like an eclipse. I refer back to the transit of Venus:

Given the significant size difference between the Sun and planet, and the massive distances between the planets themselves, it doesn't seem feasible for the umbra to even reach the next planet's orbit before it hits its focal point, which is why we don't describe such a transit as an eclipse.

NASA explains this on their website:

Like an eclipse, a transit occurs when one object appears to pass in front of another object. But in a transit, the apparent size of the first object is not large enough to cast the second into complete shadow. Instead a much smaller dark shadow makes its way across the face of the further planet or star. Perhaps the most famous recent transit was that of Venus across the face of the sun in 2012.

## Casting a shadow - intuitive explanation

There's a more intuitive way to express this. If a certain object (nearest planet) can block a light source (Sun), that means that from the point of view of the observer (on the furthest planet), the object must appear to be bigger than the light source. There's a quite well-known scene from Apollo 13 that showcases this:

Tom Hanks (as Jim Lovell) is blocking out the moon using his thumb. Even though the moon is much bigger than Tom Hanks' thumb (citation needed), the relative closeness of his thumb to his eye (compared to that of the moon) makes the thumb appear bigger than the moon.

Let's say that Tom Hanks puts down his hand, and Chris Hadfield sticks his thumb out of the window of the ISS. His thumb is perfectly aligned between the moon and Tom Hanks' eye. We assume his thumb is the same size as Tom Hanks' (citation needed).
This wouldn't block out the moon anymore from Tom. But why is that?

Very simply put, even though the thumb is the same size, the relative distances between the eye, thumb and moon have changed. And now, Chris Hadfield's thumb does not appear to be bigger than the moon, when observed by Tom Hanks. And therefore, it can no longer block the moon from Tom Hanks' view.

So, we can reformulate your eclipse question: Is there any planet which appears to be bigger than the Sun, when observed from another planet?

Edit: I've decided to make the below text easier to parse, by talking about the Earth and Venus. These are just examples, the same applies to any combination of planets, where Earth = furthest and Venus = nearest.

If you assume that the Sun isXtimes larger (in diameter!) than Venus, and the sun isYtimes further from Earth than Venus is from Earth, then Venus will appear to be bigger than the sun (when standing on Earth) whenX < Y.

In other words, when measuring the distance from Earth, the sun needs to be further away than Venus, by a larger factor than the sun is bigger than Venus.
As a simple example, if the sun were exactly 5 times bigger than Venus, it would have to be >5 times as far from Earth as Venus is, for it to appear to be smaller than Venus.

If you look up the numbers for planet diameters and orbital radii, you'll notice that this doesn't happen in our solar system. Not even close. This is because the sun is simply too big relative to any ratio of two planetary orbits.

## Is it possible for one star to orbit another?

I'm dealing with a planet very similar to Earth, except that it has two suns. So, of course, it would be an extraordinary occasion when the inhabitants of the planet see only one sun in the sky. So the idea is that it is extremely rare for there to be a moment when the two stars and the planet align perfectly so that only one sun is visible in the sky. About once every 1000 years. Apparently, a prophecy made the last time the planet and the stars align, is to be fulfilled the next time it happens which (in the story) is about a few months away.

However, stars don't exist so close to each other for the simple reason that the gravitational pull between the two stars would be so great, they might very well end up colliding. Also, even if two stars could exist close together in space, a planet could not conceivably orbit around them both.

However, if we had a primary star that is maybe about the size of our current sun, and a secondary star in orbit who's radius is twice Jupiter's radius (by the way, the sun's radius is about 10 times Jupiter's radius), the gravitational pull would be stable enough that the secondary star, like our planets, would be able to remain in orbit. Instead of, you know, crashing into the primary star.

So basically, in this solar system, I have the primary star (the size of our sun), then the secondary star (with 2 times Jupiter's radius) is in the first orbital, then the home planet in the story (the size of earth) with at least one moon in the second orbital, and then the third orbital onwards may or may not be occupied by planets, it's not really relevant now.

Is this arrangement possible? Assuming the home planet is sufficiently far away enough from the two stars to not be baked to a crisp, could a star conceivably orbit another, and still support its system of planets?

I don't need super scientific answers. I'm not an expert in astronomy, and stuff, so errr. A slightly dumbed-down answer would be fine. :)

## Scientists Find Hints Of A Giant, Hidden Planet In Our Solar System

The imagined view from Planet Nine back toward the sun. Astronomers think the huge, distant planet is likely gaseous, similar to Uranus and Neptune.

The astronomer whose work helped kick Pluto out of the pantheon of planets says he has good reason to believe there's an undiscovered planet bigger than Earth lurking in the distant reaches of our solar system.

That's quite a claim, because Mike Brown of Caltech is no stranger to this part of our cosmic neighborhood. After all, he discovered Eris, an icy world more massive than Pluto that proved our old friend wasn't special enough to be considered a full-fledged planet. He also introduced the world to Sedna, a first-of-its-kind dwarf planet that's so far out there, its region of space was long thought to be an empty no man's land.

Now Brown has teamed up with Caltech colleague Konstantin Batygin to do a new analysis of oddities in the orbits of small, icy bodies out beyond Neptune. In their report published Wednesday in The Astronomical Journal, the researchers say it looks like the orbits are all being affected by the presence of an unseen planet that's about 10 times more massive than Earth — the size astronomers refer to as a super-Earth.

"I'm willing to take bets on anyone who's not a believer," says Brown. He thinks existing telescopes have a shot at spotting this mystery planet in just a few years, since this new study points to a band of sky where astronomers should look.

### Planet Nine Wields Huge Influence

The six most distant known objects in the solar system with orbits exclusively beyond Neptune (magenta) all mysteriously line up in a single direction. Moreover, when viewed in 3-D, the orbits of all these icy little objects are tilted in the same direction, away from the plane of the solar system. "The only way to get these objects to line up in one direction, says Caltech astronomer Mike Brown, "is to have a massive planet lined up in the other direction." Many scientists are now searching the skies with powerful telescopes, hoping for a faint glimpse of "Planet Nine."

Source: Caltech/R. Hurt (IPAC)

The first suggestion that something big might be affecting the orbits of distant, icy bodies came in 2014. An international team of astronomers announced that they'd discovered a new dwarf planet, nicknamed Biden, that stays even farther out than Sedna. They also noted a strange clustering in the orbits of these objects, and in the orbits of about a dozen others. Perhaps, they hypothesized, the gravity of some unseen planet was acting as a shepherd.

"They were pointing out that there was something funny going on in the outer solar system, but nobody could really understand what it was," says Brown. "Ever since they pointed it out we've been scratching our heads."

The idea of a huge, hidden planet seemed kind of crazy. "No one really took it very seriously," says Brown. "It was ignored more than you might guess."

But he walked a few doors down to meet with Batygin and suggested they take this on. As they studied the freaky way that these objects lined up in space, Brown says, they realized that "the only way to get these objects to line up in one direction is to have a massive planet lined up in the other direction."

What's more, this planet naturally explains why the dwarf planets Sedna and Biden have weird orbits that never let them come in close to the solar system. "This wasn't something we were setting out to explain," says Brown. "This is something that just popped out of the theory."

But there was one moment that turned Brown into a believer. Their computer simulations predicted that if this hypothetical planet existed, it would twist the orbits of other small bodies in a certain way. So Brown looked through some old data to see if any icy bodies had been discovered with those kinds of orbits — and, lo and behold, he found five of them.

"They're objects that nobody has really explained or tried to explain before," says Brown. "My jaw hit the floor. That just came out of the blue. Being able to make a prediction and having it come true in five minutes is about as fun as it gets in science."

Their work suggests how big the planet must be, and more or less where it could be found. Brown has already started looking. He hopes other scientists will too.

"I want to know what it's like. I want to see that it's really there," says Brown. "It will hurt when somebody finds it and it's not me — but I assume it's going to happen, and I'm willing to feel that pain."

Caltech astronomers Mike Brown (left) and Konstantin Batygin are "willing to take bets" that a giant ninth planet is lurking in our solar system — way, way out, beyond Neptune. Lance Hayashida/Courtesy of Caltech hide caption

Caltech astronomers Mike Brown (left) and Konstantin Batygin are "willing to take bets" that a giant ninth planet is lurking in our solar system — way, way out, beyond Neptune.

Lance Hayashida/Courtesy of Caltech

It may be hard to believe that something so big would not have been seen before now. But Scott Sheppard of the Carnegie Institution for Science explains that for us to see it, sunlight has to travel all the way out there, bounce off the object, then travel all the way back.

"Objects get very faint very fast," says Sheppard. "If you do the math, if you move something twice as far away from the sun, it gets 16 times fainter."

Sheppard is one of the researchers who, after discovering Biden and the strange orbits, suggested a large planet might be the culprit.

"What we published was a very basic analysis of this clustering of objects in the outer solar system," he says. "We just did some basic stuff."

The new analysis, he says, has gone much deeper and has more rigor. "It leaves me thinking that the possibility of there being this super-Earth or mini-Neptune out there is more and more real now," says Sheppard.

Still, he's not completely convinced. "We really need to find more of these objects — more of these small objects that can lead us to the bigger object," Sheppard says. "I think it's still a tossup if it's really out there or not. I think we just need more data. Hopefully within the next few years we'll really be able to nail this down."

Dwarf planets like Sedna and Biden are not exactly household names. But Sheppard says if the solar system indeed has an honest-to-goodness ninth planet — a distant, giant planet that's bigger than Earth — "that, I think, is something that would blow the mind of anyone here on Earth."

## July Daily Guide

### July 1

Thursday, July 1. 2021, will be the first morning when the planet Mercury will appear above the horizon in the east-northeast as morning twilight begins. Mercury will not start appearing above the horizon at the time morning twilight begins until July 1.

Thursday evening, July 1, 2021, the waning Moon will appear half-full as it reaches its last quarter at 5:11 p.m. EDT.

### July 2

On Friday evening, July 2, 2021, as evening twilight ends, if you have a clear view of the horizon in the west-northwest, look for the bright planet Venus about 4 degrees above the horizon. With clear skies and a telescope or good binoculars, you should be able to see the stars of the Beehive Cluster mostly to the left of Venus. The Beehive Cluster is an open cluster of over 1,000 stars gravitationally bound together in a relatively small volume, appearing from Earth about 1.5 degrees across.

Sometime in late June through late July (2021-Jul-03 19:54 UTC with 9 days, 35 minutes uncertainty), Near-Earth Object (2015 BY3), between 28 to 62 feet (8 and 19 meters) across, will pass the Earth at between 0.2 and 120.9 lunar distances (nominally 51.9), traveling at (45,800 miles per hour (20.48 kilometers per second).

### July 4

Early Sunday morning, July 4, 2021, sometime around 2:45 a.m. EDT (2021-Jul-04 06:45 UTC with 1 hour, 27 minutes uncertainty), Near-Earth Object (2020 AD1) between 48 to 107 feet (15 and 33 meters) across, will pass the Moon at between 3.3 and 3.8 lunar distances (nominally 3.6), traveling at 2,800 miles per hour (5.72 kilometers per second).

Sunday, July 4, 2021, will be when the planet Mercury reaches its greatest angular separation from the Sun as seen from the Earth for this apparition (called greatest elongation), appearing half-lit through a large enough telescope. Because the angle of the line between the Sun and Mercury and the horizon changes with the seasons, the date when Mercury and the Sun appear farthest apart as seen from the Earth is not the same as when Mercury appears at its highest above the east-northeastern horizon as morning twilight begins, which occurs on July 10 and 11.

### July 5

Monday morning, July 5, 2021, at 10:47 a.m. EDT, the Moon will be at apogee, its farthest from the Earth for this orbit.

Monday evening, July 5, 2021, at 6:27 p.m. EDT, the Earth will be at aphelion, its farthest away from the Sun in its orbit, 3.4% farther away than when it was at perihelion in early January. Since the intensity of light drops off as the square of the distance from the source, the sunlight reaching the Earth at aphelion will be about 6.5% less bright than sunlight reaching the Earth at perihelion.

Sometime Monday night into Tuesday morning, July 5 to 6, 2021 (2021-Jul-06 06:30 UTC with 8 hours, 4 minutes uncertainty), Near-Earth Object (2021 MC), between 55 to 123 feet (17 and 38 meters) across, will pass the Earth at between 2.9 and 3.0 lunar distances (nominally 2.9), traveling at 16,000 miles per hour (7.15 kilometers per second).

### July 6

On the morning of Tuesday, July 6, 2021, the bright star Aldebaran will appear about 8 degrees below the waning crescent Moon. Aldebaran will rise after the Moon in the east-northeast at 3:48 a.m. EDT.

### July 7

On the morning of Wednesday, July 7, 2021, the planet Mercury will appear about 9 degrees below the waning crescent Moon. Mercury will rise after the Moon in the east-northeast at 4:27 a.m. EDT, just 12 minutes before morning twilight begins.

### July 8

By the morning of Thursday, July 8, 2021, the Moon will have shifted such that the planet Mercury will appear 4 degrees to the right of the waning crescent Moon low on the horizon in the east-northeast. Mercury will rise in the east-northeast at 4:27 a.m. EDT, just 5 minutes after moonrise and 12 minutes before morning twilight begins.

### July 9-10

Friday evening, July 9, 2021, at 9:17 p.m. EDT, will be the new Moon, when the Moon passes between the Earth and the Sun and will not be visible from the Earth.

The day of &ndash or the day after &ndash the New Moon marks the start of the new month for most lunisolar calendars:

• Sundown on Friday, July 9, 2021, marks the start of Av in the Hebrew calendar.
• The sixth month of the Chinese calendar starts on Saturday, July 10, 2021 (at midnight in China's time zone, which is 12 hours ahead of EDT).
• In the Islamic calendar, the months traditionally start with the first sighting of the waxing crescent Moon. Many Muslim communities now follow the Umm al-Qura Calendar of Saudi Arabia, which uses astronomical calculations to start months in a more predictable way. Using this calendar, sundown on Saturday evening, July 10, 2021, will probably mark the beginning of Dhu al-Hijjah. Dhu al-Hijjah is the 12th and final month of the Islamic year. It is one of the four sacred months during which fighting is forbidden. Dhu al-Hijjah is the month of the Hajj and the Festival of the Sacrifice. Making the Hajj or pilgrimage to Mecca at least once in your life is one of the Five Pillars of Islam. (In 2021, because of the COVID-19 pandemic, Saudi Arabia is limiting the total number of pilgrims, and is requiring they be fully vaccinated to protect the health and safety of all involved.)

### July 10-11

Saturday and Sunday morning, July 10 and 11, 2021, will be the two mornings when the planet Mercury will appear at its highest above the east-northeastern horizon (2 degrees) at the time morning twilight begins.

Beginning the evening of Saturday, July 10, 2021, the planet Saturn will begin appearing above the horizon in the east-southeast as evening twilight ends.

On the evening of Sunday, July 11, 2021, low on the west-northwestern horizon, the waxing crescent Moon will appear to the right of the bright planet Venus with the planet Mars appearing about a degree to the left of Venus. They will only be about 4 degrees above the horizon as evening twilight ends and the Moon will set first about 25 minutes later.

### July 12

The next evening, Monday, July 12, 2021, the waxing crescent Moon will have shifted to appear to the upper left of the planet Venus with the planet Mars about a half degree to the lower left of Venus and the bright star Regulus appearing about 6 degrees to the left of the Moon.

Sometime late Monday night into Tuesday morning, July 12 to 13, 2021 (2021-Jul-13 07:34 UTC with 4 hours, 6 minutes uncertainty), Near-Earth Object (2019 AT6), between 26 to 59 feet (8 and 18 meters) across, will pass the Earth at between 3.7 and 4.9 lunar distances (nominally 4.2), traveling at 11,500 miles per hour (5.15 kilometers per second).

### July 13

Tuesday evening, July 13, 2021, will be when the planets Venus and Mars will appear nearest to each other, with Mars appearing a half degree below Venus. The pair will only be about 4 degrees above the west-northwestern horizon as evening twilight ends at 9:44 p.m. EDT, and Mars will set first about 23 minutes later at 10:07 p.m. After this evening Venus will continue to shift to the left each evening, away from Mars and toward the bright star Regulus.

### July 16

On the evening of Friday, July 16, 2021, the bright star Spica will appear about 7 degrees to the lower left of the waxing half Moon. They will appear in the southwest as evening twilight ends at 9:41 p.m. EDT, and Spica will set first 2 hours, 39 minutes later (early Saturday at 12:20 a.m.).

### July 17

On Saturday morning, July 17, 2021, the Moon will appear half-full as it reaches its first quarter at 6:11 a.m. EDT.

Sometime in mid-to-late July 2021 (2021-Jul-17 19:03 UTC with 4 days, 20 hours, 30 minutes uncertainty), Near-Earth Object (2019 NB7), between 29 to 65 feet (9 and 20 meters) across, will pass the Earth at between 1.7 and 39.3 lunar distances (nominally 15.2), traveling at 30,800 miles per hour (13.76 kilometers per second).

### July 18

Sunday morning, July 18, 2021, will be the last morning for this apparition when the planet Mercury will appear above the horizon in the east-northeast at the time morning twilight begins.

### July 19-20

On Monday evening into Tuesday morning, July 19 to 20, 2021, the bright star Antares will appear about 8 degrees to the lower left of the waxing gibbous Moon. They will appear in the south as evening twilight ends at 9:39 p.m. EDT, and will set in the west-southwest at about the same time on Tuesday morning around 2:15 a.m.

By Tuesday evening into Wednesday morning, July 20 to 21, 2021, the Moon will have shifted such that Antares will appear about 8 degrees to the right of the Moon, with Antares setting first Wednesday morning at 2:10 a.m.

### July 21

During the week of July 21, 2021 (2021-Jul-21 09:48 UTC with 3 days, 1 hour, 6 minutes uncertainty), Near-Earth Object (2014 BP43), between 44 to 98 feet (13 and 30 meters) across, will pass the Earth at between 4.3 and 35.3 lunar distances (nominally 16.9), traveling at 18,900 miles per hour (8.46 kilometers per second).

Wednesday morning, July 21, 2021, at 6:25 a.m. EDT, the Moon will be at perigee, its closest to the Earth for this orbit.

Wednesday evening, July 21, 2021, will be when the bright planet Venus and the bright star Regulus will appear nearest each other, with Regulus 1 degree to the lower left of Venus. As evening twilight ends at 9:37 p.m. EDT Venus will appear about 5 degrees above the west-northwestern horizon. The planet Mars will appear farther to the lower right at only 2 degrees above the horizon. Mars will set first at 9:49 p.m., Regulus next at 10 p.m., and Venus last at 10:04 p.m. After this Venus will appear to continue toward the left and away from Regulus and Mars.

### July 23

The full Moon after next will be Friday night, July 23, 2021, at 10:37 p.m. EDT. The Moon will appear full for about 3 days around this time, from Thursday night through Sunday morning. ​

## Nasa discovers planet unlike any in our solar system

Nasa has found three new planets – including a kind of world unseen in our own solar system.

The mysterious planets, part of the TOI-270 system, are ”missing link” worlds and could be a huge gift to researchers looking for alien worlds, they said.

The three planets orbit a star that is only 73 light years away. It makes them among the closest exoplanets ever found, as well as being among the smallest.

They were discovered by researchers using Nasa’s Exoplanet Survey Satellite (TESS), which was shot into space in 2018 and has been scanning the universe for stars and planets that could support alien life.

TOI-270 has a rocky super-Earth, which is slightly bigger than our planet, and two gaseous planets that are lightly larger. That makes them a “missing link” – sitting between the smaller rocky worlds like our own Earth or Mars, and the much larger gaseous planets such as Saturn and Jupiter.

Best Nasa pictures of the month - July 2019

### Best Nasa pictures of the month - July 2019

Researchers hope to use the solar system to understand why there are so few worlds of that size, as well as helping how the planets in our solar system were found.

“TOI-270 will soon allow us to study this ‘missing link’ between rocky Earth-like planets and gas-dominant mini-Neptunes, because here all of these types formed in the same system,” said lead researcher Maximilian Gunther, from Massachusetts Institute of Technology.

### Recommended

Alongside the Earth-like exoplanet are two gaseous worlds, just over twice the size of our own. One of those, the furthest from the star, is thought to sit in the temperature range that could allow it to support alien life – but its atmosphere is expected to be so thick and dense that it stores lots of heat, and the surface is probably too warm.

Scientists hope to learn more about the relatively nearby worlds. As well as being close by and playing host to such unusual worlds, the star is unusually bright, without the solar flares and storms that can sometimes get in the way of observations.

“TOI-270 is a true Disneyland for exoplanet science, and one of the prime systems TESS was set out to discover,” Mr Gunther continued.

“It is an exceptional laboratory for not one, but many reasons – it really ticks all the boxes.”

There might be yet more planets in this solar system waiting to be found, researchers said.

While the planets are interesting enough in themselves, they also link together in a “resonant chain”, according to the researchers who found them. That means that their orbits are lined up neatly in whole integers, meaning that they are in “resonance” with each other.

“For TOI-270, these planets line up like pearls on a string,” Mr Gunther said. “That’s a very interesting thing, because it lets us study their dynamical behaviour. And you can almost expect, if there are more planets, the next one would be somewhere further out, at another integer ratio.”

Other solar systems have been found with planets that line up in these “resonant” formations. In our solar system, the moons of Jupiter are lined up in this interesting way, too.

## ‘Ring of fire’ annular solar eclipse to occur June 10, partially visible in US

The "ring of fire" is a type of solar eclipse where the moon partially blocks the sun's light on Earth.

Many Americans will have a chance to witness another astronomical phenomenon on June 10 during the "ring of fire" annular solar eclipse.

EarthSky believes most people around the world will be able to view the spectacle. In the United States, scientists said the eclipse will be partially visible in the upper Midwest and the East Coast, with the exception of Florida.

Americans will see the eclipse at sunrise, according to timeandate.com, a site providing time, weather, astronomy and other information.

An eclipse "occurs when one heavenly body such as a moon or planet moves into the shadow of another heavenly body," NASA states on its website.

One type is a solar eclipse, where the moon moves between the sun and the Earth. This blocks the sun’s light, casting a shadow on the planet.  There are three types of solar eclipses: total, where the moon completely blocks the sun partial, where the sun appears to have a small dark shadow on its surface, and annular, where the moon blocks the sun but the sun’s outer rim is still visible.

#### Photo of sun eclipsed by moon bears uncanny resemblance to 'devil horns'

The visual phenomenon seen in the photo is the result of the moon eclipsing the sun at the same time that there was an inversion layer of unusually warm air in the Persian Gulf.

The "ring of fire" is an annular solar eclipse. NASA says it happens because the moon is too far away from Earth and cannot completely block out the sun’s view, leaving what appears to be a ring around the moon.  "Annular" comes from the Latin word for "ring."

The last total solar eclipse, visible in the U.S., was in August 2017. An estimated 20 million people watched as the moon completely covered the sun.

The next total eclipse will occur on April 8, 2024, visible from Texas to Maine, weather permitting.

The American Astronomical Society cautions people to wear eye protection, such as eclipse glasses, when trying to view the eclipse, saying it’s never safe to look directly into the sun’s rays.

## X-Ray Images

And the source of these X-rays — the black hole or neutron star — is tiny. That means a Saturn-sized planet orbiting a billion kilometers away can completely eclipse the X-ray source, should it pass directly in front in the line of sight with Earth.

On Sep. 20, 2012, that’s exactly what appears to have happened. Fortuitously, the orbiting Chandra X-ray Observatory was watching at the time. The X-ray source dimmed to nothing and then reappeared, the entire transit lasting about 3 hours.

At the time, nobody noticed because the data sets from Chandra weren’t being searched for such short variations. But when Di Stefano and colleagues looked, the tell tale signs were clear to see.

There are various reasons why an X-ray source can dim in this way. One is the presence of another small star, such as a white dwarf, that eclipses the X-ray source. The team says M51-ULS-1b cannot be a white dwarf or other type of star because the binary system is too young for such an object to have evolved nearby.

Another potential explanation is natural variation, perhaps because of an interruption to the material falling into the black hole or neutron star. Di Stefano and co say in these cases, the luminosity changes in a characteristic way, with higher energy light frequencies changing more quickly than lower energy ones, and switching back on in a different way.

September 3, 2020 at 5:04 pm

Actually, the Sun can be covered much closer to perfection by Galilean moons as seen from fellow moons seasons of mutual events.

Roger W. Sinnott Post Author

September 3, 2020 at 7:46 pm

Good point! The Sun's angular size, seen from the vicinity of Jupiter, is about 0.10°, and the outermost Galilean moon, Callisto, when viewed from Jupiter itself, appears almost half again larger than the Sun. But if, instead, you could view Callisto from Ganymede at a time when they were nearly on opposite sides of Jupiter, you'd see Callisto subtending just 0.09°, a tad smaller than the Sun. So there can be rare occasions (rarer than total solar eclipses seen from Earth) when Callisto and the Sun are very nearly matched in size as seen from another Jovian moon. -- Roger

## Planets with Two Suns Likely Common

In the Star Warssaga, the Skywalker clan has its roots on Tatooine - a desert-covered planet revolvingaround two suns. A theoretical investigation has explored the likelihood forworlds like this to exist.

And it looks like thenearest Tatooine may be closer than a galaxy far, far away.

That's because more thanhalf of the stars in our galaxy have a stellar companion. And yet, of the 130or so currently known exoplanets (none of which are Earth-like),only about 20 of them are around so-called binaries. The percentage may grow higher.The current ratio is affected by an observational bias: planet hunters tend toavoid binaries because the star-star interactions can hide the planetsignatures.

Scientists discussed theissue earlier this month at a gathering of exoplanet hunters at the SpaceTelescope Science Institute in Baltimore.

"A few years ago, itwas thought that [binaries] were a very bad site to search for planets,"says Michel Mayor of the Observatoire de Geneve. "So we carefullyeliminated all binary stars from our sample."

But planets may be just aslikely around binaries as around single stars. Recent numerical simulationshave shown that Earth-like planets, known as terrestrials, form readily indouble star systems.

"The most significantthing we found is that terrestrial planets around certain close and widebinaries can look similar to planets around a single star," said JackLissauer of the NASA Ames Research Center.

Wide binaries are those inwhich the two stars are separated by several astronomical units (AU), which isthe distance between the Sun and the Earth. Planets could orbit around one ofthe pair, or each separately. So far, all the stellar binaries with exoplanetsare wide binaries.

But close binaries, wherethe stars are less than about an AU apart, can potentially have planets inorbit around both stars - presumably as is the case for Tatooine. Theseplanets, however, will be much harder to detect.

Lissauer and his collaboratorshave explored what binary star systems are favorable for planet formation.These limits could be useful in future planet searches.

Simulations

The researchers usedcomputer models that start with 14 large planet "embryos" and 140smaller planetesimals in orbit around one star or both stars of a binary.Evolution of this material is influenced by gravity and collisions. The modelsare followed for the equivalent of about one billion years.

"All of oursimulations have been able to form terrestrial planets," said Amesresearcher Elisa Quintana, who presented a poster on these results at thesymposium.

But not all of the modelsproduce planets around 1 AU, which is often thought to be the most likely habitable zone for life. Quintana varied how thetwo stars revolve around each other to see what configurations allowed forstable planet orbits inside 1 AU.

For wide binaries,Earth-like planets formed as long as the two stars came no closer than 7 AU.Quintana said that about 50 percent of known binaries meet this constraint.

The research group also ransimulations that mimicked Alpha Centauri - the nearest binary system toEarth, where the closest the two stars come is about 11 AU. The secondary starapparently acts like Jupiter does in our solar system - limiting how far outplanets can form. The results showed several terrestrial planets were possiblearound either of the stars.

Planets have not yet beenseen in the Alpha Centauri system, but small mass planets cannot yet be ruledout.

For close binaries, if thetwo stars are about 0.1 AU apart, the planets that form are indistinguishablefrom those seen in simulations with only one star. But as this separationincreases, or the orbit becomes highly non-circular, it is harder forEarth-like planets to exist.

"Perturbations fromthe stellar motions can eject matter into space or into one of the stars,"Quintana said.

The simulation results caninform observers which binaries might be better targets for their telescopes.

Observational hurdles

That said, it will not beeasy to see a planet around a binary, especially those where the stars areclose to each other. Most planets have been found by the radial velocity technique that searches for Doppler shifts inthe light spectra of stars.

"Finding the wobblefrom a planet in a stellar spectrum is hard enough without having another starorbiting the one you are looking at," Quintana said.

An alternative way ofdetecting planets is to look for the eclipse, or transit, of a planet in front of a star.Lissauer said that transiting searches could potentially discover planetsaround close binaries, but "there are complications."

For one thing, two starsare putting out light, so the eclipse of one star is less noticeable. Also, thetransit searches look for certain patterns of dimming and brightening of astar. If there are two stars in a tight orbit, this pattern will be different,so special algorithms will be needed.

But there are situationswhere a binary could provide an advantage for detecting planets. If the twostars eclipse each other, a planet could change when this eclipse happens.

"If the timing of theeclipses is not periodic, maybe a planet is to blame," Lissauer said.

Besides the possibility of transit timing, eclipsing binaries make goodtargets because planets - if they exist - will likely orbit in the same planeas the two stars - meaning they will also eclipse the stars at some point.

Which of these detectionmethods will be most likely to find the first Tatooine-like planet? Lissauer isunwilling to say.