Is it possible to have a planet orbit two stars, like Tatooine?

How does that two sun thing work in Star Wars: A New Hope? Is that possible?
This artist's concept shows a hypothetical planet covered in water around the binary star system of Kepler-35A and B.  Image credit: NASA/JPL-Caltech

This artist's concept shows a hypothetical planet covered in water around the binary star system of Kepler-35A and B. Image credit: NASA/JPL-Caltech

It is possible, and we’ve actually found a number of planets orbiting double stars, like Luke’s homeworld in Star Wars does. However, outside of the Star Wars Universe, there are a lot of ways for this setup to go very wrong. So far, we haven’t found an enormous number of planets orbiting double stars, which seems to speak to how rare it is for a planet to survive in an environment like Tatooine’s.

At the beginning of Star Wars, Luke Skywalker lives on a planet with a double sunrise, on a planet which orbits two stars. We can presume that the two stars are orbiting each other, and that this planet then orbits around both stars. The technical term for two stars which orbit each other is a binary system, and the easiest way for the stars to find themselves in this situation is if they both form out of the same cloud of gas, at the same time. The remainders of that cloud of gas would hang around long enough to make planets to surround the pair of stars.

This artist's concept illustrates a tight pair of stars and a surrounding disk of dust -- most likely the shattered remains of planetary smashups. Image credit:  NASA/JPL-Caltech/Harvard-Smithsonian CfA

This artist's concept illustrates a tight pair of stars and a surrounding disk of dust -- most likely the shattered remains of planetary smashups. Image credit: NASA/JPL-Caltech/Harvard-Smithsonian CfA

If a planet were orbiting far enough away from the two stars, it wouldn’t really notice a difference between orbiting the double star set, and orbiting one star of their combined mass. However, by the time you get really far away from the stars, there’s not a tremendous amount of sunlight reaching the surface of your planet. If you want your world to be habitable (and a desert world still counts), you’ll have to be on a planet that’s a little closer to the stars, and this is where things start to get tricky.

If you are a planet, it’s nicest if the two stars orbit each other closely and circularly. This kind of setup for the stars means that you’re more or less always the same distance from the stars, which guarantees you a pretty consistent amount of light from the stars. If you’re trying to be a habitable world, this is important, because it keeps your surface temperature roughly consistent as well. You’d still have some variability, because the stars will still eclipse or partially eclipse each other periodically, which would lower the amount of light you’d get on the surface.

This artist's concept illustrates Kepler-47, the first transiting circumbinary system. Image credit:  NASA Ames/JPL-Caltech/T. Pyle

This artist's concept illustrates Kepler-47, the first transiting circumbinary system. Image credit: NASA Ames/JPL-Caltech/T. Pyle

However, if you are a star, close orbits are more complicated than wide ones. Wide orbits are easier to maintain, because the two stars have a weaker gravitational influence on each other. In a smaller orbit, the two stars will exert a reasonably strong tidal force on each other, and will change each other’s orbits over time. When the orbits of the stars begin to change around, the planets’ orbits also change, and you are in prime conditions for what’s called a three-body interaction.

The three body interaction happens when you have three objects orbiting each other in relatively close range. This could be three stars or two stars and a planet, and in either case, the lowest mass object can wind up getting flung suddenly out of the solar system entirely. The other outcome is for the planet to wind up crashing into one of the two stars - not a habitable outcome there, either. The three-body interaction is of particular concern for two stars and a planet, as this means that if your planet is close enough to the star to get caught up in one of these interactions, it won’t stay as a planet in the solar system for a particularly long time.  This might partially explain the relatively low number of circumbinary planets we’ve seen so far with Kepler - these planets are prone to either being ejected or consumed by their parent stars.

So it’s not impossible for a Tatooine-like planet to orbit a binary system, but given how rare they are in our solar system, everything has to be exactly so, or Tatooine will wind up on a one-way trip out of its solar system on a journey through its home galaxy.


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What Would Have Happened To That Interstellar Object If It Had Hit The Sun?

What would have happened if A/2017 U1 had hit or grazed the sun? Would we have noticed?
This animation shows the path of A/2017 U1, which is an asteroid — or perhaps a comet — as it passed through our inner solar system in September and October 2017. From analysis of its motion, scientists calculate that it probably originated from outside of our solar system. Image credit:  NASA/JPL-Caltech

This animation shows the path of A/2017 U1, which is an asteroid — or perhaps a comet — as it passed through our inner solar system in September and October 2017. From analysis of its motion, scientists calculate that it probably originated from outside of our solar system. Image credit: NASA/JPL-Caltech

Originally posted on Forbes!

The snappily-named object A/2017 U1 may be more familiar to you as the interstellar visitor that zipped through our solar system at nearly 16 miles per second, discovered in mid-October. It has now been given a less alphanumeric name by the Minor Planet Center: ‘Oumuamua. That Hawai'ian name “reflects the way this object is like a scout or messenger sent from the distant past to reach out to us (ʻou means reach out for, and mua, with the second mua placing emphasis, means first, in advance of)

At 400 meters (about a quarter mile) across, ‘Oumuamua is a relatively small visitor to our solar system. Though it passed through the innermost regions of the Solar system, closer to the Sun than Mercury, that’s not nearly close enough to be considered a sun-grazing comet, and well too far away to hit the Sun directly.

At 400 meters across A/2017 U1 is considerably larger than the vast majority of the comets spotted by the SOHO satellite, one of our Sun-monitoring satellites. SOHO’s main goal is to watch out for solar flares and other events on the surface of the Sun which could pose a hazard to the Earth, but its continual monitoring of the sun has also discovered a huge number of comets - in 2015, NASA celebrated SOHO’s 3,000th comet discovery.  These comets are usually only a few tens of meters across, ten times smaller than our interstellar visitor. SOHO has also spotted objects which blur the boundaries between comets and asteroids, probably a fairer comparison to our interstellar wanderer. One such discovery, comet 322P, is estimated to be around 100m in diameter, not so far off of the estimated size of 'Oumuamua.

If the object had hit the Sun directly, it would have been astoundingly bad luck for our interstellar wanderer. Imagine travelling for billions of years, only to run smack into a star - that’s like skiing into the only tree on the entire mountain. If that had happened, though, that’s a straightforward end to this interstellar object. Plunging into the incredible heat of our Sun would have destroyed that object, however rocky it was.

A sun grazing comet as witnessed by the ESA/NASA Solar and Heliospheric Observatory, or SOHO, as it dived toward the sun on July 5 and July 6, 2011. SOHO is the overwhelming leader in spotting sungrazers, with almost 3000 spotted to date. SOHO can see the faint light of a comet, because the much brighter light of the sun is blocked by what's known as a coronograph. Image credit: ESA&NASA/SOHO

A sun grazing comet as witnessed by the ESA/NASA Solar and Heliospheric Observatory, or SOHO, as it dived toward the sun on July 5 and July 6, 2011. SOHO is the overwhelming leader in spotting sungrazers, with almost 3000 spotted to date. SOHO can see the faint light of a comet, because the much brighter light of the sun is blocked by what's known as a coronograph. Image credit: ESA&NASA/SOHO

Grazing the Sun involves swinging past the Sun at such a close distance that your object is traveling within a contour that’s less twice the size of the Sun. Generally, from observations by satellites like SOHO, it seems that only comets which are more than a few kilometers across will survive the intense environment that close to the Sun - comets smaller than that will evaporate entirely away, reaching the same fate as their plunge-diving cousins. Asteroids and other rocky objects are a little more durable than the ice of a comet, but the harshness of the space immediately surrounding the Sun will abrade away the surface of even very durable materials.

Would we have been able to spot this abrasion of a small rock? The more comet-like our visiting object were, the easier it would be, since SOHO easily spots comets a tenth the size of our visitor. Rocky objects are harder to spot because they tend not to form large tails, but they will still reflect light into any waiting cameras, and as the detection of 322P proves, intermediate objects are still readily detectable at the size of 'Oumuamua. If the object were 100% rock, it reflects so little light that it would be much more difficult to observe with SOHO unless the object were another factor of ten or so larger - kilometers instead of hundreds of meters across. However, since it seems that 'Oumuamua was one of these mysterious, rocky/icy objects like the objects in our own Kuiper belt, it might have been more analogous to the hybrid comets we've spotted so far. In that case, as long as it had gone within SOHO’s field of view, we might have had a good chance of seeing the reflected sunlight from its surface. SOHO can spot objects a little beyond the surface of the Sun out to 30 times the radius of the Sun (the very surface of the Sun is too bright, and so it’s blocked from view). It might have been harder, given the brief flash of observation time we would have had before it annihilated, to determine exactly where it had come from, and we certainly wouldn’t have had time to get more information on our first interstellar visitor, like its color (red)!

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Can A Star Ever Turn Its Spin Backwards?

Can a star reverse its rotational direction during some time in their life, and if so, how would it affect any planets around it?
This artist’s impression of the water snowline around the young star V883 Orionis, as detected with ALMA. Image credit: A. Angelich (NRAO/AUI/NSF)/ALMA (ESO/NAOJ/NRAO)

This artist’s impression of the water snowline around the young star V883 Orionis, as detected with ALMA. Image credit: A. Angelich (NRAO/AUI/NSF)/ALMA (ESO/NAOJ/NRAO)

Originally posted on Forbes! 

The stars in the night sky all have their preferred direction of rotation, which depends directly on the exact way that the cloud of gas and dust that the star formed out of collapsed. If there was slightly more random motion in a clockwise or a counterclockwise direction, as the cloud of gas collapsed, the star would have magnified that hint of rotation, spinning up the same way that a figure-skater does, pulling in their arms and legs.

Once spun up this way, another piece of fundamental physics comes into play - inertia. Inertia tells us that objects in motion tend to stay in motion unless there’s something else that’s causing that object to slow down. On Earth, that something else can come in many forms - we have the mass of the Earth, whose gravity will pull objects down towards the surface, an atmosphere to move through, which will slow objects moving through it, or quite simply mountains and buildings which objects can bounce off of and away from.

If you’re in space, these sorts of Earthly obstacles which can serve to stop a moving object aren’t around. There are many fewer things which could serve to stop an object’s motion - without a thick atmosphere to move through, the planets and our spacecraft continue at their current speeds without any impediments. The most common method of slowing down (or speeding up) an object in space is by traveling near another large object (ones that are of a similar mass to yourself are the most effective) and letting the force of gravity alter your path.

These requirements for an external force hold both for motion as we normally think about it (a forward or sideways motion) and for spin. So if we think about spinning a bicycle tire, that wheel will continue to spin until the forces of friction in the axle (primarily) will slow it down, or until you clamp down on the brakes. An object which is spinning in space has no friction-containing axle around which to spin, and so if it’s isolated, without any external objects which can act as a braking force, that object should continue to spin as it is, ad infinitum. This is basically the situation that stars find themselves in. Stars do not reverse their spins as a standard part of their lifetimes.

Planet formation begins with a brilliant young star at the center of what’s called a protoplanetary disk. Collisions within the disk form rocks that act as planetary building blocks. They settle into orbit around the star, creating gaps in the disk. Image credit: NASA's Goddard Space Flight Center Video and images courtesy of NASA/JPL-Caltech

Planet formation begins with a brilliant young star at the center of what’s called a protoplanetary disk. Collisions within the disk form rocks that act as planetary building blocks. They settle into orbit around the star, creating gaps in the disk. Image credit: NASA's Goddard Space Flight Center Video and images courtesy of NASA/JPL-Caltech

A reversal in the rotation of a star is extremely difficult to accomplish without something external to the star punching it backwards in the other direction, to slow down each rotating particle that makes up the star. If you could, using magic, reverse the rotation of our own star, without changing anything else, the planets surrounding our Sun wouldn’t be influenced at all - the orbits of the planets are determined by the gravitational pull of the Sun, which hasn’t changed if we haven’t changed its mass, plus the planets’ own velocities.

What sort of objects could act as a brake in space? The is easiest is always a good old large-scale collision. This is how we think the Earth formed the Moon, how Uranus got tipped onto its side, and Venus got tipped completely upside down. So if we wanted to reverse the spin of the Sun, we’d have to hit it with something pretty catastrophic in order to both stop the rotation of the gas & plasma which currently makes up the Sun, and reverse the direction of the spin. Any impact on that scale would definitely impact the planets. If it was another star that hit our own, we’d have gravitational and temperature-related chaos even before the impact, setting aside whatever cataclysm of energy would be unleashed during the collision.

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Why Does The Earth Pull On One Side Of The Moon More? Is The Moon Lopsided?

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Who's Going To Find Planet Nine?

Could or will a serious amateur find “Planet 9” if it really exists? I get the impression from NASA that they are leaving the search for others and not going to devote any Hubble time to the search but will jump in as soon as likely object is found.
This artist's conception shows the closest known planetary system to our own, called Epsilon Eridani. Observations from NASA's Spitzer Space Telescope show that the system hosts two asteroid belts, in addition to previously identified candidate planets and an outer comet ring. Image credit: NASA/JPL-Caltech

This artist's conception shows the closest known planetary system to our own, called Epsilon Eridani. Observations from NASA's Spitzer Space Telescope show that the system hosts two asteroid belts, in addition to previously identified candidate planets and an outer comet ring. Image credit: NASA/JPL-Caltech

Originally posted on Forbes!

Planet Nine has been making the rounds again, largely because the Division of Planetary Science had their annual meeting last week, and if you’ve had some new thoughts on planets in the past year, this is the time to announce them. So in the past week, articles have popped up informing us that Planet Nine might be responsible for why the Sun's rotation appears slightly tilted relative to the orbits of the rest of our planets, that we’ve found a few more distant objects which have odd orbits best explained by another large planet, and the very long orbit of a new distant world was also claimed for Planet 9 (though the scientists who discovered it prefer a non-Planet 9 explanation).

All of these victories of Planet Nine are based on simulations of what our solar system would look like, if Planet Nine were there, given our current best guesses for what Planet Nine should look like. At the moment, Planet Nine has to be at least several times the size of Earth, and several more times the mass of Earth. Planet Nine happens to be at the farthest point in its orbit from the Sun, as it slowly putters around the Sun in a very elliptical orbit, once every ten to twenty thousand years, at 700 times the distance of the Earth to the Sun.

Artist's Conception of a Kuiper Belt Object. Credit: NASA/JPL-Caltech/T. Pyle (SSC)

Artist's Conception of a Kuiper Belt Object. Credit: NASA/JPL-Caltech/T. Pyle (SSC)

This distance from the Sun means that the hypothetical Planet Nine is much fainter than it would be if it were at its closest approach to the Sun. That faintness, in turn, means that your best chance for spotting it lies with the biggest telescopes we can turn towards the sky. However, these massive telescopes are in high demand, with scientists all over the world competing for the use of them for a few nights. To get to use them, your proposed science must beat out the proposed science of many other scientists, who also have good ideas for what to do with the telescope. If you go to the telescope and say 'I need to use a lot of the telescope time to survey a huge area of the sky, to find a single object that I hope is there', you are not going to get that telescope time. That’s much too risky a way of spending telescope time – what have you learned if it isn’t there?

A quick note on NASA’s role in all this - while NASA is involved in the USA’s space based telescopes, they’re mostly involved in the construction, launch, and management of the data that is taken with them - major tasks. The science that is done with those telescopes is done by teams of people - scientists - who have made a case for why they should be entrusted with the instruments for a few hours, to point at their favorite patches of sky. These are not necessarily NASA employees, and many of them aren't. For Planet Nine purposes, NASA’s space telescopes are not the ideal facilities to do a major planetary hunt. That honor goes instead to the Subaru telescope on Mauna Kea, which is operated by the National Astronomical Observatory of Japan.

The Subaru Telescope on Mauna Kea, Hawai'i. Image credit wikimedia user Denys, CC BY 3.0.

The Subaru Telescope on Mauna Kea, Hawai'i. Image credit wikimedia user Denys, CC BY 3.0.

There are a number of groups of scientists already hunting for Planet Nine. Critically, the hunts are not all being done in exactly the same way. Some scientists are trying to narrow down the area of sky which needs to be surveyed. Some are looking for other, smaller objects in the outer solar system, which might help to rule out some possible orbits of Planet Nine. (These authors prefer an orbit of 17,000 years specifically.) Some are running more simulations to see if the data we already have is enough to put constraints on whether or not the planet could be there (not so far, is the answer). Some have dug out all the old Pluto observations that we have, to see if Pluto's orbit has been jostled by Planet 9. And some are hunting for Planet Nine itself, while also looking for other moving objects in the solar system - that way, even if they don’t find it, they’ve found a new piece of the outer solar system to fit into the jigsaw puzzle.

This hunt has already turned up some new objects. These particular new discoveries are among the most distant from the sun on their closest visits to the inner solar system. They may not travel the furthest away from our Sun, but they’re also never coming in. The more information scientists can get about what’s happening at the edge of the solar system, the more we will understand how much an extra giant planet, wandering frigidly and slowly, would change those happenings.

If Planet Nine is found, credit will go not just to the group of scientists which finally got photons from the distant world into their telescope’s camera, but to all those who led the way to get there - other scientists who have laid the foundations of understanding of how our solar system works very much included. To be able to notice oddities in the very outskirts of our solar system is an achievement already. We will either find Planet Nine, or a reason that it cannot exist.

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