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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How Do We Spot White Dwarf Stars Orbiting Red Giants?

Hi, I just read your article on the red giant/white dwarf binaries and loved it! Thanks for the post! My question is, do we have any recorded examples of a RG/WD binary? I sifted through about 20 descriptions of popular binary systems, figuring it would be in a list with that title, but didn’t see any.
New ultraviolet images from NASA’s Galaxy Evolution Explorer shows a speeding star that is leaving an enormous trail of “seeds” for new solar systems. The star, named Mira (pronounced my-rah) after the latin word for “wonderful,” is shedding material that will be recycled into new stars, planets and possibly even life as it hurls through our galaxy. Mira, also known as Mira A, is not alone in its travels through space. It has a distant companion star called Mira B that is thought to be the burnt-out, dead core of a star, called a white dwarf. Mira A and B circle around each other slowly, making one orbit about every 500 years. Astronomers believe that Mira B has no effect on Mira’s tail. Image credit: NASA/JPL-Caltech

New ultraviolet images from NASA’s Galaxy Evolution Explorer shows a speeding star that is leaving an enormous trail of “seeds” for new solar systems. The star, named Mira (pronounced my-rah) after the latin word for “wonderful,” is shedding material that will be recycled into new stars, planets and possibly even life as it hurls through our galaxy. Mira, also known as Mira A, is not alone in its travels through space. It has a distant companion star called Mira B that is thought to be the burnt-out, dead core of a star, called a white dwarf. Mira A and B circle around each other slowly, making one orbit about every 500 years. Astronomers believe that Mira B has no effect on Mira’s tail. Image credit: NASA/JPL-Caltech

Originally posted at Forbes!

There are lots of red giants and white dwarf stars orbiting each other out in our galaxy, and probably just as many in every other galaxy as well. Any set of binary stars usually forms at the same time, but since the lifetime of a star is tied very closely to its mass, if the two stars aren’t the exact same mass, then they reach the ends of their lives at different times.

A red giant star hasn’t completely finished its lifetime as a star just yet - it’s no longer burning hydrogen in its core the way our star is, but it’s still going - just more loosely held together, and burning helium instead of hydrogen. A white dwarf, on the other hand, has already reached the end of its pathway. White dwarfs are formed after the star runs out of helium to burn, and the outer layers of the star are lost into the formation of a planetary nebula. Only a central nugget of the former star remains after it loses its grip on its outer layers, which remains as a white-hot ball of atoms, slowly cooling to the temperature of the void of space. This is also the fate of our star, several billion years from now.

So a binary star system with a red giant and a white dwarf is a system where one star has already lost the majority of its mass into a planetary nebula, and the other star, being of a different mass, has progressed through its evolution at a different speed, and hasn’t come to the end of its helium burning phase just yet. If the stars are far enough apart, they will simply continue to orbit each other, until the red giant star reaches the end of its life and either creates its own planetary nebula and white dwarf, or explodes as a supernova, leaving behind a black hole or neutron star.

A team of researchers pointed the telescope at GK Persei, an object that became a sensation in the astronomical world in 1901 when it suddenly appeared as one of the brightest stars in the sky for a few days, before gradually fading away in brightness. Today, astronomers cite GK Persei as an example of a "classical nova," an outburst produced by a thermonuclear explosion on the surface of a white dwarf star, the dense remnant of a Sun-like star. This new image of GK Persei contains X-rays from Chandra (blue), optical data from NASA's Hubble Space Telescope (yellow), and radio data from the National Science Foundation's Very Large Array (pink). Image credit: X-ray: NASA/CXC/RIKEN/D.Takei et al; Optical: NASA/STScI; Radio: NRAO/VLA

A team of researchers pointed the telescope at GK Persei, an object that became a sensation in the astronomical world in 1901 when it suddenly appeared as one of the brightest stars in the sky for a few days, before gradually fading away in brightness. Today, astronomers cite GK Persei as an example of a "classical nova," an outburst produced by a thermonuclear explosion on the surface of a white dwarf star, the dense remnant of a Sun-like star. This new image of GK Persei contains X-rays from Chandra (blue), optical data from NASA's Hubble Space Telescope (yellow), and radio data from the National Science Foundation's Very Large Array (pink). Image credit: X-ray: NASA/CXC/RIKEN/D.Takei et al; Optical: NASA/STScI; Radio: NRAO/VLA

If the stars are close enough together, some other fun things start to happen. White dwarfs can begin to steal away some of the red giant star’s gas, pulling it in towards itself and growing in mass. After enough time, and after enough material has been thieved from its companion, the white dwarf can explode in either a nova (which is a surface explosion, which doesn’t destroy the star) or in a supernova (which is a complete detonation of the white dwarf). Not all supernovae are triggered this way; some are triggered by two white dwarfs colliding, and some are triggered by very high mass stars reaching an explosive end to their life. But novae seem to be exclusively the land of large companions and white dwarfs, and given that the white dwarf has already aged its way to the end of its lifetime, that means many of the companions will be red giants.

A stunning amount of energy is unleashed when a star goes nova. Image credit: NASA

A stunning amount of energy is unleashed when a star goes nova. Image credit: NASA

There’s at least one very well known red giant/white dwarf system in the night sky of the northern hemisphere. Unfortunately, it is found in the constellation Cetus, which has no particularly bright stars, but you can point your face in its direction if you choose. Mira, the star at the top of the page, is a red giant star plowing through the thin gas between stars in our galaxy, producing a shock wave and a long tail. But hiding in these images is a very faint white dwarf, visible to us in X-rays, pulling material in towards itself. If you want to go hunting for white dwarfs lurking around red giants, try looking for stars which have exploded in novae.

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What happens when one star in a binary turns into a red giant?

When you have a binary star system with the stars close together, what happens when one of the stars starts to turn into a red giant?


Binary stars are actually pretty common (about a third of the stars in our galaxy are in some kind of binary), although the very close pairs are a little less common.

For the majority of the lifetime of the stars in a binary, both stars just spin around each other, burning their own hydrogen and orbiting tranquilly. However, unless the two stars are exactly the same size when they’re formed (not generally the case), then one star will run out of hydrogen before the other, and will transform into a red giant star.

Red giants are pretty fuzzy stars, only loosely held together by their own gravity. For some scale, our own Sun will become a red giant in its future. It will expand from its current (quite large) radius of 695,500 kilometers to something 200 times that large. But it won’t have gained any material to grow that much bigger - it just spreads out what’s already there.

If the star is on its own, this newfound wispiness doesn’t change much. But if there’s another star nearby, the other star can begin to tug on the outer layers of the red giant, pulling some of the outer layers towards itself. (This is the same thing that the moon does to our oceans.) Because the red giant is so big, the outer layers are only weakly tied to the centre of the star. If the companion star in the binary is close enough or massive enough, it can begin to tear the outer layers of the red giant away, and pull them towards itself. This will change the shape of the red giant from a diffuse sphere into a diffuse teardrop, with the point of the teardrop facing the companion star.

What the companion star does with that surface material depends on what kind of star the companion is. By the time one star has aged its way to being a red giant, its companion is in one of two states, which will be dictated by how massive the companion was when it began its life.

If the companion started out with a lower mass than its red giant partner, it will still be burning hydrogen in its core when the red giant begins to form. As it siphons gas off of the red giant, this companion star will grow in mass. Depending on how rapidly the red giant is growing, and how quickly the companion is siphoning material, the lower mass star can actually grow so large that it becomes more massive than the red giant. After having so much of its material drawn away from itself and onto its neighbor, the red giant will slow its donation of mass. This can result in a fairly stable configuration - the red giant, having lost a good amount of its atmosphere to its neighbor, will continue to slowly bleed gas into its neighbor’s gravitational well, and the neighbor will continue burning hydrogen until it has exhausted its own resources.

The other option for these binaries is if the red giant was the smaller of the two stars when they started their lives. This means that the star now turning into a red giant took much longer to reach the end of its life than its neighbor. (The bigger your mass, the shorter your life, if you’re a star.) So the companion star has already gone through its death throes, and can be one of a number of interesting stellar remnants.

The main options for your stellar companion in this case are: a white dwarf, a neutron star, or a black hole.



If your red giant is pouring gas down onto a white dwarf, you will eventually trigger some kind of explosion: either a nova or a supernova. A nova is a thermonuclear detonation on the surface of a white dwarf, and can recur multiple times, as it’s just a surface explosion. This kind of behavior makes these binaries fairly noticeable, because the brightness of the star will flare to many times its original brightness. A supernova, on the other hand, will detonate the entire white dwarf, blasting itself apart, and leaving nothing behind (also quite noticeable). This kind of supernova occurs when the white dwarf gains too much material to be stable (these stars are balancing gravity against an electron’s unwillingness to be pushed too close to another electron), and some trigger in the core sparks a runaway burning of material.

If the other object is a black hole or a neutron star, you’ll wind up with what’s called an X-ray binary, for the somewhat boring reason that it produces a lot of X-rays. For these objects, as the gas from the red giant is pulled off of the red giant star, it gets pulled into a very thin disk, surrounding the black hole or neutron star. The disk forms because it’s very hard for gas to lose a lot of momentum all at once and plunge straight down onto the black hole, but as a result, the gas winds up heating up to an incredible temperature before it makes it all the way to the neutron star or black hole. This heat causes the X-ray glow, and keeps the disk itself almost invisible in optical light.

So there you have it! A binary system of stars with one red giant will result in the companion tearing the outer layers of the red giant star away. From there, you wind up growing the object nearby, or causing a nova, a supernova, or the creation of a lot of X-rays.

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