The fate of worlds orbiting 2 suns

Artist view of a planet orbiting two aging stars that exchange material and spiral closer together. Image via Jon Lomberg/ York University.
Artist’s concept of a planet orbiting two aging stars that exchange material and spiral closer together. Image via Jon Lomberg/ York University.

When our sun gets old, it’ll swell into a red giant whose outer layers will swallow the sun’s innermost planets, Venus and Mercury, and maybe Earth, too. But a new study – published in the peer-reviewed Astrophysical Journal on October 12, 2016 – suggests that planets orbiting two suns will have a different fate. According to the study, these so-called “Tatooine worlds,” which are named for the iconic planetary home of Luke Skywalker in Star Wars, might be expected to escape death and destruction by moving out to wider orbits.

Veselin Kostov at the NASA Goddard Space Flight Center led the study, in collaboration with Keavin Moore and Ray Jayawardhana, both of York University in Toronto, Canada. Kostov said in a statement:

This is very different from what will happen in our own solar system a few billion years from now, when our sun starts to evolve and expand to such a tremendous size that it will engulf the inner planets, like Mercury and Venus and possibly Earth too, faster than they can migrate out to larger orbits.

It seems that if we had a second star in the center of our solar system, things might go differently.

Classic shot from the first Star Wars movie, when we knew we weren't in Kansas anymore. Image via Wikimedia Commons.
Classic shot from the first Star Wars movie, on Tatooine, Luke Skywalker’s home planet, a world with 2 suns. Image via Wikimedia Commons.

Why do we care about planets orbiting two stars? Because there may be so many of them! Multiple star systems are common in our Milky Way galaxy, and presumably beyond it.

In a binary system – where two stars orbit around a common center of gravity – if the two stars are close enough to each other, when one starts evolving and expanding into a giant, they exchange material and spiral towards each other. The result is what astronomers called a common envelope, a shared common atmosphere. In the process, the binary star system ends up losing a large amount of mass. It may even be destroyed in a supernova explosion.

But what about its planets?

These researchers simulated the fate of nine actual planets, each orbiting two suns, recently discovered by NASA’s Kepler mission. They found that even planets orbiting close to their stars will predominantly survive the common envelope (or shared solar atmosphere) phase.

One outcome, the researchers said, is that the planets can migrate to farther orbits:

… similar to what it would be like if Venus moved out to where Uranus orbits our sun. In some cases, planets can even reach more than twice the distance to Pluto.

Interestingly, when there are multiple planets orbiting a binary star, some can be ejected from the system, while others can switch places or even collide with their stars.

Ray Jayawardhana said:

Given the exciting recent discoveries of planets circling binary stars, some with orbits similar in size to that of Mercury around the Sun, we were curious to explore the ultimate fate of these Tatooine worlds.

We found that many such planets are likely to survive the messy and violent late stages of their stars’ lives by moving farther out.

Artist's impression of the simultaneous stellar eclipse and planetary transit events on Kepler-1647. Such a double eclipse event is known as a syzygy. Image via Lynette Cook
Artist’s concept of a simultaneous stellar eclipse and planetary transit event in the double star system Kepler-1647. This system contains one of the actual planets found by Kepler, in this case the largest of these sorts of planets known so far, found earlier this year. Image by Lynette Cook via SDSU.

Bottom line: Astronomers are beginning to find planets orbiting two suns. A research group recently explored the fate of such planets as their suns age. They learned that these Tatooine worlds, as they’re called, might be the ultimate survivors, moving farther out in stellar systems where two aging stars might be aging, exchanging material, spiraling together, and even possibly exploding as supernovae.

October 12, 2016

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