How Two-Sunned ‘Tatooine’ Planets Escape Being Crushed By Their Host Stars

People like Elon Musk and Stephen Hawking want humans to become an interplanetary species, and a few billion years from now—if our ancestors are still around—we'll be glad we moved on. That's because our aging Sun will have expanded to such a point that it will gobble up its closest planets, Mercury and Venus, and maybe Earth, too.

Planets that revolve around two suns, though, might avoid this fate. That's the conclusion of a new paper, which will be published in The Astrophysical Journal, and finds that double-sunned Tatooines migrate out to wider orbits, and can avoid being destroyed.

Not so long ago, a planet orbiting two stars was a Star Wars fantasy. "Only since 2011 is there evidence of planets [that orbit] two stars," study author Ray Jayawardhana, dean of science and professor of physics and astronomy at York University in Toronto, told me.

Thanks to NASA's Kepler space telescope, which was created with the goal of taking a census of our galactic neighbourhood, we now know of several planets orbiting binary star systems. And while we can't yet say how common this type of planetary system is, binary stars are everywhere we look for them. "Close to half of all stars in the galaxy are binary systems," Jayawardhana said. "So it's really relevant to know whether binary systems can have stable planets," especially as scientists try to sort out whether life could exist anywhere other than on Earth.

No life as we know it could live on one of the planets modelled in this study, though. They orbit their stars too closely. "They're even closer than Mercury is to the Sun," he said.

In the study, Jayawardhana and his co-authors, led by Veselin Kostov of the NASA Goddard Space Flight Center, looked at what happens as these stars age and evolve. This happens "at a different pace, depending on their mass," he explained: the bigger star will burn brighter, and run out of fuel faster. As the star expands, the two begin to swap material back and forth until a shell of gas surrounds them, he continued. The stars' orbits are altered.

In some cases, all of this will result in a splashy supernova.

Researchers simulated what would happen to nine planets Kepler has found that orbit two stars, and found that they could survive the "common envelope" phase, where the gas is wrapping both stars. "The final phases of evolution happen rather quickly and are dramatic," Jayawardhana said. "There's quite a bit of messiness in the interaction between the two stars, and it also changes the planetary orbits," sometimes throwing the planets out farther from the star.

In systems with multiple planets orbiting the same binary star system, "even wilder things can happen," he continued. Planets might actually swap places in a sort of dance. "The outer planet could become the inner planet, and the inner planet could get hurled out."

In extreme cases, a planet might get chucked so far from the gravitational pull of its host stars that it becomes untethered and free-floating in space.

Scientists are sure to find other exotic planetary systems. If something does manage to live on a double-sunned Tatooine, "it's interesting to know that life might last longer [there] than it would on a planet circling a single sun," he said. And the view would be fantastic.

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