'Tatooine' planets survive violent escapades of ageing stars

Planets that revolve around two suns may surprisingly survive the violent late stages of the stars' lives, according to new research. The finding is surprising because planets orbiting close to a single sun, like Mercury and Venus in our solar system, would be destroyed when the ageing star swells into a red giant, researchers said.

The study found that planets orbiting two (binary) stars - also referred to as circumbinary planets or "Tatooine worlds" after the iconic planetary home of Luke Skywalker in Star Wars - often escape death and destruction by moving out to wider orbits. "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," said Veselin Kostov at the NASA Goddard Space Flight Centre. "It seems that if we had a second star in the centre of our solar system, things might go differently."

Binary star systems are ubiquitous in the universe and consist of two stars that orbit around a common centre 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 resulting in their sharing a common atmosphere (also called a common envelope). The binary star system ends up losing a large amount of mass, or might be destroyed in a supernova explosion.

"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," said Professor Ray Jayawardhana from York University in Canada. "We found that many such planets are likely to survive the messy and violent late stages of their stars' lives by moving farther out."

They found that the planets will predominantly survive the common envelope phase - even those orbiting very close to their stars. In addition, 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. "The reconfiguration can be quite dramatic when there are several planets," said Keavin Moore from York University. "Although all of the known circumbinary planets are gas giants, it is possible that somewhere out there is a terrestrial circumbinary planet that migrates to an orbit that now makes the planet potentially habitable for a little while." The research appears in The Astrophysical Journal.