According to astrophysicists Dr Scott Kenyon of the Smithsonian Astrophysical Observatory and Dr Ben Bromley of the University of Utah, terrestrial, Earth-like exoplanets around binary stars may be common.
Artist’s impression of the terrestrial exoplanet Gliese 667 Cb with the Gliese 667 A/B binary in the background. Image credit: L. Calçada / ESO.
For years, scientists believed that rocky planets could not form around most binary stars, at least not close enough to support life. The problem is that planetesimals need to merge gently together to grow.
Around a single star, planetesimals tend to follow circular paths – concentric rings that do not cross. If planetesimals do approach each other, they can merge together gently.
But if planetesimals orbit a pair of stars, their paths get mixed up by the to-and-fro pull of the binary stars. Their orbits can get so tangled that they cross each other’s paths at high speeds, dooming them to destructive collisions, not growth.
Previous studies started with circular orbits when pondering planet formation around binary stars, while the new study shows that planets, when they are small, will naturally seek these oval orbits and never start off on circular ones.
“If the planetesimals are in an oval-shaped orbit instead of a circle, their orbits can be nested and they won’t bash into each other. They can find orbits where planets can form,” said Dr Bromley, who is a co-author of the paper submitted for publication in the Astrophysical Journal (arXiv.org preprint).
“Tatooine sunsets may be common after all. Our main result is that outside a small region near a binary star, planet formation can proceed in much the same was as around a single star,” Dr Bromley and Dr Kenyon said.
“In our scenario, planets are as prevalent around binaries as around single stars.”
The team showed mathematically and by computer simulations that terrestrial exoplanets can form around binary stars if they have the oval ‘most circular’ orbit.
The scientists didn’t conduct their simulations to the point of planet formation, but showed that planetesimals could survive without collisions for tens of thousands of years in concentric, oval-shaped orbits around binary stars.
“An Earth-like Tatooine would have no problem forming right where it needs to be to host life,” Dr Bromley said.
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B. C. Bromley & S. J. Kenyon. 2015. Planet formation around binary stars: Tatooine made easy. ApJ, submitted for publication; arXiv: 1503.03876
