Rocky Planets Can Form In Binary Systems

The SDSS 1557 in an artist impression. Mark Garlick, UCL, University of Warwick and University of Sheffield. 

Star Wars introduced the idea of a planet orbiting two stars into the public imagination, but astronomers weren’t too convinced that a rocky planet like Tatooine could exist in real life. New observations, however, challenge that idea.

Researchers have discovered that the binary system SDSS 1557 about 1,000 light-years away is surrounded by a debris disk rich in metallic and rocky asteroids. Astronomers suspect that half the planets in the universe are in binary systems, but all of those found so far have been gas giants; this is the strongest evidence so far that a rocky world can form around close binaries. The discovery is published in Nature Astronomy.

“Building rocky planets around two suns is a challenge because the gravity of both stars can push and pull tremendously, preventing bits of rock and dust from sticking together and growing into full-fledged planets,” lead author Dr Jay Farihi, from the University College London, said in a statement. “With the discovery of asteroid debris in the SDSS 1557 system, we see clear signatures of rocky planet assembly via large asteroids that formed, helping us understand how rocky exoplanets are made in double star systems.”

SDSS 1557 contains a white dwarf that is 45 percent the mass of our Sun and a brown dwarf located about 490,000 kilometers (305,000 miles) away. The team used the Very Large Telescope and the Gemini South Observatory, both in Chile, to look at the chemical composition of the white dwarf. They found that since its discovery in 2010, it has been polluted by 100 billion tons (110 billion US tons) of planetary debris, cumulatively about the size of a large asteroid.

“Any metals we see in the white dwarf will disappear within a few weeks, and sink down into the interior, unless the debris is continuously flowing onto the star," added co-author Professor Boris Gänsicke from the University of Warwick. "We'll be looking at SDSS 1557 next with Hubble, to conclusively show the dust is made of rock rather than ice.”

White dwarfs are stars at the end of their lives and these disks of debris are suspected of being the remains of minor planet disrupted by gravity. A fully fledged rocky planet has yet to be found orbiting a two-star system, but this is a good indication that we just need to keep looking.  

The SDSS 1557 in an artist impression. Mark Garlick, UCL, University of Warwick and University of Sheffield. 

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