Evidence of planetary debris surrounding a white dwarf and its substellar companion has been found by a University College London-led international team of astronomers. The discovery is remarkable because the debris appears to be rocky and suggests that terrestrial planets might exist in this exotic system.
A disc of rocky debris from a disrupted planetesimal surrounds the white dwarf-brown dwarf binary system SDSS 1557. Image credit: Mark Garlick, University College London, University of Warwick and University of Sheffield.
The binary system in question is called SDSS J155720.77+091624.6 (hereafter SDSS 1557).
It is 1,592 light-years away from Earth and consists of a low-mass white dwarf (0.45 solar masses) and a brown dwarf (66 Jupiter’s masses).
To date, all exoplanets discovered in orbit around binary stars are gas giants and are thought to form in the icy regions of their systems.
In contrast to the carbon-rich icy material found in other binaries, the planetary material identified in SDSS 1557 has a high metal content, including silicon and magnesium.
These elements were identified as the debris flowed from its orbit onto the surface of the white dwarf, polluting it temporarily with at least 1017 g (or 1.1 trillion US tons) of matter, equating it to an asteroid at least 4 km in size.
“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,” said lead author Dr. Jay Farihi, a researcher in the Department of Physics and Astronomy at the University College London.
“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.”
In the Solar System, the main asteroid belt contains the leftover building blocks for the terrestrial planets Mercury, Venus, Earth, and Mars, so planetary researchers study the asteroids to gain a better understanding of how terrestrial, and potentially habitable, planets are formed.
The same approach was used by Dr. Farihi and co-authors to study the SDSS 1557 system as any planets within it cannot yet be detected directly but the debris is spread in a large belt around the double stars, which is a much larger target for analysis.
“We know of thousands of binaries similar to SDSS 1557 but this is the first time we’ve seen asteroid debris and pollution,” said co-author Dr. Steven Parsons, from the University of Valparaíso and the University of Sheffield.
“The brown dwarf was effectively hidden by the dust until we looked with the right instrument, but when we observed SDSS 1557 in detail we recognized the brown dwarf’s subtle gravitational pull on the white dwarf.”
The astronomers studied SDSS 1557 and the chemical composition of the debris by measuring the absorption of different wavelengths of light or ‘spectra,’ using the Gemini Observatory South telescope and ESO’s Very Large Telescope.
“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,” said co-author Prof. 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.”
A paper reporting this discovery is published in the journal Nature Astronomy (arXiv.org preprint).
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J. Farihi et al. A circumbinary debris disk in a polluted white dwarf system. Nature Astronomy, published online February 27, 2017; doi: 10.1038/s41550-016-0032
