1st August 2013 First planet discovered orbiting a brown dwarf Astronomers have discovered the first exoplanet orbiting a brown dwarf, 6000 light years from Earth.
Brown dwarfs are a class of substellar object lying mid-way between a star and a planet. Unable to sustain hydrogen-1 fusion reactions in their cores, they are often referred to as "failed stars". A typical brown dwarf has a mass between 15 and 90 times that of Jupiter, with a surface temperature ranging from 500-1,900ºC (930-3,450ºF). By contrast, our Sun has a mass around 1,000 times that of Jupiter, with an outer shell temperature of 5,500ºC (9,930ºF). Brown dwarfs are a fairly recent discovery, the first known example being Teide 1, in 1995. Earlier this year, a pair was confirmed to be just 6.5 light years from Earth – the closest star system found in almost a century. It had been assumed that brown dwarfs could harbour planets. Indeed, three planetary-mass objects were found to be orbiting brown dwarfs in 2004, 2005 and 2010 respectively. These are thought to have formed by cloud collapse, however – rather than accretion – due to relatively similar mass ratios to their companion, along with huge separation distances, making them more likely to be sub-brown dwarfs. Another object, MOA-2007-BLG-192Lb, was found in 2008, but it is unclear whether its companion is a red dwarf or brown dwarf.
A team of international astronomers has now produced definitive proof of a low-mass planet, in a close orbit (~0.87 AU) around a brown dwarf. Located 6,000 light years away in the constellation Scorpio, this planet (OGLE-2012-BLG-0358L b) was detected by gravitational lensing and is thought to have 1.9 Jupiter masses. The brown dwarf itself is about 10 times larger than its planetary companion. Gravitational lensing is a method which tracks one body passing in front of another. The gravity of the foremost object focusses the light of the background object towards Earth. It works regardless of how bright the object is, making it particularly well-suited to observing brown dwarfs, which are much dimmer and cooler than ordinary stars. This is the first known example of an object around a brown dwarf that can truly be described as a planet, formed in the parent's protoplanetary disk. Astronomers will now be working to determine the habitable zone which might be suitable for liquid water. Far from being "failed stars", brown dwarfs might actually be able to support alien life. The study is available through Cornell University's online library: Microlensing Discovery of a Tight, Low Mass-ratio Planetary-mass Object around an Old, Field Brown Dwarf.
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