An international team of astronomers has found a brown dwarf that is made up of about 99 percent hydrogen and helium. Described as ultra-cool, its temperature is just 400 degrees Celsius and its discovery could be a key step forward in helping astronomers tell the difference between brown dwarfs and giant planets.
Brown dwarfs are starlike objects with not enough mass to ignite hydrogen fusion in their cores. Over time they cool to temperatures of just a few hundred degrees Celsius. Formed like stars from the collapse of a giant molecular cloud a few hundred light-years across, brown dwarfs in binary systems such as this have the same atmospheric chemistry as their host star.
In contrast, giant planets form with a more diverse chemistry. Those in our own solar system (such as Jupiter and Saturn) first formed as large solid cores, which then accreted gas from the disk around them. This led to a different chemistry in their outer layers. For example, when the Galileo spacecraft entered Jupiter's atmosphere in 1995, it found the proportion of heavier elements (astronomers call these ‘metals') to be three times higher than in the sun. Such differences allow astronomers to discriminate between planets and brown dwarfs and reveal their formation mechanisms.
The newly discovered object, known as BD+01 2920B, is about 35 times more massive than Jupiter. It orbits its host star at a distance of 390 billion km or about 2,600 times the average distance from the Earth to the sun.
Searches for planets around other stars find many possible planets through the gravitational pull of the candidate objects on the stars they orbit as well as direct imaging using the latest (and future) optical technology on the largest telescopes. The problem is that compact brown dwarfs share many characteristics with giant planets, so astronomers struggle to confirm the nature of what they detect.
The new work has been made possible by combining data from ground- and space-based surveys.
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