Spot the difference? Super-cool brown dwarf helps distinguish dwarfs and giant planets
An international team of EU-funded astronomers has just discovered a brown dwarf that is more than 99% hydrogen and helium. With a temperature of just 400 degrees Celsius, its discovery could be crucial to helping astronomers distinguish between brown dwarfs and giant planets.
When astronomers carry out searches for planets around other stars, they often spot many possible planets through the gravitational pull of the candidate objects on the stars they orbit. It can be tricky to distinguish between compact brown dwarfs and giant planets, as they share many characteristics.
Writing in the journal Monthly Notices of the Royal Astronomical Society, the team explains how they made the observation using data from the Wide-field Infrared Survey Explorer (WISE) satellite, the United Kingdom Infra-Red Telescope (UKIRT) in Hawaii and the Visible and Infrared Survey Telescope for Astronomy (VISTA) in Chile.
The study brings together scientists from Australia, Chile, France, Italy, Spain and the United Kingdom. It is funded in part by an EU-funded Marie Curie Initial Training Network (ITN) called 'Rocky Planets Around Cool Stars' (RoPACS) which is supported by more than EUR 3 million of funding under the 'People' Theme of the EU's Seventh Framework Programme (FP7).
Brown dwarfs are star-like objects with insufficient mass to ignite hydrogen fusion in their cores; over time, they cool to temperatures of just a few hundred degrees. They form like stars from the collapse of a giant molecular cloud a few hundred light years across, and brown dwarfs in binary systems share the same atmospheric chemistry as their host star.
Giant planets, on the other hand, first formed as large solid cores, which then accreted gas from the disk around them and led to them having different chemistry in their outer layers. For giant planets, the proportion of heavier elements, or 'metals', is three times higher than in the Sun. It is these types of differences that allow astronomers to discriminate between planets and brown dwarfs, and reveal their formation mechanisms.
The newly discovered object, known simply as BD+01° 2920B, is about 35 times bigger than Jupiter, and 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.
Lead study author, Dr Pinfield from the Centre for Astrophysics Research at the University of Hertfordshire in the United Kingdom, explains that this new work has been made possible by combining data from ground- and space-based surveys: 'Surveys from telescopes like VISTA and UKIRT and orbiting observatories like WISE are giving us an unprecedented view of "ultra-cool" bodies in our neighbourhood. By finding these rare objects in orbit around nearby stars, we get a handle on the bigger picture, that we live in a galaxy where both giant planets and brown dwarfs are commonplace.'
The main objective of the RoPACS network is to discover and study extra-solar planets that surround cool stars, by far the most common potential planet hosts.
The team aim to get to the bottom of several questions: how planet formation depends on the host star; the full range of stars around which habitable planets may be found; how our Solar System relates to the full diversity of planetary systems; and how we can best use future technology to discover and measure the full variety of potentially habitable extra-solar planets.
The network trains early stage researchers (ESRs) and equips young experienced researchers (ERs) with astronomical, computational, mathematical and technical skills so they can measure, simulate and interpret scientific observations of extra-solar planets and the technologies used to observe and characterise them.
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Document Reference: Pinfield, D. J., et al., 'Discovery of the benchmark metal poor T8 dwarf BD+01° 2920B', Monthly Notices of the Royal Astronomical Society, 2012. doi:10.1111/j.1365-2966.2012.20549.x
Subject Index: Coordination, Cooperation; Scientific Research; Space & satellite research