Ultra-cool Companion Helps Reveal Giant Planets

An international team of astronomers led by David Pinfield of the University of Hertfordshire has found a brown dwarf that is more than 99% hydrogen and helium. Described as ultra-cool, it has a temperature of just 400 degrees Celsius and its discovery could be a key step forward in helping astronomers distinguish between brown dwarfs and giant planets.

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. 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 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.

Dr. Pinfield and his team detected the brown dwarf using data from the Wide-field Infrared Explorer (WISE) satellite, the UK Infrared Telescope (UKIRT) in Hawaii and the Visible and Infrared Survey Telescope for Astronomy (VISTA) in Chile. He carried out this work as part of his European (FP7) Initial Training Network RoPACS which studies planets around cool stars.

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 2600 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, says Dr. Pinfield. "Surveys from telescopes like VISTA and UKIRT and orbiting observatories like WISE are giving us an unprecedented view of 'ultra-cool' bodies in our neighborhood.

"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."

Image: http://star-www.herts.ac.uk/~dpi/brown_dwarf_companion.png

Caption: This artist's impression shows BD+01 2920B in the foreground (on the right-hand side), with its host star in the background. The color and banded atmosphere of the brown dwarf result from atmospheric gases and turbulence.

Credit: J. Pinfield, for the RoPACS network at the University of Hertfordshire

Media Contact:
Robert Massey
Royal Astronomical Society
+44 (0)20 7734 3307 x214, cell: +44 (0)794 124 8035

Science Contact:
David Pinfield
Centre for Astrophysics Research
University of Hertfordshire
+44 (0)1707 284171, cell: +44(0)770 765 7453

Dr. Pinfield leads the European network Rocky Planets around Cool Stars (RoPACS, http://star.herts.ac.uk/RoPACS/).

The new work appears in, "Discovery of the benchmark metal poor T8 dwarf BD+01 2920B", D. J. Pinfield et al., Monthly Notices of the Royal Astronomical Society.

Preprint: http://arxiv.org/abs/1201.3243

Final version: http://adsabs.harvard.edu/doi/10.1111/j.1365-2966.2012.20549.x

The Wide-field Infrared Survey Explorer (WISE, http://wise.ssl.berkeley.edu/) is a space-based telescope orbiting several hundred km above the Earth. Over its 6 month mission after launch in December 2010, it scanned the sky at four different mid-infrared wavelengths.

The UKIRT Infrared Deep Sky Survey (UKIDSS, http://www.ukidss.org/) observes the near infrared northern sky (just beyond the wavelength range of the human eye) using the UK Infrared Telescope and its wide-field camera atop the dormant volcano Mauna Kea in Hawaii.

The Visible and Infrared Survey Telescope for Astronomy (VISTA, http://www.vista.ac.uk/) measures the near infrared Southern sky using its state-of-the-art 67 million pixel camera, from Mount Paranal in the Atacama desert, Chile.

The Royal Astronomical Society (RAS, http://www.ras.org.uk), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organizes scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3500 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others.

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