The coolest brown dwarf
BY EMILY BALDWIN
ASTRONOMY NOW
Posted: April 15, 2008

An international team of astronomers has discovered the coldest brown dwarf ever observed, bringing scientists one step closer to bridging the gap between stars and planets.

The new brown dwarf, which has been assigned the index number CFBDS J005910.83-011401.3, was discovered in the framework of the Canada France Brown Dwarfs survey, and was observed using the Canada France Hawaii Telescope and the Gemini North Telescope, both located in Hawaii, and the ESO/NTT in Chile. It has a temperature of about 350 degrees Celsius and mass 15-30 times that of Jupiter and is an isolated object that doesn’t orbit another star. Because of their low masses, the central temperatures of brown dwarfs are not high enough to maintain thermonuclear fusion reactions over a prolonged period. Our Sun, in contrast, spends most of its life burning hydrogen, maintaining a constant internal temperature, whereas brown dwarfs get cooler and cooler after being formed.

Three-colour image of the star field in which the brown dwarf has been discovered. The brown dwarf us the very red object seen at the top of the image. Image: Canada-France-Brown-Dwarf-Survey 2008.

Since the first detection of brown dwarfs in 1995, scientists have noticed that they share certain characteristics with gas planets. For example, clouds of dust, aerosols and methane were detected in their atmospheres, just as for Jupiter and Saturn. However, there are still major differences: in the brown dwarf atmospheres water is always in the gaseous state, whereas it condenses into water ice in giant planets, and ammonia has never previously been detected in brown dwarfs, while it is major component of Jupiter’s atmosphere. The newly discovered brown dwarf bears the closest resemblance to a giant planet because of its low temperature and presence of ammonia.

Two classes of brown dwarf have currently been identified: L dwarfs (temperature 1200-2000 degrees Celsius and clouds of dust and aerosols in their high atmosphere) and T dwarfs (temperatures lower than 1200 degrees Celsius, and methane in their atmosphere). Because it contains ammonia and has an even lower temperature, CFBDS0059 might be the prototype of a new class, to be called Y dwarfs, and would provide the next rung on the ladder towards the giant planets, which have temperatures of less than -100 degrees.

Spectrum of the brown dwarf from the Gemini-North telescope showing a peak around the wavelength for ammonia. Image: Canada-France-Brown-Dwarf-Survey 2008.