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If it's not a planet or a star, what is it?

Astronomers have found the coldest brown dwarf to date. And the failed star might represent a new class of objects that are a missing link between planets and stars.
Image: Brown dwarf comparison
This graphic compares the approximate size of our sun (at left) with Jupiter (at right) and a brown dwarf that is 15 to 40 times the mass of Jupiter (in the middle).M. Weiss / CXC / NASA
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Brown dwarfs are the oddballs of the cosmos, more massive than planets but not heavy enough to generate the thermonuclear fusion that powers real stars. Now astronomers have found the coldest brown dwarf to date.

The failed star might represent a new class of objects that are a missing link between planets and stars.

The cold brown dwarf floats freely in space, not bound to a star. Its mass is somewhere between 15 and 30 times that of Jupiter. And it is about 660 degrees F (350 C), cooler than any other known object in its class.

The surface of the sun is about 11,000 degrees Fahrenheit (6,000 degrees Celsius). The temperature at the top of Jupiter's clouds is about -230 degrees F (-145 degrees C), though at its core the mercury soars to 43,000 degrees F (24,000 degrees C).

The brown dwarf, named CFBDS J005910.83-011401.3, is about 40 light-years from our solar system. It was found by an international team using the Canada France Hawaii Telescope and Gemini North Telescope, both located in Hawaii, and the a European Southern Observatory telescope in Chile.

The mass of brown dwarfs is usually less than 70 Jupiter masses. In contrast to a star like our sun, which spends most of its lifetime burning hydrogen and keeping a constant internal temperature, a brown dwarf spends its lifetime getting colder and colder.

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The first brown dwarfs were detected in 1995. Since then, they have been found to share common properties with giant planets, while differences remain. For example, clouds of dust and aerosols, as well as large amounts of methane, were detected in the atmospheres of the coolest brown dwarfs, just as in the atmosphere of Jupiter and Saturn.

However, there were still two major differences. In the brown dwarf atmospheres, water is always in gaseous state, while it condenses into water ice in giant planets; and ammonia has never been detected in the brown dwarf, while it is a major component from Jupiter's atmosphere.

Cooler still
The newly-discovered brown dwarf looks much more like a giant planet than the known classes of brown dwarfs, both because of its low temperature and because of the presence of ammonia.

To date, two classes of brown dwarfs have been known: the L dwarfs (temperature of 2,100 to 3,600 Fahrenheit (1,200-2,000 C), which have clouds of dust and aerosols in their high atmosphere, and the T dwarfs (temperature lower than 2,100 F), which have a very different spectrum because of methane forming in their atmosphere.

Because it contains ammonia and has a much lower temperature than do L and T dwarfs, CFBDS0059 might be the protoype of a new class of brown dwarfs to be called the Y dwarfs, the researchers propose. This new class would become the coldest stellar objects, hence the missing link toward giant planets.

Almost a planet
This discovery also has important implications in the study of extrasolar planets.

The atmosphere of brown dwarfs looks very much like that of giant planets, therefore the same models are used to reproduce their physical conditions. Such modeling requires to be constrained with observations. Observing the atmospheres of extrasolar planets is indeed very hard because the light from the planets is embedded in the much stronger light from their parent star. Because brown dwarfs are sometimes isolated bodies, with no stars nearby, they are much easier to observe.

So looking to brown dwarfs with a temperature close to that of the giant planets will help in constraining the models of extrasolar planets' atmospheres, the researchers said.

The discovery, led by researchers at Observatoire de Grenoble in France, the Canada France Hawaii Telescope, the University of Ottowa and other institutions will be detailed in the journal Astronomy & Astrophysics.

An earlier version of this report misstated a temperature reading for Jupiter's core.