Ultra-Cold Brown Dwarf Discovered?

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Not to be outdone by the Keck II telescope in Hawaii, NASA’s infrared Spitzer Space Telescope may have discovered a brown dwarf even cooler than the brown dwarf Discovery News reported on last week.

Keck spotted “CFBDSIR J1458+1013B,” a brown dwarf 75 light-years from Earth with a mass of 6-15 times that of Jupiter, and from observations realized that this very dim object must have a temperature of less than 100 degrees Celsius (212 F) — about as hot as boiling water.

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This discovery has very important ramifications for star and planetary physics, because a brown dwarf is neither a star or a planet, it is an object that bridges the gap between planets and stars. For this reason, and because brown dwarfs do not possess enough mass to sustain nuclear fusion in their cores, that they are often dubbed “failed stars.”

So, the potential Spitzer discovery of an “ultra-cold” brown dwarf has excited scientists even more. Spitzer’s candidate brown dwarf, detected 63 light-years from Earth with a mass of approximately seven times the mass of Jupiter, appears to have a temperature of 30 degrees Celsius (86 F)! This “room temperature” brown dwarf is called “WD 0806-661B.”

Interestingly, WD 0806-661B orbits a white dwarf star (called, unsurprisingly, WD 0806-661) and could be considered an exoplanet. But it orbits at a huge distance: 2,500 AU — or 2,500 times the distance between the sun and Earth. For it to be a planet, it would need to be orbiting far, far closer to the white dwarf.

However, there is another possibility. This white dwarf star was once a star two-times the mass of our sun. White dwarfs are what remains of an old star after it has run out of fuel and died.

Indeed, our sun has a similar fate in approximately 4 billion years time. After it has run out of hydrogen fuel, puffed up as an angry “red giant” and swallowed the Earth as it expands, our red giant sun will eject its outer layers, leaving a white dwarf shining in the core of the resulting “planetary nebula.” (I’ve written about this fascinating scenario before: “What Will Happen When the Sun Turns into a White Dwarf?“)

Models predict that once the sun has shed huge quantities of mass after its red giant phase, the remaining planets of the solar systems will drift into wider orbits. Is this what happened to WD 0806-661B? Was it once a very massive planet orbiting a star twice the mass of the sun? Further measurements of the object are obviously needed.

The discoveries of CFBDSIR J1458+1013B and WD 0806-661B (if it is indeed a brown dwarf) are unprecedented; they could represent a population of low-temperature brown dwarfs that have more “planet-like” than “star-like” characteristics.

They are so cool in fact that water vapor in their atmospheres could condense to form clouds of water droplets. Brown dwarfs that are capable of this belong to a specific “Y”-class family of these objects. Y-class brown dwarfs have only been theorized, but these two new discoveries are prime Y-class candidates.

Publication: “Discovery of a Candidate for the Coolest Known Brown Dwarf,” Luhman et al., 2011, ApJL. doi: 10.1088/2041-8205/730/1/L9

via New Scientist

Image: NASA, edit by Ian O’Neill, Discovery News