Michael Liu/Institute for Astronomy, University of Hawaii

Cold 'Star' No Hotter Than a Summer's Day

Stars are stars, and planets are planets, and never the twain shall meet, right? Not quite. Brown dwarfs—so-called failed stars that are too small to sustain the stable burning of hydrogen—fall somewhere in between stars and planets when it comes to mass and temperature. Now, researchers have found two brown dwarfs that are colder than any previously seen—so cold and so small that they are almost like giant planets. In a paper published 20 March in The Astrophysical Journal Letters, a team reports on a brown dwarf about 63 light-years away whose temperature is barely 300 kelvin. That's 200 K cooler than the previous record holder and about as warm as a bright summer day on Earth. In a second study, to be published in an upcoming issue of The Astrophysical Journal, researchers describe another very cold brown dwarf (shown here in an infrared telescope image), whose estimated temperature is about 370 Kelvin. The two objects could be the first examples of a proposed class of ultra-cool brown dwarfs known as the Y-class. And because they are almost as cold as "gas giant" planets—Jupiter is about 150 K—studying them could offer a better handle on what the atmospheres of alien worlds look like.

For more information about these discoveries, see the 18 March issue of Science.

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