The infrared deep-sky survey now being carried out by the United Kingdom Infrared Telescope is only 5% done, but already astronomers looking at its data have found the coolest solitary brown dwarf ever seen.

The object, named ULAS J0034–00, is located in Cetus. A followup analysis by the Gemini Observatory of steam and methane features in its infrared spectrum pegs its temperature at just 600 to 700 kelvins (330° to 430°C, or 620° to 800°F). This puts it at the very bottom of spectral class T — or perhaps in the still-cooler proposed spectral class Y, for which no other object has yet been found.

Estimated to be 50 light-years away, ULAS J0034–00 has a near-infrared magnitude of about 18.5 and a likely mass of 15 to 30 Jupiters. Like all brown dwarfs, it should have just about Jupiter’s diameter.
This image is from the infrared survey. Paradoxically, T dwarfs appear blue in composite-color infrared images; the molecular absorption bands in their spectra block the longer (“redder”) infrared wavelengths.

About Alan MacRobert

Alan M. MacRobert became an avid Sky & Telescope subscriber in 1966 at age 14, joined the editorial staff in 1982, and is now a senior contributing editor, semi-retired. He played a role in practically every part of the magazine and the company's other products for more than a generation, both on the amateur-observing side and the science-reporting side. In 1994 a book collection of his observing how-tos and telescopic sky tours was published as Star Hopping for Backyard Astronomers. He has produced This Week's Sky at a Glance online every week since 1989.

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