A bunch of really cool objects.

A mammoth sky survey designed to map the location of more than 1 million galaxies and 100,000 distant quasars has uncovered two extremely faint objects--right in our own astronomical backyard. The two dim red dots imaged by the Sloan Digital Sky Survey belong to a class of brown dwarfs, objects too big to be planets but too small to be stars. Another large survey, known as 2MASS (2-Micron All Sky Survey), has discovered an additional four of these enigmatic objects.

Brown dwarfs don't have enough mass to sustain full-scale nuclear fusion, the process that keeps stars burning. However, they are massive enough--10 to 70 times as heavy as Jupiter--to burn deuterium, endowing them with a gentle glow for the early part of their long lives.

Follow-up observations of the newly discovered brown dwarfs reveal that all six contain methane, a sign that they are extremely cool. Methane cannot form in objects warmer than about 900 [degrees] C, notes Adam Burgasser of the California Institute of Technology in Pasadena. The low temperatures suggest that the dwarfs have had a long time to cool down and are more than 300,000 years old.

Only one other so-called methane brown dwarf has ever been found. That object, GI229B, orbits a star. In contrast, each of the six new methane dwarfs travels alone in our galaxy.

Burgasser, with J. Davy Kirkpatrick of NASA's Jet Propulsion Laboratory in Pasadena, discovered four of the dwarfs among millions of celestial objects imaged with the pair of near-infrared telescopes, one in Arizona and the other in Chile, that comprise 2MASS. Spectra taken with one of the Keck telescopes on Hawaii's Mauna Kea revealed the methane, the 2MASS team reported on May 31.

Kirkpatrick estimates that the dwarfs found with 2MASS are only 30 light-years from Earth. "Because our telescopes can only see the closest examples, this means the Milky Way must be brimming with objects like these," he says. David Golimowski of Johns Hopkins University in Baltimore, a member of the Sloan team, calculates that 200 to 400 methane brown dwarfs may lie within 30 light-years of Earth.

Astronomer Gibor S. Basri of the University of California, Berkeley agrees that methane brown dwarfs could be plentiful. They are difficult to detect only because they are extremely faint and show up only in the infrared, he suggests.

Using the Sloan survey's 3.5-meter telescope at Apache Point, N.M., Xiaohui Fan and Michael A. Strauss of Princeton University found one of the brown dwarfs earlier this spring, and last month, Zlatan Tsvetanov, Golimowski, and their Johns Hopkins colleagues spied a second one. Other researchers discovered the methane in these two dwarfs by examining spectra taken at the United Kingdom Infrared Telescope on Mauna Kea. The Sloan team reported its findings on May 31 at a meeting in Chicago of the American Astronomical Society.