MIAMI—A satellite launched by NASA in 2009 to map the sky in infrared wavelengths is beginning to deliver on one of its ancillary promises, the mission's scientific leader said here at the semiannual meeting of the American Astronomical Society. Edward Wright, a University of California, Los Angeles, astronomer who serves as principal investigator for the Wide-field Infrared Survey Explorer (WISE), said in a talk Monday that the spacecraft has already discovered at least two cool, substellar objects known as brown dwarfs.

One of the brown dwarfs, dubbed WISE 2, appears to be as cold as any that are known. It may even be colder, Wright said, than the brown dwarfs recently found by the UKIDSS survey, which are estimated to be in the neighborhood of 500 Kelvin, but the exact temperature of WISE 2 is uncertain (as are the temperatures of the UKIDSS objects). WISE 1 is a bit warmer, Wright said: "We think this is about an 800-Kelvin object."

Brown dwarfs are objects larger than planets and smaller than true stars, although the boundaries between the three groups are somewhat blurry. A strictly mass-based definition holds that brown dwarfs are too small (less than about 75 times Jupiter's mass) to fuse hydrogen in their cores, as stars do, but are large enough (more than about 13 Jupiter masses) to fuse deuterium, a heavy isotope of hydrogen.

They are thought to be numerous, and indeed hundreds of brown dwarfs have already been found. But surveys to date have not been able to find the dimmest, coolest brown dwarfs that theory predicts—those that have temperatures of just a few hundred Kelvin. But WISE's infrared channels are sensitive to that population and should be able to assess just how common those cool dwarfs are.

Wright later said that whereas the spectra of WISE 1 and WISE 2 are unambiguous, the spacecraft has found many more objects that may also be brown dwarfs. Confirmation of those will await follow-up observations, which the group has proposed on the Spitzer Space Telescope. Distances to the two new brown dwarfs are not known, Wright added, but WISE should be able to turn up bundles of such objects in its 10-month mission, some of which may be closer to the solar system than Proxima Centauri, the nearest known star to the sun.

Artist's conception of (left to right) the sun, a low-mass star, a warm brown dwarf, a cool brown dwarf and Jupiter: NASA/IPAC/R. Hurt