Brown dwarfs, sometimes called failed stars, are the coldest known free-floating celestial bodies but are warmer than previously thought, U.S. astronomers say.

The new class of star-like objects was discovered two years ago by researchers using NASA's WISE space telescope, and some evidence suggested their surfaces might be as cold as room temperature -- but that's not so, report astronomers at the Harvard-Smithsonian Center for Astrophysics

A new study shows some brown dwarfs have temperatures of about 250-350 degrees Fahrenheit; our sun, by comparison, is about 10,000 degrees F at its surface.

To be so relatively cool after billions of years means that these objects can only have about 5 to 20 times the mass of Jupiter, the researchers said, and unlike our sun their only source of energy is from their gravitational contraction -- which depends directly on their mass.

Brown dwarfs are often considered failed stars because they are larger than planets but with too little mass to trigger nuclear fusion and ignite into the brilliance of a full-fledged star.

"If one of these objects were found orbiting a star, there is a good chance that it would be called a planet," Trent Dupuy, a Hubble Fellow at the Harvard-Smithsonian Center said. But because they probably formed on their own and not in a proto-planetary disk, astronomers still call these objects brown dwarfs even if they are "planetary mass," he said.

The new study has implications for the current official definition of a brown dwarf, experts said, which places the boundary between the biggest planets and the smallest brown dwarf stars at 13 times the mass of Jupiter.

"I personally think that's a stupid definition because it's not based on an understanding of how brown dwarfs form, or how planets form, it's just an arbitrary spot that's been drawn to divide between them," Chris Tinney of the University of New South Wales said.

"These objects could be formed by planetary processes around another star by core accretion ... or they could have formed just like stars in the center of their own accretion disk, and just never really gotten very big, becoming a small aborted star," he said.