Study of Coldest Known Brown Dwarfs Offers Insight into Planetary, Star Formation

The closest look yet at a handful of misfits known as brown dwarfs reveals the space bodies represent something of a cosmic identity crisis, hovering between a star and a planet.

The findings, researchers say, could reveal insight into the evolution of planets and stars.

Among the study's discoveries was that, despite being the coldest known free-floating celestial bodies, the brown dwarfs under examination are much warmer than previously believed, ranging from about 250 to 350 degrees Fahrenheit. Unlike the Sun, which is powered by the conversion of hydrogen and helium, brown dwarfs derive their energy from their gravitational contraction, which is directly affected by their mass. In order to reach these surface temperatures after billions of years spent cooling, the researchers estimate that these objects would have to possess a mass just five to 20 times that of Jupiter.

"If one of these objects were found orbiting a star, there is a good chance that it would be called a planet," said Trent Dupuy, a Hubble Fellow at the Harvard-Smithsonian Center for Astrophysics and a co-author of the study published in the journal Science Express. As it is, scientists believe they probably formed on their own and not in a planet-forming disk.

Before they could obtain accurate temperature readings of the space bodies, however, astronomers first had to determine the distance of the objects.

"We wanted to find out if they were colder, fainter and nearby, or if they were warmer, brighter and more distant," Dupuy explained.

To do this, the researchers relied on a principle called parallax, or the apparent change of an object's position when seen from two different lines of sight. 

"To be able to determine accurate distances, our measurements had to be the same precision as knowing the position of a firefly to within 1 inch (2.5 centimeters) from 200 miles (320 kilometers) away," said Adam Kraus, professor at the University of Texas at Austin and the study's other co-author.

Only by using NASA's Spitzer Space Telescope were the astronomers able to measure the distances precisely and, as a result, determine the space bodies' temperature, brightness and mass with a great deal of accuracy.

One mystery that still remains, which is why the brown dwarfs' observable properties don't appear to correlate with their temperatures, suggesting that other factors, such as convective mixing, play major roles in driving the chemistry at the surface.

Given that the most recent study focused solely on the coldest brown dwarfs discovered by NASA's WISE survey, however, researchers say they hope future studies on additional objects discovered over the past two years may help resolve these incongruities.