Astronomers have used data from the Spitzer Space Telescope to measure the distances, and hence temperatures, of so-called “brown dwarfs” (see Section 9.5).
These brown dwarfs, the coldest known free-floating celestial bodies, were found to be warmer than previously thought, with surface temperatures ranging from about 250 to 350 degrees Fahrenheit (125 to 175 degrees Celsius). By comparison, the Sun has a surface temperature of about 10,000 degrees Fahrenheit (around 6000 degrees Celsius).
To reach these surface temperatures after cooling for billions of years, these objects would have to have masses of only 5 to 20 times that of Jupiter. Unlike the Sun, the only source of energy for these coldest of brown dwarfs is from their gravitational contraction, which depends directly on their mass. The Sun is powered by the conversion of hydrogen to helium; these brown dwarfs are not hot enough for this type of “nuclear burning” to occur.
The findings help researchers understand how planets and stars form, but also present new puzzles to astronomers who study cool, planet-like atmospheres, as the observable properties don’t correlate with temperature in a straight-forward way. Ongoing studies of newly discovered brown dwarfs may shed some light (and heat) on these outstanding issues.
Read the JPL press release for more detail and additional images.