Hubble Takes Census of Elusive Brown Dwarf Stars

August 24, 2000

Astronomers using NASA's Hubble Space Telescope have carried out the most complete inventory to date of brown dwarfs, one of the universe's most elusive types of objects, which dwell in limbo between stars and planets.

The Hubble census provides new and compelling evidence that stars and planets form in different ways. "Because the brown dwarfs 'bridge the gap' between stars and planets, their properties reveal new and unique insights into how stars and planets form," says Joan Najita of the National Optical Astronomy Observatory (NOAO) in Tucson, AZ, whose study with Glenn Tiede (NOAO) and John Carr (Naval Research Laboratory, Washington, DC) will appear in the October Astrophysical Journal.

Considered an astronomical oddity only a few years ago, brown dwarfs are intriguing objects that, unlike stars, are too low in mass to burn hydrogen, but are more massive than planets. At 15 to 80 times the mass of Jupiter, the light that they do emit is so faint that it has been hard to tell how many of them are scattered throughout the galaxy, and whether their formation process is similar to that of stars, planets, or neither of these.

The Hubble census finds that like stars, there are more low mass brown dwarfs than high mass ones, and this trend continues down to low, nearly planetary masses. "In this respect, the isolated ("free-floating") brown dwarfs found by Hubble appear to represent the low mass counterparts of the more massive stars. This suggests that stars and free-floating brown dwarfs form in the same way," says Najita.

However, the Hubble finding also offers the strongest evidence to date that free-floating brown dwarfs are a completely different population from the recently discovered planets that orbit nearby stars. Najita's team found that brown dwarfs are, remarkably, far more common in isolation than in orbit around other stars. "This suggests that the extra-solar planets and, by extension, the planets in our own solar system, formed very differently from how the Sun and other stars formed," Najita says.

For example, these results support the idea that stars form through the gravitational collapse of molecular clouds, whereas planets grow through the sticking of micron-sized particles of stardust, creating larger rocks, and finally planets.

Only a few years ago, it was commonly believed that brown dwarfs are rare, perhaps because the process that makes stars "stops working" at lower masses. "The Hubble result is a resounding 'NOT!' Nature does not discriminate between stars that can shine by fusion and lower mass objects that are unable to do so," says Najita. "In fact, the universe easily makes brown dwarfs of all masses, from the most massive to the least," Najita says.

The study also found that brown dwarfs, elusive though they have been, are unlikely to contribute significantly to the mysterious, unseen "dark matter" that dominates the mass of our galaxy and the universe. Although Hubble found that brown dwarfs are abundant, it turns out that they are not common enough to explain the unseen "dark matter" that dominates the mass of our galaxy and the universe. Najita and her colleagues conclude that brown dwarfs probably contribute less than 0.1 percent of the mass of the Milky Way's halo.

The inventory was carried out using Hubble's infrared vision to measure the brightness and temperature of stars in the young cluster IC 348, located in the constellation Perseus. Because the cluster is young, the brown dwarfs in the cluster are intrinsically brighter, which made it easy to detect approximately 30 brown dwarfs dwelling in the cluster.

A critical step in the observation was picking out the cluster brown dwarfs from the clutter of background stars. To tackle this problem, Najita and her colleagues developed a new technique. They used Hubble's NICMOS camera to measure the strength of an infrared water absorption band in the atmospheres of the stars. The strength of the band is a sensitive measure of each star's temperature. Since strong water absorption in the Earth's own atmosphere made this experiment impossible from the ground, using Hubble and NICMOS was the ideal solution.

"The ability to measure the temperature of each star solved several problems simultaneously," says Najita. "In addition to helping us distinguish the cluster brown dwarfs from background stars, we were also able to measure the masses of the brown dwarfs without having to assume their age. This greatly improved our mass estimates."

As a result of the high sensitivity of Hubble and NICMOS, Najita and her colleagues were able to accurately identify and study brown dwarfs to very low masses. Their study is the first to be complete down to 15 Jupiter masses, a fiducial boundary between brown dwarfs and planets.

What are brown dwarfs?

Considered an astronomical oddity only a few years ago, brown dwarfs are intriguing objects that, unlike stars, are too low in mass to burn hydrogen, but are more massive than planets. At 15 to 80 times the mass of Jupiter, the light they do emit is so faint that it has been hard to tell how many of them are scattered throughout the galaxy, and whether their formation process is similar to that of stars, planets, or neither of these. Only a few years ago astronomers commonly believed that brown dwarfs are rare, perhaps because the process that makes stars "stops working" at lower masses. The Hubble results provide evidence that this isn't the case.


What are the results of the census?

The Hubble finding shows stars and brown dwarfs form in the same way. Like stars, low-mass brown dwarfs are more plentiful than heavyweights. This trend continues down to very low, almost planetary masses. The Hubble results also offer the strongest evidence to date that brown dwarfs are a completely different population from the recently discovered planets that orbit nearby stars. The census showed that brown dwarfs are, remarkably, far more common in isolation than in orbit around other stars. This finding suggests that the planets, whether in our solar system or beyond, formed very differently than the Sun and other stars. These results support the idea that stars form through the gravitational collapse of cold, dense clouds of gas, whereas planets grow through the buildup of tiny particles of stardust.