Milky Way could have 100 billion brown dwarfs

Brown dwarfs, sometimes known as "failed stars", are the link between low-mass stars and gas giant planets. They are weak objects and, therefore, difficult to study, so many of their characteristics are still unknown. A study, with the participation of the Institute of Astrophysics of Andalusia (IAA-CSIC), points out that the Milky Way could contain between twenty five and one hundred billion brown dwarfs.

It is believed that brown dwarfs follow a formation process similar to that of stars, which begins with the fragmentation and contraction of an interstellar gas cloud. However, these objects barely reach 10% of the mass of the Sun, which prevents the nuclear reactions that feed the brightness of the stars and cause them to weaken over time. Since the discovery of the first brown dwarfs, in 1995, more than two thousand have been detected, mainly in regions of star formation nearby and with a low density of stars.

Now an international team of researchers has looked for brown dwarfs in the young star cluster RCW 38, which has a very high stellar density and many massive stars. It is a totally different environment from those where brown dwarfs have been studied, and researchers looked to see if their birth place affects the rate of formation of brown dwarfs -which in the nearby clusters may amount to a brown dwarf for each two stars-.

"This is the first study of its kind in a massive star cluster at a distance greater than one kiloparsec (3,262 light-years), and represents a great step forward in this field," says Rainer Schoedel, a researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC) involved in the research. The large angular resolution in adaptive optics of the NACO instrument of the Very Large Telescope (ESO) was fundamental to this study, and my experience with this kind of images has been my main contribution".

Researchers have found that RCW 38, which lies 5,500 light-years away, shows a similar proportion of brown dwarfs and stars than other nearby and less massive clusters, suggesting that the conditions where they form do not affect the number of brown dwarfs.

Researchers estimate that in the Milky Way there could be between twenty-five and a hundred billion brown dwarfs. Considering that our galaxy contains between one hundred and four hundred billion stars, it is a very high proportion. "And since these objects are extremely weak, we may just be seeing the tip of the iceberg," Rainer Schoedel concludes.