Brown Dwarf

Brown Dwarf, type of body in space that is smaller than a star but more massive than a planet. Brown dwarfs do not have enough mass to start thermonuclear reactions that fuse normal hydrogen (a hydrogen atom has 1 proton) into helium (2 protons) the way true stars do. For this process to take place, they would need to have about 8 percent of the mass of the Sun or about 80 times the mass of Jupiter. Brown dwarfs with at least 13 times the mass of Jupiter are thought to fuse a form of hydrogen called deuterium (1 proton + 1 neutron) into helium in their early history. Some brown dwarfs may be large enough to have reactions that burn lithium (3 protons). Such fusion processes last only a few million years, however. Brown dwarfs remain at a lower temperature than stars and radiate mostly infrared energy generated as they slowly contract under the influence of their own gravity. Often called failed stars, brown dwarfs may become more planetlike as they age, and develop clouds and dust in their atmospheres.

How brown dwarfs form is a subject of debate. One theory is that they begin in the same manner as stars—that is, from the gravitational contraction of a mass of interstellar gas. However, the parent clump of gas does not contain enough material to create a true star. Another theory is that brown dwarfs start as normal star embryos in larger clouds. Gravitational interactions with nearby stars or other star embryos may throw the forming star out of the cloud before it can reach the critical size needed to become a true star.

In 2007 astronomers reported that a brown dwarf with 24 times the mass of Jupiter was ejecting jets of material from both poles, a process known to occur in many young stars. The same brown dwarf, called 2MASS 1207-3932, also has a disk of material around it and a large planet in orbit. These findings support the idea that at least some brown dwarfs form in a similar way to stars.

Astronomers predicted the possible existence of objects intermediate in mass between stars and planets, but it was not until October 1995 that they announced the first discovery of a brown dwarf. The object, known as GL 229B, is located in the constellation Lepus and is the small companion of a known star 18.6 light years from Earth. GL 229B is 20 to 50 times the mass of Jupiter and contains methane at its surface—a definite sign that it is cooler than a true star. Methane cannot survive temperatures much higher than 725°C (1337°F), about 1000°C (1832°F) colder than the surface of the coolest stars. GL 229B is too faint and close to its companion star for astronomers to observe it directly with Earth-bound telescopes, but the object was photographed by the Hubble Space Telescope in November 1995.

Astronomers have since found hundreds of brown dwarfs. Some float free in space while others orbit around larger true stars. A few form double systems with another brown dwarf. At least some brown dwarfs have planets. The first extrasolar planet that scientists could see in an image was found around the brown dwarf 2MASS 1207-3932 in 2005. The planet has five times the mass of Jupiter.

Brown dwarfs are now thought to be about as numerous as normal stars. However, brown dwarfs are not massive enough to account for much of the “missing mass” in the universe—that is, mass that has been theorized to exist but has not yet been detected (see Dark Matter).