Some of the most mysterious objects in our galaxy are also among the most numerous. And it turns out that there's an estimated 100 billion mysterious brown dwarfs scattered among the stars.
They are so ubiquitous that there could be one closer to the Earth than the nearest star system, Alpha Centauri. If a brown dwarf is ever discovered nearby, it would likely be the target of our first interstellar mission.
Brown dwarfs are smaller than the lowest-mass stars but can be dozens of times more massive than Jupiter. They are too low mass to sustain hydrogen fusion and so technically they are not stars by definition. (The term brown dwarf is also misleading because brown is not a color in the visible spectrum. Something like ultra-red dwarfs would be a more appropriate name.)
Among the biggest questions is why brown dwarfs aren't commonly found orbiting normal stars. However, some tend to hang out together in binary pairs of dwarfs. That said, they are found in the vicinity of normal stars but not gravitationally bound to them, as seen in the above photo of the Pleiades star cluster.
A newly published set of dynamical simulations points to brown dwarfs being born as knots of gas in protoplanetary disks around normal stars. They are then rudely ejected into interstellar space as lone drifters.
"We conclude that gas clump ejection and the formation of low-mass and substellar objects is a common occurrence, with important implications for understanding the formation of stars," writes Shantanu Basu of the University of Western Ontario and Eduard I. Vorobyov of the University of Vienna.
The team’s simulations show a lumpy, fragmenting gas disk whirling around a forming star (as seen here, arrow points to brown dwarf progenitor clump). Several other large gas clumps play a gravitational game of bumper cars. In a typical "three's-a-crowd" interaction, a clump is gravitationally ejected from the system. The clumps left behind may fall into the star or get tidally shredded.
The runaway clump later forms its own separate accretion disk and gravitationally collapses down to a compact object ranging from one-tenth to one-third the mass of our sun. In some cases two brown dwarfs contract out of the clump to make a binary system.
I'm fascinated by this theoretical work because I wonder if it would help explain one of the most legendary and strangest space photos ever taken — of an unexplained object called TMR-1c:
In 1998, the Hubble Space Telescope made an infrared photo of a very red pinpoint object (seen at bottom right) that is at the end of a ghostly finger of illuminated dust stretching 135 billion miles from a young binary star system. The telltale finger was interpreted as being formed after a large, hot planet was gravitationally ejected from the binary.
It was later dismissed as simply a chance juxtaposition of the dust-reddened light from an old background star with a foreground linear nebulous feature. However, ground-based telescopic observations in 2009 show that the mystery object had gotten brighter and bluer. This is something a normal main sequence star could never do.
The variability in brightness and color is interpreted as a young substellar object surrounded by a spinning thick disk of dust. The colors of TMR-1c could be explained by the presence of a condensed atmosphere, as commonly observed among brown dwarfs.
These detailed computer simulations underscore that star-making, planet-making and even brown dwarf birth is a much more chaotic and messy business than ever before imagined.
Photo credits: ESO, S. Basu, NASA