If the universe were a fairy tale, the celestial objects called brown dwarfs would be the ugly ducklings. Small and dim as they are, brown dwarfs are informally known as “failed stars.” But some scientists have proposed that brown dwarfs possess unrecognized majesty—that they are in fact gargantuan planets. Alas, a new study suggests that the story is not destined for a happy ending. Astronomers have detected the first direct evidence that these cosmic misfits are forged in a miniature version of star formation. The study was published in The Astrophysical Journal on July 1.
Brown dwarfs have, at most, 8 percent of the mass of our sun, so their interiors lack the high heat and pressure necessary to fuse hydrogen into helium—the thermonuclear process that powers regular stars. But brown dwarfs do not fit comfortably in the planet category, either. They are tens of times more massive than even heavyweights such as Jupiter, and keep much hotter cores by contracting in on themselves and fusing deuterium, explains James Di Francesco, an astrophysicist at the National Research Council Canada who was not involved with the study.
Because brown dwarfs seem to reside in this no-man’s-land between planets and stars, astronomers have long wondered about their origins. On the one hand, brown dwarfs could form like stars, through the collapse of vast clouds of gas and dust. The protostar born in such a condensation wraps itself in a disk of material, and interactions between this accretion disk and the baby star’s magnetic field launch two jets of material from opposite sides of the disk. On the other hand, planets form when bits of material in a disk around a star glom onto one another. Some astronomers even proposed that if a big enough chunk of material broke off the disk, it could seed a brown dwarf.
Over the last 15 years astronomers have found that young brown dwarfs share similar properties with young normal stars, particularly the jets and accretion disks, says Emma Whelan, an astrophysicist at University of Tübingen in Germany. But these surveys primarily examined brown dwarfs whose surrounding envelope of gas and dust had already begun to disperse. In the new study an international team of scientists led by Oscar Morata, an astronomer at the Academia Sinica in Taiwan, examined even younger proto–brown dwarfs to see if they found the jets of material that could only be explained by a process akin to that of normal star formation. “Basically we thought, you know, ‘if it walks like a duck, talks like a duck,’ maybe stars and brown dwarfs are the same kind of thing,” Morata says.
Glimpsing brown dwarfs at such an early stage of formation had previously proved difficult because brown dwarfs are so dim. But Morata’s team scoured data from the Spitzer and Herschel space telescopes for young, faint objects. They eventually settled on 10 proto–brown dwarf candidates and examined them using the Very Large Array (VLA) of radio telescopes in Socorro, New Mexico.
Among their sample, the astronomers spotted four proto–brown dwarfs spitting out the jets characteristic of regular star formation. Moreover, Morata’s team found that the brightness of these jets depended on the brightness of the proto–brown dwarf itself. There is a similar relationship between the brightness of protostars and their associated jets. These results lend the first direct observational evidence to the idea that brown dwarfs are produced by a scaled-down version of the process that forms stars. “I think that this team has done a terrific job of advancing our understanding of how brown dwarfs form,” Di Francesco says.
But what about the other six objects—the majority? According to Morata, the accretion process that drives jets on young stars and brown dwarfs is not continuous, so jets spurt on and off and may have been missed on the other six objects. Or the jets of these objects are simply too faint to see. Morata also allows for the possibility that these objects were not proto–brown dwarfs at all, but rather background galaxies. He says that the team’s next steps will be analyzing data on their proto–brown dwarf candidates that they have collected with the Atacama Large Millimeter/submillimeter Array (ALMA) of radio telescopes and requesting more VLA observation time to reexamine the candidates in better detail. Morata and his colleagues also hope to find other newborn brown dwarfs to study. “Four—well, it’s not that much,” Morata says. “It would be nice to have something more robust.”
Whelan says that many interesting questions arise from the conclusion that brown dwarfs form like stars, particularly regarding whether these objects could host their own planetary systems—and what those systems might be like. If so, the ugly ducklings might be home to great beauty after all.