Carnegie Institution of Washington
News Release
Tuesday April 3, 2001
CALL: Alan Boss at 202-478-8858 (, or Tina McDowell (CIW News Office) at 202-939-1120

Are They Planets or What?

Recent observations of star-forming regions by several different groups have uncovered evidence for free-floating objects with masses inferred to be as low as 5 to 13 times the mass of Jupiter. Because such objects are incapable of burning deuterium, they have been labeled "planets," creating considerable controversy over this use of the "p-word." It has also been thought to be unlikely that objects so low in mass could have formed in the same way in which stars form, leading to the suggestion that these objects were tossed out of planetary systems. In a paper accepted for publication in the Astrophysical Journal (Letters), astrophysicist Alan P. Boss of the Carnegie Institution shows that magnetic fields may allow stars to form with minimum masses as low as about one Jupiter mass. In this case, free-floating objects with masses below 13 Jupiter masses would be best termed "sub-brown dwarfs," not "planets."

Searches for very low mass objects free-floating in young star clusters such as Orion have revealed hundreds of brown dwarf candidates with estimated masses below the hydrogen-burning limit of about 75 Jupiter masses. Free-floating objects have even been found with inferred masses below the deuterium-burning limit of about 13 Jupiter masses, prompting their designation as "planets" by some. Radial velocity surveys have detected over 50 likely planetary companions to sun-like stars, with minimum masses in the range from below one Saturn mass to over 15 Jupiter masses. Evidently the least massive, isolated objects found in young star clusters could be less massive than the most massive planetary companions to sun-like stars, blurring a mass-based distinction between stars and planets. Such observations raise an important theoretical question: Can very low mass, free-floating objects be formed directly in star-forming regions, or must they form in planetary systems and later be ejected?

Theoretical estimates of the minimum mass of an object formed by the process that forms stars had predicted that no star could have a mass less than about 3 to 10 Jupiter masses, and most likely such an object would end up with a considerably higher mass, because it would continue to gain mass after it first formed. Stars form when dense clouds of gas and dust are driven to collapse in upon themselves by their own self-gravity. During this collapse phase, protostellar clouds can sub-divide (fragment) into smaller and smaller mass objects, until such time as the cloud begins to heat above its initial temperature, increasing the pressure of the gas and helping to stifle any further fragmentation. Boss performed detailed computer calculations of this process over a decade ago, suggesting that the lowest mass object formed by protostellar collapse should be well over 10 Jupiter masses.

However, all of the previous estimates of the minimum mass neglected the effects of magnetic fields. In a new set of detailed computer calculations, Boss has included the effects of magnetic fields on protostellar fragmentation in a crude approximation, but one that appears to capture the essence of the physical effects. Magnetic fields can be thought of as stretched rubber bands, with a tension force that resists their being pinched together. During the star-formation process, magnetic fields help stop the cloud from collapsing into a single object at the center of the cloud, because of this tension force. As a result, the clouds remain more distended, and thus more able to break-up and fragment into smaller mass objects. Magnetic tension also helps the cloud to rebound away from the center once it begins to heat, leading to decompressional cooling, and the formation of even smaller mass fragments. Boss finds that four fragments as low as about one Saturn mass may form in a single collapsing cloud in this way. The system of four fragments is expected to be highly unstable and should decay by ejecting single fragments, which would then appear as isolated objects. These fragments would continue to gain mass rapidly only until they were ejected, and so could end up with masses in the range inferred for the Orion free-floaters.
Boss suggests calling the free-floating objects "sub-brown dwarfs," because they probably formed in the same way that stars and brown dwarfs form, but ended up with less mass than brown dwarfs, and as a result would be less luminous, all other things being equal.
A color jpeg image of an unstable quadruple protostar system with sub-Jupiter mass components is available at: Boss's paper is scheduled to be published in the April 20 issue of the Astrophysical Journal (Letters).
Alan Boss is a staff member of the Carnegie Institution's Department of Terrestrial Magnetism in Washington, D.C. His work on this topic is supported in part by grants from NSF's Stellar Astronomy and Astrophysics Program and NSF's Major Research Instrumentation Program.
Carnegie Institution of Washington was founded in 1902 by Andrew Carnegie as his institution for discovery. Today, the Institution operates five research centers: the Department of Embryology in Baltimore, the Department of Plant Biology in Stanford, California, the Department of Terrestrial Magnetism and the Geophysical Laboratory, both in Washington, D.C., and the Carnegie Observatories, based in Pasadena, California with principal observing location at the Institution's Las Campanas Observatory, Chile. The DTM geochemists, seismologists, and astrophysicists are led by the Department's director, Sean Solomon. The president of Carnegie Institution is the biologist Maxine Singer. For more information about the Carnegie institution, see the web site