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 Published by the American Geological Institute
November 2000
Newsmagazine of the Earth Sciences

News Notes

Who needs a star?

Eighteen celestial objects were caught loitering recently in a young stellar cluster near Orion’s belt, rather than orbiting stars as most planets do, reports a team of astronomers in the Oct. 6 Science. The astronomical community is divided over what to do with these misfit youths.

Based on our own solar system, a massive body is defined as a planet if it has no nuclear reaction in its interior and orbits a star, says Maria Rosa Zapatero Osorio, who led the team at the Astrophysics Institute of the Canary Islands. But that definition leaves the new-found free-floaters, which have planet-like interiors, in semantic limbo. They hover in the star cluster without orbiting anything.

While some astronomers would like to initiate a change in policy to allow planets the freedom to drift when young, “most astronomers still insist that planets have to be in orbit about stars,” says Jibor Basri of the University of California at Berkeley.

At five to 15 times the mass of Jupiter, these new kind of isolated giant planets, as the Canary Island team calls them, breach the size of brown dwarfs. A failed star, a brown dwarf burns deuterium in its core and not hydrogen, as other stars do. It must be at least 13 times the mass of Jupiter before it can burn deuterium, but never reach 75 times Jupiter’s mass, the point at which hydrogen fusion begins.

Location of the Orionis star cluster 
(region around the brightest star in the
bottom right side of the figure) and 
finding charts (45” by 45”) for three 
spectroscopically confirmed, very 
young cluster membranes with masses
between 5 to 15 times the mass of Jupiter.

And at three orders of magnitude younger than our sun, these strange objects are challenging the theory of how planetary bodies form. One theory might have the objects forming after the breakup of a large molecular cloud as the smaller fragments collapse under their own gravity, similar to the way brown dwarfs form, Zapatero Osorio says.

“No matter what terminology astronomers eventually settle on, it’s already clear that the study of free floating, low-mass objects — stars, brown dwarfs, and now planetary mass objects — can lend significant insight to our understanding of how stars and planets form,” says Joan Najita of the National Optical Astronomical Observatories. “I think that will be the ‘legacy’, so to speak, of the reported observations.”

Christina Reed

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