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Wandering gas balls: planets or brown dwarfs?

Orphan objects recently found in Orion may help scientists redefine just what is and isn't a planet.

The recent discovery of 18 planet-size gas balls in the constellation Orion has scientists puzzling anew over what constitutes a planet.

The free-floating objects have masses from five to 15 times that of Jupiter. Generally, objects 13 times the mass of Jupiter or less are classified as planets. But unlike a planet, the orphans are unattached to any central star, such as Earth's sun.

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The discovery - in a star field 1,200 light years from Earth - marks the first time astronomers have gotten a detailed look at objects this small in regions of space where young stars form. Astronomers say the find is raising fundamental questions about the origins of planet-size objects.

"These results are surprising," says Scott Kenyon, who studies solar-system formation at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. Noting that astronomers earlier discovered bodies with Jupiter-like masses orbiting stars, he said that now, with discovery of these bodies floating freely in interstellar space, "the next questions would be: Do they form in the same way or in a different way? And what does that tell us about how our solar system formed?"

Other astronomers have found about 50 planets outside the solar system, but those planets were in the orbit of a central star. It was obvious that the nomad objects failed to pass muster as stars, but several also failed as "brown dwarfs" - star wannabes that never grow massive enough to ignite hydrogen-fusion reactions at their cores. Like stars, they are thought to form when interstellar gas clouds contract and fragment to become the seeds for star birth. Less-powerful nuclear reactions can heat larger brown dwarfs, but below a certain mass, even those fires don't ignite and the dwarfs are heated only because gravity compresses them.

To some astronomers, such an absence of nuclear reactions represents a defining point between planet and brown dwarf, according to Adam Burrows, an astrophysicist at the University of Arizona in Tucson. But, he says, "this is an artificial distinction." Instead, he says, origins should define the two types of objects. Thus, planets form from flattened disks of dust and gas found around young stars, while brown dwarfs form from fragmentation of interstellar gas clouds, just as stars do.

That definition of a planet has been placed in draft form before the International Astronomical Union's Working Group on Extrasolar Planets.

One test for the new-found objects would be to observe their positions in another 10 years to see if they have shifted. Such movement, some astronomers say, could suggest that the objects formed around stars, but were somehow ejected from their orbits.

If the new objects fall into the brown-dwarf category, as many astronomers say they do, they still tax explanations of how they would form, says astronomer Maria Rosa Zapatero Osorio of the Astrophysical Institute of the Canary Islands, who led the team that made the discovery, published in the current issue of Science. Theories describing how gas clouds fragment are unclear about the smallest-sized object that can result from this process. Given the surprisingly large number of these objects in its survey, the discovery team says, such low-mass bodies could be quite common in other young star clusters. As detection methods improve, astronomers should begin to pick out more of them, says Dr. Zapatero Osorio.

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(c) Copyright 2000. The Christian Science Publishing Society

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