Free-floating Planets in Orion

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Science  05 May 2000:
Vol. 288, Issue 5467, pp. 773
DOI: 10.1126/science.288.5467.773c

Extrasolar planets and brown dwarfs have been discovered with increasing frequency around young stars, yet still more discoveries will be needed to understand conditions under which a star or a planet will form from the collapse of a molecular cloud or other processes. Evolutionary diagrams of star formation track the change in color, luminosity, and mass of a star from its youth to old age, but there are so few substellar objects that such diagrams are poorly defined.

Lucas and Roche completed a deep infrared photometric survey over a small area of the Trapezium cluster in the Orion nebula with the UKIRT telescope on Mauna Kea, Hawaii. They identified over 500 sources that could be plotted on an evolutionary diagram, and about 30% of these sources were brown dwarfs, while 13 sources were planets. Such a large population of substellar sources has not been observed previously in Orion and provides vital parameters for deriving evolutionary tracks. The planets are called free-floating because they appear isolated from other stars; however, these planets are assumed to have formed by cloud core fragmentation and collapse, the same process that accounts for star and brown dwarf formation. One unusual break in the substellar object track is the lack of planets of less than 8 × 103 solar masses, which the authors attribute to local O stars with strong stellar winds that disperse the collapsing core gases before the smaller planets have formed.—LR

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