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Wandering Lonely as a Planet

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Image: ROE/Anglo-Australian Observatory

The classic theory of how planets are born takes a big hit from a paper in today's issue of Science. There researchers from Spain, the U.S. and Germany report the discovery of 18 planet-size objects that, instead of circling a single star, drift free in the Orion constellation. Most planets are thought to form over tens of millions of years, as the gas and dust swirling around one star slowly condenses into distinct clumps. But the new objects have no central star, and their nearest cluster is a mere five million years old. "The formation of young, free-floating, planetary-mass objects like these is difficult to explain," said lead author Maria Rosa Zapatero Osorio of the Instituto de Astofisica de Canarias.

Most extrasolar planet hunters would have missed these new candidates based on the way they search: to find a planet, they look for evidence of its gravitational tug on the star it's orbiting--namely a little starlight wobble. But Zapatero Osorio and her colleagues instead detected dim, red light emitted directly from the objects in question, using visible- and infrared-light telescope cameras in Spain, the Canary Islands and Hawaii. Thus, they were also able to collect spectrographic information indicating the objects' compositions. And because heavier molecules form under cooler conditions, they learned of the objects' temperatures as well.

"The spectrographic results corresponded to our expectations that these were young giant planets," Zapatero Osorio said. The researchers further estimated the objects' masses to be between five times the mass of Jupiter and 15, depending on their age. Generally, bodies less than 13 Jupiter-masses are considered planets, whereas those between 13 and 75 Jupiter-masses are called brown dwarfs. Although the new candidates could be either, Zapatero Osario notes that earlier brown dwarf surveys suggest that finding 18 of them in one small area is highly unlikely. "This is only a problem of terminology," she said. "If planets can only exist around a star, then our candidates are very low-mass brown dwarfs. But if planets must be a certain mass, then these objects are planets."

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