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From MSNBC News

Marcy speculated that some interaction might even slingshot planets completely out of their solar systems. Scientists believe that such a "rogue planet" already has been spotted.

Question Over Planet Discovery
From the BBC

A scientist from the world famous Royal Greenwich Observatory says Nasa scientists claiming to have found the first planet seen outside our solar system are jumping the gun. Dr Robin Catchpole believes a lot more work needs to be done before such conclusions are drawn. The new object, in the constellation of Taurus, was found by the Hubble space telescope. Nasa believes it may be a planet two or three times the mass of Jupiter, the largest planet in our solar system. Alternatively it could be a brown dwarf star, one that was too small to sustain the nuclear reactions in its core that normal stars need to shine. But although Dr Catchpole admits that the brown dwarf explanation is feasible, he says more work is needed before further conclusions can be drawn. "I really don't think there's any evidence to say that what we are seeing here is a planet", he told BBC World Service, adding that scientists needed to use another instrument to determine what sort of light the object was giving out. A spectrum of the object would tell researchers whether it was hot and luminous like a star or cool and reflective like a planet. "This one looks far too bright to be reflecting the light of the stars nearby," he said.

Although he accepted the discovery had potential, he said: "A lot more work has to be done before it's right to go out and tell everybody you've found a planet." This could be done with the biggest telescope in the world which this week produced its first pictures of space. The new instrument built by eight European nations in northern Chile would be able to gather the light, analyse it and tell us exactly what it is made of. Dr Catchpole believes Nasa's release of the story was symptomatic of an increasing tendency for scientists "to grab the headlines as soon as possible." He said this was driven by the need to get funding for research and searching for life in other worlds was an area where there was a lot of competition. "The big question that everybody wants to know about is, is there life out there? Are there planets out there? So the first person who really gets one is going to make a major discovery."

Free-Floating Planets and Stellar Clusters
American Scientist, Vol 90, Mar-Apr, 2002

For centuries a planet has been defined as an object that orbits a star. This notion was recently upended when several groups of astronomers reported the discovery of planet-sized objects wandering through space on their own, with no parent star in sight. The discovery of these objects within dense stellar clusters has unsettled the astronomical community and raised questions about the nature of planets and how they might form. Jarrod R. Hurley and Michael M. Shara review these recent discoveries and consider how the dynamic interactions between the stars in a dense stellar cluster may free planets from the gravitational bondage of their parent stars. Jarrod Hurley is a postdoctoral investigator at the American Museum of Natural History. His research involves studying the evolution of star clusters through computer simulations. His models have helped to explain the formation of blue-straggler stars in the open cluster M67, and he has recently begun to investigate the behavior of planetary systems in star clusters. Michael Shara is curator and chair of the Department of Astrophysics at the American Museum of Natural History. His research interests include the structure and evolution of novae and supernovae, collisions between stars, and the nature of stellar populations in star clusters and galaxies.