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HubbleSite Small Companion to Brown Dwarf Challenges Simple Definition

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Space Telescope Science

As our telescopes grow more powerful, astronomers are uncovering objects that defy conventional wisdom. This latest example is the discovery of a planet-like object circling a brown dwarf. It's the right size for a planet, estimated to be 5-10 times the mass of Jupiter. There has been a lot of discussion in the context of the Pluto debate over how small an object can be and still be called a planet. This new observation addresses the question at the other end of the size spectrum: How small can an object be and still be a brown dwarf rather than a planet? This new companion is within the range of masses observed for planets around stars – less than 15 Jupiter masses. But should it be called a planet? The answer is strongly connected to the mechanism by which the companion most likely formed. What's even more puzzling is that the object formed in just 1 million years, a very short time to make a planet according to conventional theory.

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