What's in a planet?

Numerous discoveries are putting a strain on astronomers’ notion of a
“planet”. Brown dwarfs not much bigger than Jupiter, a huge asteroid a quarter
the size of Pluto and enormous gas giants close up to nearby stars are forcing a
rethink. The International Astronomical Union came close to finalising the
issue, only to be thrown into confusion when researchers spotted planet-like
objects floating through empty space
(New Scientist, 14 October, p 20).
“Nature’s making this a little bit tough,” says Alan Boss, an astrophysicist at
the Carnegie Institution of Washington DC.