Planet? I just make et up as I go along! 
In my discussion about whether planet is a natural kind, I focus just on our solar system. Although we expect there to be planets around other stars as well, I say we don't know enough about the structure of other solar systems to generalize too much. I also comment that a gas giant which didn't orbit a star would not be a planet.

There's lots of interesting recent astronomy which makes my claims about the details out of date.

Further work has allowed astronomers to identify dozens of planets orbiting other stars, so-called exoplanets. The amateur techniques for discovery are sufficiently routine that many of the discoveries have been made by amateur astronomers. Of course, we don't have - and may never have - ways of detecting smaller objects like Trojan asteroids in the orbits of exoplanets.

Last Fall, astronomers discovered what most news sources have described as a "rogue planet" and which the paper announcing it calls a "free-floating planet". Despite such loose talk, sources are clear that it is still an open question whether the object formed around a star (and so was born like a planet) or formed out on its own (and lacked sufficient mass to ignite as a star). In the introduction to the paper, the authors use the more cautious phrase 'Isolated Planetary Mass Object' (IPMO).

These are cool findings which I would discuss if I were writing the book now, rather than two years ago. The difference would just be in the details, though, and I think my general conclusions still hold.

Since natural kinds are contingent and identifying them is fallible, though, I fully expect there to eventually be some discoveries which entirely undo something that I say in the book. I would be deeply suspicious of any philosophy of science which was immunized against such revision.

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