My thanks to R. Martin, who points out that while the IAU never had a formal definition of planet, it does have a “working definition” that it released in February 2001.
I only totally wrote a paper that is in part about this very definition… How soon we forget.
Anyway, here it is. (The WGESP [Working Group on Extrasolar Planets] is part of the IAU):
1) Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are “planets” (no matter how they formed). The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our solar system.
2) Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are “brown dwarfs,” no matter how they formed nor where they are located.
3) Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not “planets,” but are “sub-brown dwarfs” (or whatever name is most appropriate).These statements are a compromise between definitions based purely on the deuterium-burning mass or on the formation mechanism, and as such do not fully satisfy anyone on the WGESP. However, the WGESP agrees that these statements constitute the basis for a reasonable working definition of a “planet” at this time. We can expect this definition to evolve as our knowledge improves.