So now we know what a planet is. As confirmed by the passage of a revised resolution at the International Astronomical Union’s general assembly today in Prague, a planet meets the following criteria:
The third item, of course, is the interesting part, for it rules out Ceres, about which there had been some controversy. I mean, it was one thing to consider 2003 UB313 as a planet, but to delve into the middle of the Solar System and define a new planet in medias res seemed a stretch too far for some people (though not for me). Pluto is also ruled out because it moves for part of its orbit inside the orbit of Neptune; Charon likewise is left without planetary designation.
What does happen to Pluto is that it becomes a ‘dwarf planet,’ a new kind of object that also includes Ceres and 2003 UB313, and of course we will be finding numerous other dwarf planets as we continue to refine our observations of the Kuiper Belt and surrounding space (as many as a dozen candidate dwarves are already on the IAU watchlist — click here for a list of known dwarf planets).
So there you have it, eight planets that can be considered ‘classical’ and a new category that subsumes all the rest. Which for the traditionalist in me is OK, while the logician says that the third criteria involving clearing out a planetary orbit is likewise sensible, and we can forget about the Pluto/Charon barycenter. Sigh.
There is precedent for planetary demotion, incidentally. After its discovery in 1801, Ceres was generally thought to be a planet, but by the early 1850s so many of what we now call ‘asteroids’ were being discovered that planetary status for all was ruled out. It shouldn’t be long before the IAU ruling becomes accepted and similarly relegated to a historical footnote, but the controversy was intense while it lasted.
But let’s move on to other things, which tomorrow includes a new planet around Mu Arae, a system that is getting more crowded (and better characterized) all the time, and the methods used to find it.
Update: A telling comment from Hal Weaver at Johns Hopkins’ Applied Physics Laboratory. apropos of dynamically cleared orbits:
“Regarding the resolution itself, I’m with Andy Cheng in concluding that the situation is still somewhat muddled. What exactly is meant by a planet ‘clearing its neighborhood?’ Since many ‘plutinos’ … (including Pluto) …cross Neptune’s orbit, I’d say Neptune’s neighborhood still needs some clearing! … It just seems a bit risky to me to base a definition on a theoretical construct (‘dynamically cleared regions’) that’s only approximate at best and may change significantly as our understanding of planet formation evolves over time.
“I further note that there have been particularly large swings in the theories of outer solar system dynamical evolution during the past decade. What was ‘conventional wisdom’ five years ago has been replaced with the latest fad, and I don’t expect that situation to change any time soon.”