No longer a lonely planet. Photo: Miguel Ugalde
The fall out from the IAU gets more and more amusing from a non-astronomer’s point of view. The definition has some major problems in it. The two important parts of Resolution 5A are
(1) A “planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
(2) A “dwarf planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape , (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
There’s big argument you could have about “How round is nearly round?” but that will be sorted out at another meeting. No, the real rake in the grass is section c:
(c) (has / has not) cleared the neighbourhood around its orbit.
Pluto is no longer a planet because it hasn’t cleared Neptune from its neighbourhood. Neighbourhood is another problem word, but it’s the logic that’s the real problem. If Pluto hasn’t cleared Neptune then no can Neptune have cleared Pluto – it can’t be a planet either. I worked this out as I read Phil Plait’s entry on the topic and was feeling pretty smug about this line of reasoning until I saw further down the page that he’d noticed it too. It is an obvious hole. But where does this leave Earth?
Near Earth asteroids are potentially a huge danger. That’s why Spaceguard was set up. Therefore it’s clear that Earth “has not cleared the neighbourhood around its orbit” and thus isn’t a planet. Nor is Jupiter. Among other things Jupiter is known for its accompanying Trojan asteroids. So it can’t be a planet either. In fact I’m not sure any of the named eight planets in the release meet all criteria for being a planet.
The argument is going to continue until astronomers have an idea about what they want the term “planet” to do. The ongoing wrangling seems to be symptom of various different space scientists arguing tangentially rather than address the issues that bother them.