Saturday, August 26, 2006

Would aliens from Jupiter consider Earth a planet . . . ?

(Hat tip: Reality Carnival.)


W.M. Bear said...

Very cute. However, apparently, size was not a major factor in the IAU's "demotion" of Pluto. In my (addmitedly limited) understanding of their criteria, their main problem with Pluto was that it hadn't "swept its orbit clear" of other bodies nor circularized it very well either, since a portion of Pluto's orbit lies inside Neptune's orbit. (Well, folks, that would seem to mean that NEPTUNE HASN'T SWEPT ITS ORBIT CLEAN EITHER, wouldn't it now?) They also disliked the fact that the center of mass of the Pluto/Charon system -- the point around which Pluto and its satellite revolve -- lies outside of Pluto itself. (Well, exc-u-u-u-se me!) These criteria will, however, lead to a conundrum if a Pluto-sized body is discovered around another star which does NOT have a satellite system with the center of mass outside of the main body and which DOES have a circular orbit which it HAS swept clean! PLANET?

Given the somewhat arbitrary character of definitions anyway, my solution to the whole mess would have been simply to "grandfather in" Pluto as a planet and to make ITS size the arbitrary criterion for planethood: as large as or larger than Pluto, AND orbiting its primary star and not a larger planet -- PLANET. Not meeting these criteria, NOT A PLANET. The size and characteristics of ITS satellites make no difference. If two bodies larger than Pluto are orbiting each other, call it a "binary planet" on the model of a binary star. But this is NOT the case with Charon, so no need to worry about Charon. Pluto -- PLANET. Ceres -- NOT A PLANET. Charon -- NOT A PLANET. Xena -- POSSIBLY A PLANET. Why is this difficult?

W.M. Bear said...

And it looks like I've got plenty of company!

Backlash against the Pluto demotion