The Daily Duck has a post about the current proposal by the International Astronomical Union to promote other solar orbiting objects to be planets. I managed to get emotionally involved in this issue, to the extent that I want to make some extended remarks that are too much work to be buried in the Daily Duck comments.
While I would personally prefer to up the requirements for being a planet and dump Pluto from those ranks (it having turned out to be a much smaller object than originally thought when it was made a planet), I must admit that the new standard is more objective and rational than the existing “whatever we felt like” definition. The key properties are:
So perhaps I will, in time, come to accept the new definition, presuming it passes muster with the IAU.
On specific points, one of the issues brought up was Pluto’s moon Charon, which would become a planet, making the Pluto / Charon couplet a double planet. Why, then, wouldn’t Luna become a planet as well?
The answer is whether the presumed primary itself is orbiting. If not, then clearly you have a primary and a satellite.
The definition of orbiting is revolution about an external point. For the Earth / Luna system, the center of mass (the point about which both Earth and Luna revolve) is inside Earth, so the Earth wobbles (or rotates) but doesn’t revolve / orbit. Since physically you can’t have the center of mass inside both objects (or there would only be one object), this makes a very clean answer of whether you have a primary / satellite or a double. By this definition, then, Luna is a satellite and Charon is not because the center of mass for Pluto / Charon is roughly 1.8 times the radius of Pluto from Pluto’s center, i.e. outside Pluto.
Ah, but Brit says
If Jupiter were to disappear, its moons would go in all directions, so they are moons.
Whereas if the Earth disappeared, the Moon would continue orbiting the sun, so it is not really a moon, but the Earth and Moon are a ‘double planet’.
I wondered whether that was true. Clearly, the issue here is relative orbital velocities, that of the system about the Sun (“solar orbital velocity”) and that of the satellite about the primary. If the latter is close to or larger than the former, then you would have the scatter effect should the primary disappear. Conversely, if the satellite orbital velocity is small compared to the solar orbital velocity, removing the primary wouldn’t have much effect.
As it turns out, Brit’s base claim is true. Approximately, Lunar orbital velocity is 1 km/s, with a solar orbital velocity of 30 km /s. In contrast, Io has an orbital velocity of 17 km/s with a solar orbital velocity of 13 km /s. So Luna would orbit the Sun roughly the same without Earth, but Io could do just about anything from continuing unchanged to dropping straight in to the Sun.
However, I do not like that definition as it depends on factors that I don’t think should matter, such as the absolute (not relative) mass of the primary, and the distance of the primary from the sun. For instance, should Luna be labeled a planet or moon based solely on how far Earth is from the Sun? After all, if you moved Earth far enough out, its solar orbital velocity could be reduced to roughly 1 km / s, or equal to Luna’s orbital velocity, demoting Luna from planet to moon. I just don’t like that sort of property in my definitional rules.
UPDATE: Via a deep black blog, I have recieved word that Pluto is no longer a planet. I am fine with that, in fact I think demoting Pluto is probably the best choice. I simply willing to accept Pluto as a planet in exchange for having an objective categorization based on the physical properties of the objects. I think the “it owns its orbit” is interesting and probably a good way to judge whether a planet is serious or not. The “200 year” orbit idea struck me as quite silly in contrast. I do wonder, though, about how Pluto gets bonked for not clearing out Neptune without Neptune getting bonked for not clearing out Pluto.
I suspect one is supposed to look at the zone from perihelion and aphelion and ask “is there any other body with a solar orbit in that zone?”. If yes, it’s not a planet. By that criteria, Neptune wins (because Pluto’s orbit crosses in and out of Neptune’s zone) but Pluto loses (because Neptune’s orbit is entirely within Pluto’s zone). This seems reasonable both at that level and because otherwise you would’t have any planets, since various comets and planetoids cross the orbits of the “classic” planets.
It looks like a good decision to me.