Chris Peterson wrote: Startreader wrote:
Unless the IAU has changed its tiny mind again and eliminated the Earth.
Five planets, six if you include Earth. There's no reasonable way to say seven unless you are going back to the ancient tradition of including the Moon (which the APOD editors were not).
Of course, you could make a case for Pluto, which is also in this image, contributing a few photons just below Mercury... although they are most certainly lost in the noise.
If we take the IAU to be the final arbiters of planethood, and there is no reason we should, then the Earth and the Moon are either both planets or both something else. The Moon sort of doesn't really orbit the Earth, it co-orbits Sol with the Earth and the Moon is larger, by far, in relation to its co-orbiting partner than any satellite of any planet.
Yes, some asteroids (dwarf planets, minor planets) have partners that are closer together in relative sizes as does Pluto but the IAU insist that none of those are planets
Luna and Earth have, together, mostly cleared their common orbit, certainly more than Jupiter has and they are both large enough to be nearly rounded by gravity. What more could we wish of a planet?
If Luna isn't a planet, neither is Earth.
Anyway, Luna probably used to be a totally independent planet before the two orbits got slowly synchronised.
Full disclosure: I think the IAU are wrong, Pluto is a planet. Ceres and Vesta are also planets. Eris and the others are not. Why? Because that makes me happy.
I also consider Luna, the Moon, to be at least so much a planet as the Earth.