Ok, so it's 12 now?

As reported here and in many other places, we will possibly end August with 12 planets in our Solar System: the current set of nine (including Pluto), plus Ceres, 2003UB₃₁₃ and Charon. The unofficial definition, to be voted next Thursday, is that a planet is any body that orbits a star and is large enough that gravity makes it spherical (or almost spherical).

Ceres is one of the bodies in the asteroid belt, between the orbits of Mars and Jupiter, and it joins the group because it is, indeed, round. It was briefly considered a planet shortly after it was discovered, so it actually regains the status.

2003UB₃₁₃ is the large trans-Neptunian body discovered in 2003 and recognised last year as being larger than Pluto. Despite a very weird orbit, it actually makes sense to consider it as a planet is Pluto is one.

Now, Charon is a weird one. It is, until now, considered one of the moons of Pluto (and, until recently, the only one); so, it might seem that it does not orbit the Sun, but Pluto. However, Charon is so large compared to Pluto that the center of gravity of the combined system is actually outside Pluto; that is, it's not so much Charon orbiting Pluto, but both orbiting each other. By the "new rules", this makes the system a binary planet, and therefore both count as planets.

Interestingly enough, our own Moon is also quite large, in comparison with the moons of other planets. It's not that large, though, and the centre of gravity of the system is very much inside Earth. But the Moon is getting away from Earth: the average distance increases by some 4cm a year. This means that, in a few million years, the centre of gravity will be outside Earth, and the Moon will be a planet. I really don't think the IAU rules will still exist (or be relevant) by then, though.

The order of planets our kids will have to learn will, then, be Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon and 2003UB₃₁₃ (which will by then have a new name). Now, I don't know why Sedna and Quaoar were not invited to the party. Maybe that will come later.