It’s official: Pluto is a planet!
well, nearly official, there’s still some rubber stamping to be done, but this week the proposal was made at the IAU conference, which is expected to be unanimously passed, that a planet is defined as follows:
- A body that orbits a star but is not itself a star or a satellite of another planet
- Such a body that has sufficient mass that its own gravity causes it to take an approximately spherical shape
- Where a body is orbiting another body that has been defined as a planet, the orbiting body is defined as a satellite if the barycenter of the two bodies lies within the primary body; else, the secondary body is not a satellite but also a planet.
By this definition, Pluto is absolutely a planet. But so are at least three other bodies in the solar system, and potentially several more! As of this week, we have at least 12 planets!
The Bad Astronomer has a detailed explanation of what this all means, and what some of the loopholes could be, but essentially we now have three classes of planets: Terrestrials, Jovians, and Plutons. The first two kinds are collectively referred to as the ‘classical planets’.
Now here’s some of the weirdness that arises from this definition: firstly, the asteroid Ceres, which orbits between Mars and Jupiter, is classified as a planet. This means plutons aren’t necessarily trans-neptunian. Secondly, Pluto’s moon Charon is also a planet. This makes Pluto-Charon a binary planet system. Thirdly, due to tidal interactions our moon is steadily moving away from us. Eventually, it will be distant enough that the barycenter of our system is outside the surface of the earth, making the Moon a planet!
Another of our additional planets is UB313, which is yet to be given an official name. The nickname given to it by its discoverers is Xena, but I doubt that’ll stick. This is the planet that finally pushed the IAU into redefining the term, because it’s bigger than Pluto, which pretty much meant that the IAU had to either declare Xena the 10th planet or remove planetary status from Pluto. Frankly, I think they went in the more interesting, but less scientifically accurate direction. It’ll sell papers, that’s for sure, and it’s not bruising any egos.
So get used to it: soon schoolbooks may be teaching kids that the planets are Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon and (Something). Well, except for the fact that sometimes Charon is closer to the sun than Pluto. And sometimes both of them are closer to the sun than Neptune…And then there are all the other bodies under review, like Sedna and Quaoar, and all the many bodies thought to be just beyond the reach of our telescopes… exciting times for the solar system, indeed. Granted, this is all semantics and taxonomy, and has nothing to do with the science of solar system formation. But it certainly is fun.
I can’t wait for the astrologers to start falling over themselves to explain why their horoscopes don’t reflect the fact that there are more planets. They can’t argue size – Xena is bigger than Pluto. They can’t argue proximity – Ceres is closer. This is going to be hilarious.