There’s some interesting news news from the International Astronomical Union this morning. They may be about to settle on the definition of a planet…and there are more of them than you think. (For those of you who don’t remember this stuff, you’re supposed to think there are nine of them.)
The sticking point has been the most distant planet, Pluto. We’ve been calling it a planet for a long time, but with a diameter of only about 1400 miles, it’s actually smaller than the Earth’s moon and is closer in size to an asteroid. Some people don’t think that’s significant enough to call a planet.
The problem was exacerbated with the discovery of something cataloged as 2003 UB313 which is even further away, and ever so slightly larger than Pluto.
This isn’t a vitally important issue—not even to astronomers—but they’d like to settle it to avoid the embarrassment of having children in different countries learning different lists of planets.
The IAU appears to be settling on a two-part definition. The first part, which settles the matter of Pluto, is that planets must be round. That is, for an object to be considered a planet, it must be large enough that its own gravity has shaped it into a sphere. (The IAU definition actually talks about “hydrostatic equilibrium” which accounts for some variation in shape, most significantly that spinning planets bulge at the equator.) In practice, this means it must be at least 500 miles in diameter with a mass of at least 1/12000 of Earth.
Pluto is round, therefore Pluto is a planet.
Unsurprisingly, UB313 is also a planet under this rule, but they’ll probably give it a real name. Actually, its discoverer, Michael Brown, has nick-named it Xena, as in Xena: Warrior Princess, and naming rights usually go to the discoverer, but the IAU says it will probably be named something else later. (I hope not.)
Slightly more surprising is that the asteroid Ceres, orbiting between Mars and Jupiter, will become a planet under this rule. It’s 900 miles in diameter and mostly round. It was considered a planet when it was discovered, but downgraded to an asteroid when all the other asteroids were discovered in roughly the same orbit (i.e. the asteroid belt).
“What about the moon?” you may be asking. (Or not.) It’s round and it’s bigger than Pluto, Ceres, and UB313. Shouldn’t it be a planet? For that matter, Jupiter’s four largest moons are all pretty big, shouldn’t they be planets too?
That brings us to the second part of the definition: Planets orbit the Sun. Not other planets. Thus, the Moon remains a moon. And the planet-sized moons of Jupiter remain moons too.
Pluto is once again a troublemaker. Its moon, Charon, is about half the size of Pluto itself and is quite massive. That’s a problem because a planet’s moon doesn’t technically have an orbit centered on its planet. Rather, the planet and the moon orbit a common center of gravity. When the moon is a small rock, like the moons of Mars, that common center of gravity is very nearly the center of the planet. But with a larger moon the center of gravity shifts away from the planet’s center. The Earth’s moon is so large that the center of gravity is 3000 miles from the center of the Earth, but that still puts it 1000 miles below the Earth’s surface, so it’s still accurate to say the Moon orbits the Earth. That’s why it will remain a moon.
Not so with Pluto and Charon. The center of gravity of the Pluto-Charon system is a point between Pluto and Charon. This means that Charon doesn’t technically orbit Pluto. So Charon is a planet too.
If you’re counting, that’s twelve planets: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon, Xena.
Let’s see now…I learned the memory mnemonic for the order of the planets as “Mary’s Violet Eyes Make John Sometimes Unusually Nervous Perhaps.” I guess we’ll have to change that to something like “Mary’s Violet Eyes Can Make John Sometimes Unusually Nervous Perhaps Concerning Xylophones.” No, that doesn’t work…
Update: They decided it’s not a planet after all.
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