Word of the Week: Planemo!

Hey, everyone! What a gorgeous day it’s turned into. Other a few clouds, the kind that the TV weather would call “a little fair-weather cloudiness” if the weather were a little warmer, and maybe there was a baseball game about to start, the skies are clear. As I struggle to get these words down, I can see the hills across the river. Earlier, I watched the moon, which looked like it was etched into the sky, like on one of those cameo boxes your grandmother used to keep on a just-out-of reach shelf, setting over the hills. We have a lot of snow, but now, the shoveling is done, and, the kids are finally back at school, the hard part is over. I hope, if you were also hit by it, you came through okay.

There’s been so much going on in astronomy news lately that it’s hard to keep up.Not all of it good, unfortunately. Today’s federal budget announcement paints a very bleak, unfortunate, and depressing picture for arts and sciences over the next few years. We have to have hope, though.

Of the good, one of the things that’s grabbed me most of all are these photos that NASA’s Cassini probe was able to grab  of Saturn’s moon tiny Pan. Pan is Saturn’s closest moon; it orbits so close to Saturn that it makes it all the way around in just 14 hours, within Saturn’s rings. Remember, Earth’s moon takes 29 days to get all the way around once.

Have you seen these photos? It’s the one that looks kind of like a hat, or a ravioli, or if you want to be picky about it, a ravilo. Either way, it’s one of the most delicious-looking moons in the solar system, and the people who run the cameras on Cassini should get a raise. The work they’ve done has been incredible for years and years.

Pan is Saturn's innermost moon
Pan is Saturn’s innermost moon

Pan is an interesting and amazing place, but it’s not round! So, here’s this week’s word of the week!


I kind of picture Batman, the 1960s one, clobbering the Joker when I hear that one.

One of the parts of the definition of a planet, as maligned as it is, that the International Astronomical Union (IAU) decided on, when it ultimately changed Pluto’s status to a dwarf planet, was that an object needs to be round. It needs to be have what’s called hydrostatic equilibrium. That is, an object needs to be massive enough, big enough, to not just hold itself together, but also be big enough and have enough gravity to force it into more or less a sphere. I say more or less because most of the round objects in the solar system are close, but aren’t. Jupiter, Saturn, and even Earth are oblate; they’re squashed a bit at the poles, and not perfectly round.

The trouble is, just because something is round doesn’t make it a planet. Take Mercury (please!). It’s one of the eight planets, yeah, but it’s only the 11th biggest object in the solar system. The sun is one of those three bigger things. Do you know the other two?

These things that are big enough to be round, but don’t orbit the sun directly, and haven’t cleared their orbit of competing objects, and, therefore, aren’t considered planets, are collectively called Planetary Mass Objects, PMOs for short. Planemo is what happens when you take “planetary mass object” and smash it down into one back-formed portmanteau.

A planemo is any object that has enough mass to be round, but not enough mass for fusion to start; not enough mass to become a star.

Our solar system is full of them. We have the eight planets, Ceres, Pluto, Eris, Vesta, Titan, Triton, Titania; planemos, the lot of them.

So, if you’re out tonight, you can see the moon, our own, nearby planemo, and wonder if any of the stars calling to us from off in the distance have systems of their own, with their own exoplanemos. There’s a good word. You heard it here first.

Clear skies, everyone!


11 thoughts on “Word of the Week: Planemo!

    1. Great! I’m glad you’ll find it handy. I totally know what you mean about those two moons being planety, though, in the case of those two, wouldn’t “Titanic” and “European” be good? 🙂 It’s amazing that these enormous moons are orbiting planets. It’s hard to conceive how big everything is sometimes. As much as I know about how these things are arranged, I sometimes have to remind myself just because something is a moon doesn’t mean it’s small.

      Liked by 1 person

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