Thursday, July 16, 2015

planet--and remind me why Pluto isn't one again

I was at a party over the weekend and a bunch of us were sitting around afterwards and our most space oriented friend mentioned the news about upcoming proximity to Pluto via spacecraft.The rest of us fell into a chorus of lamentation for its exile from our planetary solar system and how we thought we might call it a planet anyway. She smiled politely and diplomatically at us and said, "Except that it isn't, you know." Apparently on beyond Pluto there are a bunch of other Plutolike satellites and at least one of them is even bigger than Pluto. In my mind, though, I wasn't clear on why we wouldn't just include them in rather than exclude the one we know. True it would be harder to do the styrofoam planet projects of my youth, but kids have computers now, people!

Anyway. What is a planet? If we start with the  etymology, it isn't exactly clarifying. According to the Online Etymology Dictionary, The Old English was planete, going back through the Old French of the same spelling through Late Latin planeta to the Greek planetes, which was actually a kind of condensation of astares planetai, or wandering stars, planasthai meaning 'to wander'. This was to distinguish them from the "fixed" stars, because they appear to move and the actual stars don't. For this reason, the sun and moon were also called planets. The "modern scientific sense of "world that orbits a star" is from 1630s."

None of this would seem to leave Pluto out.

As NASA's own Mission Science site points out: "Technically, there was never a scientific definition of the term Planet before 2006." Evolving technology allowed scientist to see further out and in 1991, they discovered that beyond Pluto, there was a disc shaped cloud of objects, which they named the Kuiper Belt. Gerard Kuiper was an astronomer who mentioned some objects out there beyond Pluto in 1951, but according to this pretty contrarian article, he kind of got it wrong. Anyway, the problem with the Kuiper Belt wasthat it had a lot of objects in it and at least one was bigger than Pluto. What to do?

Oddly enough, it strikes me as a little bit like Fox News's decision about the Republican debates. If you don't know, the Republican field is currently at fifteen candidates, but Fox has decided that the top ten in recent polls will be the only ones invited to the first debate. Astronomers similarly have decided that including every object in the Kuiper belt as a possible planet would be too much information to deal with. I don't really see why. I'd assume that since we didn't really know what all was there for the rest of human history, we could maybe take a leisurely approach to learning about it.

The (very) new definition of a planet is:

1.) It must orbit around the sun.

2.) It must be big enough for gravity to squash into a round ball.

3.) It must be big enough to clear other objects (like asteroids) from its neighborhood, either by sucking them in or hurling them off into space.

But of course, as the NASA article goes on to say, "Define neighborhood." In any case, it's requirement three where Pluto is getting the demerit points.

Frankly, I don't put a lot of stock in things that were just defined in 2006. It seems kind of likely to, uh, change. But all right. What is the new term for a Pluto like space object?

A dwarf planet.

Oh, come on. If you said someone wasn't a human being but a dwarf human being as if that was a meaningful difference, you would get sued. And rightly so.

Pluto is also called a "plutoid", which is defined as any dwarf planet that is farther out in space than Neptune. So far, there are three known ones. Pluto, the larger Eris that started this whole mess, and Makemake. I hope we get to know them too.

Judging from the response to our nearing Pluto, rendering it a non-planet hasn't lessened the thrill.

The New Horizons team     NASA/Bill Ingalls
But as someone wiser than we are has already told us, size isn't the only way to talk about a planet we love, even if that planet is "only" an asteroid.


  1. I have trouble getting used to the idea of an eight-planet solar system, and astronomy was my big interest when I was little. So yes, I have a soft spot for Pluto.

  2. I happened to see the director of the Griffith Park Observatory on Chris Hayes the next night, and to my surprise he seems largely seemed to agree that the relationship of Pluto to our thoughts of it trumps the size of the 'plutoid'. Or at least that it's okay if we think so.