Is Pluto a planet?

Since the question is raised again by the discovery of Sedna, and since the topic here is only MainlyMartian, I thought I'd post an article I did for the International edition of Newsweek a few years ago, with some very minor polishing. (I'd link to the published version, but it doesn't appear accessible). Nothing much seems to have changed -- nor will it, I expect, until a TNO larger than Pluto turns up.

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"The Little Planet that Couldn't"
Newsweek International, February 26th 2001

We live on one, we've been observing a handful of others since antiquity, we've sent space probes to almost as many as can be reached and we're currently discovering new examples around distant stars at a staggering rate. So it might come as something of a surprise that we don't really seem to know what a planet is. But we don't. The recent decision by the American Museum of Natural History to question Pluto's right to be called a planet has reopened a fascinating -- and instructive -- can of worms.

When Pluto was discovered by Clyde Tombaugh in 1930, there wasn't much doubt about it being a planet. Percival Lowell, the man who had built the observatory where Tombaugh worked, had predicted that there should be a ninth planet beyond the orbit of Neptune. Tombaugh had found something out there. What could it be other than the long sought planet? After all, the only objects in the solar system were planets (big things that orbit the sun), moons (things which orbit planets), asteroids (little stony things, mostly to be found between the orbits of Mars and Jupiter) and comets (little icy things that spend most of their time further away and which, unlike asteroids, develop a gaseous "coma" and tail when they pass close to the sun). With that menu to choose from, Pluto could only be a planet.

Today things look less clear. Since 1992, astronomers have been finding big lumps of ice in orbits very similar to Pluto's: almost 400 have been discovered to date, some as large as the largest asteroid (though in composition they're much closer to comets), and larger ones almost certainly remain to be discovered. Some say that Pluto should properly be classed as the largest of these "Trans-Neptunian Objects", or TNOs, rather than as a planet proper. There is precedent for such a move. When Giuseppe Piazzi discovered something orbiting between Mars and Jupiter 200 years ago, it was proclaimed a planet; as it became clear that there were a lot more of these objects in similar orbits, though, Piazzi's discovery was downgraded. Ceres is now seen as the largest of the asteroids and nothing more. Asteroids and comets are referred to as minor planets in some circumstances: but that doesn't mean anyone thinks of them as planets strictu sensu.

To many astronomers this is heresy. Pluto was one of the planets they learned as schoolchildren. It's undeniably larger than any other TNO so far discovered. It has great scientific interest -- and it is the only planet to have been discovered by a great American, to boot. (Pluto's demoters believe this fact is a strong, if unvoiced, factor in the Pluto-is-a-planet arguments championed by, among others, the American Astronomical Society). Changing things would be needlessly confusing to the public -- and possibly to Congress. Planetary scientists have long wanted NASA to send a mission to Pluto, but the plans have been repeatedly deferred and cancelled. A current last ditch effort at a shoestring mission is underway, and it needs all the PR it can get. Having its target demoted from its planetary status might not be helpful.

Though heartfelt, these arguments are not particularly strong. Since the Voyager missions revealed the extraordinary faces of the major moons of Jupiter, Saturn and Neptune (all six of them larger than Pluto, as is the earth's moon) no-one has been able to pretend that planets are uniquely attractive heavenly bodies; Pluto doesn't have to be a planet to be interesting. And the size question cuts both ways. Yes, Pluto may be larger than any other known TNO, but it is very much smaller than Mercury, the smallest of the eight echt planets.

As a matter of practical politics it might indeed be better to wait five years or so. By then, either a mission to Pluto will already be underway, or the once-in-many-lifetimes opportunity to mount one as good as is currently possible will be gone. For most of Pluto's two and a half century long year, its thin atmosphere, having frozen, lies on its surface. Only during its brief few decades of summer is it really worth visiting. If you think a thin envelope of gas that only appears during Pluto's closest approach to the sun makes it sound like a comet, I think you have a point. If NASA's mission to Pluto gets cancelled, Pluto will look more cometary still -- NASA has a long history of cancelling comet missions.

There is a more principled argument for allowing Pluto to stay a planet --though it is unpalatable to many astronomers. Pluto is big enough, which means it has enough gravity, to have pulled itself into a spherical shape. This differentiates it from most comets and asteroids, which are oddly shaped, and implies that it has a complex, layered internal structure. A definition of planets as gravitationally collapsed, chemically differentiated non-luminous structures would let Pluto in. But it would also let in many moons, some of the largest asteroids and quite a few of the TNOs. On this definition, the total number of planets in the solar system would rise to 30 or more: an exact tally would involve close-up looks at various asteroids and TNOs.

This is the sort of possibility that seems to worry some astronomers. They think that people are comfortable with nine planets, and don't want any more -- and that the public will think less of them if they start promoting and demoting bits of the solar system. But this is wrong. The members of the public most interested in such matters -- schoolchildren -- will be quite happy with more planets. They can keep a couple of hundred pokemons straight in their heads: a couple of dozen planets shouldn't present a problem. More important, if the denizens of the solar system are more rigorously categorized according to their natures, rather than their sizes and positions, we'll be in a better position to talk about the denizens of other planetary systems as they are discovered. But most important of all, seeing scientists change their minds is an important lesson in and of itself. It reinforces that scientific ideas are often provisional and that new data changes the way things are viewed. It shows that science is about self-criticism and growth, about being open to the new ideas a serious study of the universe will always bring with it. And that's a lesson worth learning at any age.

Update: Alan Stern provides a more detailed account of the "if it's collapsed into a sphere it's a planet" argument on Spacedaily (though his daughter seems to gainsay my point about schoolchildren). And this column from Ray Arvidson's hometown paper is cute.

Comments

[T. E. Williams]

Yeah! That last paragraph is right on. Kids are perfectly capable of understanding how things get changed and renamed, and it would be good for the adult public at large to understand how fundamentally our understanding of planetology has changed since they went to school in decades past.

[NelC]

Is there a provisional list somewhere of the 30-odd spherical-bodies-orbiting-the-Sun?

[Oliver Morton]

I guess my provisional list was the nine canonical "planets", seven big moons, and the same number again in big TNOs like Sedna and the largest asteroids (Ceres, Pallas, Vesta, I think, maybe more) Maybe Amalthea and Charon, too. It was a pretty loose guesstimate. Anyone who wants to put together a better one should feel welcome (but remember there will be more big TNOs to include, by and by...)

[Bruce Moomaw]

Actually, Amalthea is about 1.5 times longer than it is wide. And the real problem is the intermediate cases. Vesta is very seriously flattened at the poles; and Neptune's second biggest moon, Proteus, looks like nothing so much as a giant marshmallow. No doubt there are plenty of such intermediate cses among the KBOs, too.

[someone]

something

[Kevin Shuman]

Oh come on now,of course Pluto is a planet.It has one feature which none of the other objects mentioned has.Pluto has a moon and that makes it a planet.

[Oliver Morton]

Some asteroids -- quite possibly a lot of them -- have moons, too. They don't even have to be particularly large asteroids...

[Origg]

Only the Moon. It is close enough that, detpsie being relatively small, it can cover the disk of the Sun. The Moon casts a shadow and it is when this shadow passes over Earth (and you stand in that shadow patch) that you would see a total eclipse of the Sun.Venus and Mercury are bigger than the Moon, but they are much further away from us. Their apparent diameter is therefore too small to block off the light of the entire Sun's disk. When they pass in front of the Sun, it is called a transit .There will be a transit of Venus in June 2012 (just as there was one in June 2004).No comets or asteroids are big enough to block off the Sun, regardless of how close they pass to Earth. And no, they cannot cause earthquakes. The idea that something could pass between us and the Sun, causing an eclipse AND earthquakes comes from the Planet-X hoax (the original one, for May 13, 2003 the Big 2012 Hoax is simply a recycling of that hoax).The charlatans who played up the hoax used the story to scare people and then make money by selling them books on how to survive.

[Nifta]

Ich hatte schon fcberlegt, ob ich nicht dieses Jahr mal thenlieme an der Konferenz, aber die Organisatoren machen es einem nicht gerade einfach die Buchungs- und Bezahlmethoden von SIS sind noch nicht mal im letzten Jahrzehnt des vorigen Jahrhunderts angekommen: keine Emailadresse, keine Kreditkarten, kein PayPal :-(