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The Pluto Problem

Why has Pluto been demoted from "Planet" status? The reasoning behind the change

You might be wondering why the International Astronomical Union has declared that Pluto is no longer worthy of being "A Planet". It may seem shocking that a planet which has been known in your lifetime and has interesting statistics such as having a year lasting 248 years, has suddenly been downgraded and re-classified as "not a planet", or more precisely, "a dwarf planet". There are reasons, and although I don't agree with the decision, I can tell you why they decided to downgrade Pluto.

The decision was not reached lightly. Also, it was not a simple matter, because what was going on was a severe problem that was likely to mean other problems. The trouble started a long while ago, and has only recently become silly. I'll explain...

The Planet Neptune was discovered because the Planet Uranus had a slight peculiarity in its orbit. With Uranus being a big planet, much bigger than The Earth, it must have been perturbated in its orbit by another large planet. Calculations gave a good hint as to where the planet might be, and then, looking around that region, Planet Neptune was discovered by Le Verrier in 1846.

Looking at the orbit of Neptune, it too seemed to have something odd about its orbit, and so the idea was, maybe there's yet another large planet. Calculations showed about where the new planet (generally referred to as "Planet X") was likely to be, and Percival Lowell was very keen to find it. Managers at the Lowell laboratory gave Clyde Tombaugh the task of hunting for the hypothetical planet by means of blink-comparison. After a while, he found Pluto!

There was a problem right from then, because it was clear that Pluto was too small to have moved Neptune about. However, it was a planet, and big news. This has tended to reduce the amount of searching for a Neptune-sized planet out there, and for all we know it could still be there!

Pluto was measured using instruments of the time (1930), and it was estimated to be a single planet of a modest size, possibly as large as the Earth. Only later was it discovered that what was being looked at was Pluto and its co-orbiting body Charon, both of which are small but at a distance look large together. This and other discoveries and refinements in measurements meant that the known size of Pluto was very considerably downgraded. By 1978 it was regarded as one five-hundredth the mass of the Earth.

Other discoveries in astronomy were being made. New and distant planetesimals were being found, for example Sedna, Quaoar, Haumea, Makemake, etc and it was realised that sooner or later, someone was going to find one of these things that was bigger than Pluto, and then what?

This is exactly what happened. Eris was found, and Eris is larger than Pluto. Eris is named after the goddess of discord, which is quite appropriate.

Try to think about the difficulty the International Astronomical Union were having. The options for the were various:

1. They could have all of these objects described as "planets".

2. They could insist that Pluto was still a planet for traditional reasons but that the others were not!

3. They could come up with some carefully contrived rules which would be a scientific "definition of a planet" that was consistent.

Thinking about it, option 2 would have been silly. I would have thought option 1 was the best, because then there'd be a vast range of new planets being discovered! I believe this would be preferable (see below). However, there is a minor downside to the all-encompassing "It's a Planet" idea. It would mean that all kinds of little bits and pieces would be on the borderline and then it would be tricky to define whether any particular asteroidal chunk of rock was, or was not, a planet. (It would still have to be round though)

So, the International Astronomical Union decided to come up with a law, a definition, to circumscribe what shall be a planet and what shall be a lesser thing, or "dwarf planet". Dwarf planets are not new, and there have been known minor planets such as Ceres for some time now.

The new definition was worked on and decided by committee, and then announced to the world. My opinion is it's not a very good definition. The definition they came up with is as follows: A planet must fit with all of the following:

1. The object must be in orbit around the Sun.

2. The object must be massive enough to be a sphere by its own gravitational force. More specifically, its own gravity should pull it into a shape of hydrostatic equilibrium.

3. It must have cleared the neighbourhood around its orbit.

This definition has obvious problems right from the start. For one thing, The Sun. The definition is only for planets in The Solar System, and an entirely different set of rules apply for extrasolar planets! You can see this is already problematical!

The second condition is fair enough. That's basically "It's got to be round". My opinion is this would have been sufficient.

The third condition is decidedly dodgy. It's like saying that you're not a respected member of the community until you've scared off all the peasants in your neighbourhood. Besides having very little to do with the status of planetariness of an object, it also produces some bizarre anomalies. For example, if The Earth were out there where Pluto is, The Earth would not qualify as a planet!

Also, if the Moon disappeared, the Earth would still be a planet, but if the Earth disappeared but the Moon carried on going around the sun, The Moon would be a planet. However, if both the Moon and Earth continued to exist and became separated, both orbiting in solar orbit, NEITHER of them would be classed as a planet!

Some astronomers have claimed that the definition also discounts most of the outer planets as not really being planets, because they have not really cleared their orbits. Even Jupiter has all kinds of stuff going around with it, Trojans.

In Avatar, the Na'vi are a civilisation living on the planet Pandora, and if someone told them it was not a planet they'd probably be a bit miffed. However, as Pandora is in orbit around the planet Polyphemus rather than the star Alpha Centauri, it fails the Earthies' "planet definition". I would speculate that of all the planets with advanced life-forms on them, a relatively high percentage are on satellites of large planets rather an on planets in stellar orbit.

I agree that something needed to be done about the Pluto Problem, but the 2006 decision by the International Astronomical Union is not a quality decision, and it undermines the credibility of the International Astronomical Union, which is a shame, because they do a lot of good, promoting Astronomy, and campaigning against light pollution etc. Unfortunately the edict about Pluto makes them look like a religious council, almost like The Magisterium in The Golden Compass, or some of the earth-centric authorities in bygone times, the sorts of people Galileo had a problem with.

Some people have refused to accept the new law, and are going on calling Pluto a planet. The folks who are steering a spacecraft on the way to Pluto are especially dischuffed with their spacecraft's destination being demoted before it gets there! See New Horizons

My own ideas about what counts as a planet are different from those of the International Astronomical Union. I say if it's round by gravitational hydrostasis, it's a planet, and that's regardless of whether it's going around the sun, anywhere else, or nowhere in particular. I have sometimes termed the Moon "The Planet Luna"!

In my own definition, an object is whatever type of object regardless of whether it's in orbit around a star, or around a planet, or just carrying on on its own. I consider there are small planets such as the Earth, and a different set that are large and have huge atmospheres, for example Jupiter, Saturn, Uranus, Neptune, and there's another dividing region where the object is questionably big enough to become a star. A believe a great many of these will be discovered, and they will have planets going around them.

I like the idea of "The object must be massive enough to be a sphere by its own gravitational force. More specifically, its own gravity should pull it into a shape of hydrostatic equilibrium".

With this, however, there needs to be a proviso to cope with what happens when large objects made of liquid are found.

I think it's a shame that the International Astronomical Union have made a poor decision about planetary definitions in 2006, and I look forward to them updating their definition before too long.

It's not the only poor decision by the International Astronomical Union in my opinion. The companies that do "Star Naming" are a problem because their databases overlap and some of them are temporary. The International Astronomical Union could sweep all of that charlatanry away at a stroke, or at least for only the same budget as some of the world's Domain Name registrars. The International Astronomical Union has the world's best database of stars. They could allow people to name stars for a fee. The stars allowed to be named in that way would be the ones which are too small to see without a telescope. Funnily enough, there are quite a lot of these! The International Astronomical Union could have a database record of all the names which would allow each star to be named uniquely. People who want to use the new star names could, and those who don't want to could still use the rather prosaic numbers which the International Astronomical Union also uses. However, what would be really good about an official star naming registry is that the fees would go to the further progress of Astronomy and Space Research!. Think how much space exploration could be furthered by that!

Meanwhile, Pluto. I hope that when the New Horizons spacecraft gets there, the news is so spectacular it is Big News!

Also see some earlier fuss about the dwarf planet Vulcan