So, news stories are flying around claiming that Pluto is a planet again, and whole crowds of people are feeling vindicated that what they knew to be true is true again. Hooray, high fives all around! Those scientists finally came to their senses!
Or did they?
What actually happened was a debate at the Harvard-Smithsonian Center for Astrophysics (or Harvard-CfA) where three astronomers gave their views on what constituted a planet, and the audience voted. One, Dr. Owen Gingrich of the Minor Planet Center, argued that the word “planet” is, as a part of language, defined by the consensus of people, and that consensus is that there are nine planets, of which Pluto is one. Dr. Gareth Williams defended the IAU definition, for which Pluto does not qualify, and Dr. Dimitar Sasselov argued that a planet was a spherical lump of matter orbiting a star, which would certainly mean Pluto is a planet. However, regardless of the vote, an audience of people who are mostly there because they believe Pluto ought to be a planet do not have the authority to overturn the IAU decision. So this really resolves nothing.
The fact is, regardless of the actual arguments, Pluto is a pretty terrible planet in a lot of ways:
- It’s the only one whose orbit is entirely gravitationally controlled by another (gravitational tugs from Neptune, much like pushing a swing at just the right tempo, keep Pluto orbiting twice every time Neptune goes around the Sun three times.
- It’s got the least circular and most inclined orbit of any of the supposed planets. It comes closer to the Sun than Neptune for 20 out of every 240 years (most recently, 1979-1999). That makes it resemble an asteroid or a comet more than a planet.
- It’s TINY. Pluto is about 1/5 the mass of the Moon. In fact, Pluto is the 17th largest object in the Solar System, after a star, eight planets, seven moons, and Eris, the scattered disk object discovered in 2005 that finally forced the IAU to do something about this. There are probably more objects like Eris out there that are bigger than it is.
- It’s surrounded by objects very much like it. In the inner solar system, we call those objects ‘asteroids’. In the outer solar system, we now call those objects “Kuiper Belt objects” or “Scattered Disk objects”.
Calling Pluto a planet is basically a mistake. It was NEVER a planet. “Planet” just seemed like the most acceptable term to use at the time it was discovered. It wasn’t a comet, because it never comes close enough to produce a cometary tail*; it wasn’t an asteroid because it wasn’t in the asteroid belt and seemed to be much bigger, so… planet it is! But it seemed tiny, and the orbit was wrong…
In 1977, an asteroid/comet hybrid object named Chiron was discovered in an orbit outside the asteroid belt. In 1978 it was discovered that Pluto was actually two objects: Pluto, and a moon 1/3 the size of Pluto that was named Charon. They are so similar in size that they orbit around a point in space between them. In 1991, the second (or third?) object beyond Neptune was discovered: 1991 QB1. And then the floodgates opened… Tons of objects almost the same size as Pluto (some marginally famous: Sedna, Quaoar, Haumea, Makemake…) in the same kinds of orbits as Pluto, for which we developed names: “Trans-Neptunian Object”, “Kuiper Belt object” and “Scattered Disk Object”… until finally, in 2005, Eris was discovered. Pluto wasn’t alone, it wasn’t unique, and it wasn’t even the biggest thing out past Neptune.
The IAU, who are in charge of naming pretty much everything in astronomy, has proposed defining a planet many times. It came close to booting Pluto after the 1978 discovery of Charon, but continually decided not to, until the discovery of Eris forced them to say something. Were there 10 planets now? More? Where do you draw the line? What the IAU came up with has irritated a lot of people, and also a lot of scientists, chief among them Dr. Alan Stern, the lead of the New Horizons mission that’s due to fly by Pluto in July 2015. In contrast, a lot of scientists had already decided Pluto wasn’t a planet by the time the IAU decided it, including my PhD adviser, and Dr. William Hartmann, a planetary scientist whose gorgeous 1981 book “The Grand Tour: A Traveler’s Guide to the Solar System” (much beloved by me) ordered the solar system by mass, with a separate section for the 15 largest worlds, and Pluto was merely one of the “Selected Smaller Worlds”. Apparently the 2005 edition of the book now lists the 28 largest worlds, but the authors still opine that Pluto isn’t a planet.
To address the arguments of the symposium… Dr. Gingrich’s idea is right in some ways: common usage really defines what the word “planet” means. But that robs the word “planet” of all meaning. If anyone cared to, they would have to explain why Pluto was a planet, when the larger object Eris wasn’t. It wouldn’t be the first time that public usage of a word is different from the scientific meaning, but science has had enough of a problem with “theory” and “law” that this answer is probably really dangerous.
Dr. Williams argued the IAU definition, that states a planet is “A celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.”
Pluto satisfies A and B, but not C, and is not a planet. Most of the arguments have been presented regarding point C, because there are so many asteroids that cross the Earth’s orbit, but everything that crosses (or shares!) the Earth’s orbit gets knocked around by the Earth, or is held there by the Earth’s gravity. And the Earth has more mass than all the things that cross or come near its orbit, put together, Moon included. It gravitationally dominates (that is, apparently, the intended meaning of clause C, but they didn’t write that, so pthbbbt). Personally, I don’t have a problem changing Pluto from “the runtiest of the planets” to “King of the Kuiper Belt”…
The argument the audience chose, the one made by Sasselov, is that anything large enough for gravity to make it round that orbits a star (presumably: not another planet), counts as a planet. (Of course, someone can complain that it doesn’t explain the case of a planet orbiting TWO stars, but whatever…) Interestingly, the IAU nearly passed this version, which would have given us at LEAST 12 planets, including Eris and the main belt asteroid Ceres, and potentially dozens more over the years. I suppose I could be ok with this, especially because it allows for planets around ANY star, not just the Sun (go look at IAU clause A), and who knows what kinds of wacky exoplanets we’ll find**.
Whatever it is, it smacks of what I call Reverse Astrology: Belief that our lives affect the motions of heavenly bodies. Pluto is exactly what it has been for the past couple billion years: two small roundish lumps of ice with temporary atmospheres, surrounded by at least four other moonlets, locked into a gravitational resonance with Neptune that it shares with thousands of other, mostly smaller objects. And it’s still going to be exciting to find out what New Horizons sees in July, 2015.
* Actually, Pluto is thought to have an atmosphere that thaws when it reaches its closest approach to the Sun, which gradually freezes and snows out after that. It’s like a comet, except big enough to hold on to its tail.
** Three of the planets of GJ 876 may be in gravitational resonance with each other: they have orbits of 30.2, 61, and 125 days, or nearly 1:2:4. Gravitational dominance would probably imply that only the 2 Jupiter Mass one was actually a planet, even though the other two are also quite massive (0.6 Jupiter masses and 0.75 Neptune-masses)