Why Isn’t Pluto A Planet? A Matter Of Preference. | Vol. 4 / No. 18.1

Montage by Emily Lakdawalla. Data from NASA / JPL, JHUAPL/SwRI, SSI, and UCLA / MPS / DLR / IDA, processed by Gordan Ugarkovic, Ted Stryk, Bjorn Jonsson, Roman Tkachenko, and Emily Lakdawalla, CC BY-NC-SA

In which your options are either eight (possibly 9) planets or over 110 and counting.


Ever since Pluto was demoted from “planet” to “dwarf planet” people have been up in arms about it. For few people is this more true than Alan Stern, the PI of the New Horizons mission to Pluto (and parts beyond), having famously referred to the redefinition as “bullsh**”. And he’s still at it.

The current definition of a planet — the one espoused by the International Astronomical Union (IAU) — is this:

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.

This, unfortunately enough, disqualifies Pluto on the grounds that it hasn’t satisfied the third condition. And that’s on purpose.

See, there are a lot of round things orbiting the sun, and that made people uncomfortable. Why? Well, because the answer to the question “how many planets are there in the solar system” wasn’t “nine,” it was “thirteen for now, probably closer to two hundred, and maybe ten thousand or so if you count things beyond the Kuiper Belt.”

Think of it like planning a wedding. You want a small one, but you really want to invite Aunt Margaret. But if you invite her, you suddenly have to invite the other thirteen aunts and uncles and their four kids each because who cares about the environment anyway and suddenly you’ve got a very large wedding indeed. The IAU couldn’t come up with a way to invite Pluto to the party without inviting a hundred other Pluto-like objects, too.

But now, Alan Stern is suggesting that we just give up and have the very large wedding.

In a paper published earlier this month in the journal Planetary and Lunar Science, a team including Stern has proposed a new definition, one that’s basically just a more specific version of the IAU’s stipulation b:

A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters.

This, among other things, makes the moon a planet, some of Jupiter’s moons planets, Pluto and Charon planets — in fact it makes the current planetary count something like 110, and sure to rise in the coming years.

Frankly, I think orbital mechanics are an important part of the definition of a planet — but if you don’t ignore them, then you by necessity ignore size (giving planetary status to things a fraction of the size of the moon or of Europa, for instance).

It doesn’t bother me either way, and it doesn’t make Pluto special again — because it’s just not. Here’s some facts about our solar system, regardless of the specifics of how you define a planet:

  • There are four major terrestrial bodies orbiting the Sun classically referred to as the “inner planets”: Mercury, Venus, Earth, and Mars, one of which, Earth, has a spherical body orbiting it, the moon.
  • There are four “gas giants” orbiting beyond that — Jupiter, Saturn, Uranus, and Neptune — the latter two of which are referred to as “ice giants.” All of these have spherical bodies orbiting them.
  • There is a spherical terrestrial body definable as both a very large asteroid and a protoplanet in the asteroid belt — Ceres.
  • There are many roughly spherical icy bodies beyond the orbit of Neptune, including Pluto, Charon, Makemake, Haumea, Eris, Quaoar, Sedna, Orcus, Salacia (and so on).

That’s the thing: bullsh** or not, the thing that really bothers people is the fact that Pluto isn’t special. And whether there’s eight planets or hundreds, that bit isn’t going to change. Big wedding or small, Pluto is officially in a class of objects numbering in the hundreds. Pluto as “planet nine” is dead.

You can read the paper yourself at Planetary and Lunar Science.


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Richard Ford Burley is a human, writer, and doctoral candidate at Boston College, as well as Deputy Managing Editor at Ledger, the first academic journal devoted to Bitcoin and other cryptocurrencies. In his spare time he writes about science, skepticism, feminism, and futurism here at This Week In Tomorrow.