Illustration of relative sizes, colours and albedos of the large trans-Neptunian objects. 2015 RR245 is approximately the same size as Varuna. If Pluto is a planet, then so are these objects, along with around 100 others.
The IAU’s definition had just three statements:
- A planet had to be massive enough that its gravity would pull itself into hydrostatic equilibrium: round if it wasn’t rotating, but a spheroidal shape if it was.
- A planet had to orbit the Sun and no other object: Earth could be a planet but not the Moon.
- And finally, it had to clear its orbit, meaning there could be no other comparably large masses at the same orbital distance: Mars is a planet, but asteroids and Kuiper belt objects were all out.
Officially, under this definition, our Solar System was down to eight planets.
The eight planets — and a little bit more — of our Solar System, under the current definition. Dropping the ‘clear the orbit’ requirement would let in huge numbers of objects from the Kuiper belt and Oort cloud, along with potentially many moons and some asteroids, too.
For astronomers who study our own Solar System, this is fine. For astronomical purposes, these eight planets have distinct properties that all the other worlds don’t. They are united by a similar, common formation history; they influence the motion of the Sun and the other worlds more severely than any other worlds; they reflect more sunlight than other worlds; and they dominate, gravitationally, their portion of the Solar System. Eight planets, from many perspectives, is just the right number.
But this definition was dissatisfying to three very different groups: exoplanet astronomers, galactic astronomers, and planetary scientists.
The exoplanet astronomers have a very compelling argument. Why would a body orbiting our star, the Sun, be classified as a planet, but the worlds around any other star couldn’t be? It seemed like a tremendous oversight in 2006, as we had over a decade of discovered exoplanets behind us at that point. The rationale, after-the-fact, that was given by some was that there was no way to tell whether a planet had cleared its orbit or not from so far away. This may have been true in 2006, but nine years later, UCLA professor Jean-Luc Margot changed the game by devising a universal planetary test that didn’t require a spaceship! If you could learn the following three easy-to-measure parameters:
- the mass of the planet,
- its orbital distance/period around its parent star, and
- the lifetime of the planetary system in question,
you could determine, with better than 99% accuracy whether a body satisfies the three IAU criteria for being a planet.
Margot (2015), via http://arxiv.org/abs/1507.06300
If you judge whether an object is a planet or not by the IAU’s criteria, that satisfies planets in our solar system, but no others. However, by looking at a distant world’s mass, orbital parameters, and the age of the solar system, you can reproduce the IAU’s definition for 99+% of the worlds we know of.
Quite clearly, the eight planets of our Solar System are in, and the asteroids and Kuiper belt objects are out. Interestingly, if there were only the Moon rather than the Earth-Moon system orbiting the Sun, it would be right on the border between planet and non-planet. But then, for the galactic astronomers, what do you do for rogue planets? If you don’t have a parent star to orbit at all — either because you were born without one or you were ejected from your solar system — does that make you any less of a planet?
Christine Pulliam / David Aguilar / CfA
Rogue planets may have a variety of exotic origins, such as arising from shredded stars or other material, or from ejected planets from solar systems, but the majority should arise from star-forming nebula, as simply gravitational clumps that never made it to star-sized objects. There is no name for these objects that doesn’t have ‘planet’ in their title.
This is a tough question, because in astronomy — as in real estate — location is almost everything. Mercury, at the distance of Jupiter, would never clear its orbit and wouldn’t obtain planetary status. A world much smaller than Mercury could be a planet around a red dwarf star, while even Earth would fail to be a planet if it were out in the Oort cloud somewhere. And yet, we call so many of these non-planetary worlds “planets,” but with a prefix. We have dwarf planets, rogue planets, and exoplanets. In the early stages, we have protoplanets and planetesimals. But, at some level, aren’t all of them, plus more, still planets?
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
Under a size cutoff of 10,000 kilometers, there are two planets, 18 or 19 moons, 1 or 2 asteroids, and 87 trans-Neptunian objects, most of which do not yet have names. All are shown to scale, keeping in mind that for most of the trans-Neptunian objects, their sizes are only approximately known. Pluto, to the best of our knowledge, would be the 10th largest of these worlds.
This argument gets taken to an extreme by some planetary scientists, as elucidated by Stern and Grinspoon in yesterday’s Washington Post. They contend that location should be ignored, and that if you’re massive enough to pull yourself into hydrostatic equilibrium (a sphere if you aren’t spinning; a spheroid if you are), congratulations: you’re a planet. As you can see, above, this would mean that if we looked at the objects under 10,000 kilometers in diameter in our Solar System, there wouldn’t be two (Mars and Mercury) planets, but 109 of them. The Solar System’s total would rise to 115 planets, with 19 moons, one asteroid, and 87 objects beyond Neptune added to the current total of 8. And in the future, as we discover more objects in the Kuiper belt, the scattered disk, and even the Oort cloud, that number will rise further: perhaps into the thousands.
Alex Parker and the OSSOS team
The orbit of 2015 RR245, compared with the gas giants and the other known Kuiper Belt Objects. Note the insignificance of Pluto as compared with the other objects in the Kuiper belt.
On the one hand, there are advantages to the arguments they make. When we talk about the geological, atmospheric, and geophysical properties of a world, we do, in fact, call them all planets. We call the field of studying these worlds planetary science, and when we talk about a world’s oxygen content, crust, composition, or potential for habitability, we have no qualms calling these measurable quantities “planetary properties.” On Pluto alone, we’ve learned that it has clouds, weather, snow, mountains, valleys, geological layers, and likely even a subsurface ocean. It has five moons; it rotates on its axis; it has days and nights and seasons. If you ignored its astronomical location, you’d call it a planet every time.
Pluto and Charon, in enhanced color, thanks to observations from New Horizons’ Ralph/Multispectral Visual Imaging Camera (MVIC). Pluto’s frozen surface is only part of the story; an ocean of subsurface water lurks far beneath the ice.
And yet, that’s the other hand: you have to ignore all of astronomy to consider Pluto a planet. That’s the question you need to ask yourself: should an object’s position in its Solar System determine what a planet is? Or should only the intrinsic properties of the world matter? Should we ignore, completely, the connection between planets and solar systems, stars, their formation, and the gravitational dance that has always fueled our knowledge and curiosity about them?
I resent the assumption that we will become less interested in Pluto, or in asteroids, comets, centaurs, moons, and the outer reaches of the Solar System, simply if we don’t name them “planets” also. There are just as many ambiguities under the geophysical definition as there are for the astronomical one: if something stripped Saturn’s gas away, would its round core still make it a planet? Is the asteroid Vesta, which has a crust, mantle, and metallic core, but isn’t quite round, up for consideration? Would a smaller, icy world (~200 km in radius) be a planet, because it’s round, while a larger rocky world (~250 km in radius) wouldn’t be?
The simple fact is that Pluto was misclassified when it was first discovered; it was never on the same footing as the other eight worlds. The 2006 move by the IAU was an incomplete attempt to repair that mistake. The current move by Kirby Runyon, Alan Stern, and others is a step in the opposite direction: it’s a step towards making a larger, more confusing mistake that will render a definition meaningless to the majority of people who use it.
NASA / JHUAPL / New Horizons / LORRI
Pluto’s atmosphere, as imaged by New Horizons when it flew into the distant world’s eclipse shadow. The atmospheric hazes are clearly visible, and these clouds lead to periodic snow on this outer, cold world.
There are some out there who are desperate to save Pluto’s planetary status, and would be willing to open the floodgates and bestow planethood on every moon, asteroid, and ice ball out there that’s massive enough to be round. There are others who spend 100% of their time looking down at their feet on whatever world they’re considering when it comes to planethood, and to them, everything with enough mass will be a planet. But for the rest of us, where you are in the Universe is an inseparable part of what you are. Nothing in the Universe exists in a vacuum, and where you are determines a huge number of properties of you, regardless of whether you’re a planet, moon, asteroid, centaur, comet, Kuiper belt object, or Oort cloud object. If you want to ignore all of that — and proclaim, “round means planet” — then more power to you. But in planethood, as in most things, the full scientific story is far more interesting.