Alan Stern has been fighting for Pluto's planethood ever since the icy body was demoted to "dwarf planet" in 2006.
That year, the International Astronomical Union (IAU) came up with a new definition of "planet": A body that circles the sun without being another object's moon, is large enough to be rounded by its own gravity (but not so big that it undergoes nuclear fusion, like a star) and has "cleared its neighborhood" of most other orbiting bodies.
Since Pluto shares orbital space with many other objects in the Kuiper Belt the ring of icy bodies beyond Neptune it was relegated to the newly created category of dwarf planet.
The rethink was partly a response to the discovery of Eris, by Caltech astronomer Mike Brown. Eris, a rocky world circling the sun far beyond Pluto, was initially thought to be larger than its Kuiper Belt cousin. But new observations of Eris have cast doubt on its size supremacy.
Stern, a planetary scientist at the Southwest Research Institute in Boulder, Colo., and leader of NASA's New Horizons mission to Pluto, strongly disagrees with Pluto's demotion.
In an e-mail interview with SPACE.com, Stern lays out the case for Pluto's planethood and explains why it matters what we call Pluto and other objects in the solar system.
What are your major concerns with the current IAU definition of "planet?"
One problem I have with the IAU definition is that it's sloppy when it says a sufficiently large object has to "clear its orbital zone" to qualify, because none of our solar system's planets orbit in a fully cleared zone. All have some form of asteroids, comets or Kuiper Belt objects passing through their orbital zone.
So the IAU definition leaves its proponents having to argue why they didn't really mean what they wrote, that instead they really meant that zone-clearing is the "ability" to clear a zone in a perfect-world computer simulation. [POLL: Should Pluto's planet status be revisited?]
But a larger problem is that so many of its proponents said back in 2006 that the IAU definition was specifically designed to limit the number of planets to a small number. That, in my opinion, is not scientific. It's somewhere between personal preference and political.
And most importantly, I and many other planetary scientists like the almost 400 that signed a petition against the IAU in 2006 have a problem with the IAU definition because the implications of it are just nonsensical.
Here's why. The IAU's "zone-clearing" criteria, when worked out mathematically, means that to qualify as a planet at larger and larger distances from the sun, a body has to have more and more mass than it would in a closer orbit. This is in part because the zones get larger (like distance cubed, or volume) as you go outward; it's also in part because orbital speeds are slower further out, so zone-clearing takes longer.
The end result is that many objects that can clear Mercury's zone can't clear the zone at Earth's orbit (including Mercury, as I recall), and an object that can clear the zone at Earth might or might not be able to clear the zone at Pluto. In fact, Earth the one object I think everyone agrees is a planet is too small to clear Pluto's zone in the age of the solar system, and would not be a planet by the IAU's way of thinking.
So when people say Pluto can't clear its orbital zone, they should be fair and also point out that planet Earth also couldn't clear a zone this far out, so the IAU definition would exclude an Earth and a Mercury, a Venus or a Mars at Pluto's distance.
This distance-dependent planethood criterion produces situations where identical objects don't classify identically at different locations, and results in a ridiculous and chaotic classification scheme that isn't good for anyone not for school kids, not for the public or even for researchers.
You've said that the decision to strip Pluto of its planethood was motivated partly by an unscientific desire to keep planets "special." What did you mean by that?
Last week, you interviewed Mike Brown about planet definitions. Mike once said to me during an NPR interview we did together in 2006, "We can't have 50 planets. My little daughter won't be able to remember all their names."
Well, I have two daughters of my own, and really appreciate a father's love, but this is hardly a scientific rationale. Did we limit the number of stars for memorization convenience when Galileo turned his telescope to the sky and found there weren't any more a countable number of them? Did we limit the number of galaxies when a similar discovery was made about them? Do Earth scientists limit the numbers of mountains or rivers to be named? Do biologists limit the number of species?
In all these cases the answer is no, because science is about accepting new facts, independent of their convenience or inconvenience. And that should be the case with planets, too.
So I responded to Mike on NPR that evening by saying, "Well, if we can't have 50 planets but only eight, so kids can remember their names, then I guess we ought to be going back to eight states, too."
But to answer your question more fully, many proponents of the IAU's planet definition want to limit the number of planets and say so all the time. When I hear that, I think, why would anyone care to keep the number of planets small, except to protect their status as special?
But you know, data is data. In planetary science, we've learned over the past 15 or so years we were wrong our solar system does not have nine or 10 planets as we long thought it had, but more like 900. Moreover, we've learned that most of the planets in our solar system are very different from the first handful we knew about because they are much smaller, though they share virtually all other attributes in common.
It's as if we'd grown up on a desert island with only NBA-player-sized people and then, on learning that the other people of the world are mostly smaller, saying those people aren't really what we want to call human, because that would make us on the home island feel different and less special.
So when it comes to small planets outnumbering big ones (just as small stars outnumber big ones), I say, "Cool, that was unexpected, but bring it on!" After all, reacting to new data and changing one's paradigm to better fit the totality of the facts is what science is all about.
What definition of "planet" do you prefer, and why?
I like to tell audiences that we'd be better off with the "Star Trek" test for planethood than what the IAU adopted.
Whenever a starship on "Star Trek" pulls up to something in space and turns on the viewfinder, the audience and the ship's crew know, within about a second, whether it's a planet, or a star, or another spaceship, or a comet, or a nebula or an asteroid. Whatever. They don't need to know what else is nearby, they don't need to conduct a survey of the solar system and integrate orbits to determine what objects have cleared their zone, they don't even need a Ph.D astronomer to advise them.
They just know by looking. They know, because it's not all that hard to tell a planet when you see one, and they know that it doesn't matter if it's alone or in a flock: if it's big and roundish, "not on fire with fusion," and not a spaceship, it's a planet.
But more rigorously, since a 1991 article where I coined the term "dwarf planet" in analogy to dwarf stars and dwarf galaxies, I've promoted what we now call the Geophysical Planet Definition, or GPD: A planet is any object in space that is massive enough for its self-gravity to create a state of hydrostatic equilibrium, but not so massive that its core can at some point ignite in sustained nuclear fusion.
This simple definition nicely traps planets as objects that are too big to act like rocks (which control their shape by chemical bonds rather than gravity), and [too small to act like] stars (which are big enough to be in hydrostatic equilibrium but which have done or currently do nuclear fusion in their interiors).
And in GPD, nothing else matters doesn't matter where we find it, in what kind of orbit or what it's close to or far from. In fact, that's how we classify most other object types in astronomy based on what it is, not where it is or what orbit it's in.
What do you say to those who argue that Pluto is just a big Kuiper Belt object?
I say they're right! But that doesn't really relate to whether Pluto is a planet or not. Just because Pluto orbits with many other dwarf planets doesn't change what it is, just as whether an object is a mountain or not doesn't depend on whether it's in a group or in isolation.
What we see in the Kuiper Belt is a third class of planets, the dwarf planets, or DPs. Most, like Pluto and Eris, have primarily rocky compositions (like Earth), moons, and polar caps, atmospheres, seasons and other attributes like the larger planets. They're just somewhat smaller.
Back before the Kuiper Belt was discovered, Pluto did look like a misfit that didn't belong with either the terrestrials or the giant planets. Turns out that was exactly right, but now we know why: Pluto looked like a misfit because our technology back then couldn't see that it was just the brightest and easiest to detect of a large new class of planets.
In fact, that's why it's clear Ceres [the largest object in the asteroid belt] was a planet all along, but was misclassified for a time because we didn't have enough similar examples to recognize dwarf planets as their own category. Today, however, it's clear the DPs outnumber both of the other two planet classes we know of in our solar system the giants and the terrestrials. Which types look to be the misfit now, versus the norm?
Some people seem to be uncomfortable with that fact. I see it as just another step in the Copernican revolution that began by displacing the Earth from the center of the universe.
Do you think astronomers will ever come to a consensus?
I do. I expect that before long, consensus will be that our solar system was good at making planets in very large numbers, and that most of them are faraway dwarfs, rather than the closer big guys that we knew about in childhood.
Then schools will teach that, like the rivers of Earth, there is a huge number of planets in our solar system, and you only need to remember the names of the ones that are nearby or particularly famous.
Does it matter whether we call dwarf planets full-fledged planets or not? Is this just a semantic issue, or does it hit on something more important?
I think it matters. One thing scientists do is to find order among a large number of facts, and one way to do that across fields as diverse as biology, geology, physics and astronomy, is through classification.
Yes, in planetary science we have found our old classification scheme wanting because we've recently discovered a large number of small planets far out in the solar system, as well as hot Jupiters and super-Earths around other stars, and planets with strange new orbits, and planets orbiting pulsars.
I don't think our job as planetary scientists and astronomers is to react to this new data that planets are much more diverse in size, orbit, and location than we knew by saying there's no longer any reason to classify them, and that the word planet has lost all meaning.
I think our job is find a better classification scheme that makes sense in light of the new data. As I said about the "Star Trek" test, it really isn't so hard to do.
And after all, the field is called "planetary science." To say that what a planet is doesn't matter would be to imply that a planetary scientist couldn't explain to someone what the field is about. How would that be a good thing?
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You can follow SPACE.com senior writer Mike Wall on Twitter: @michaeldwall.