Nine years after the International Astronomical Union famously demoted Pluto from planethood, debate still rages among astronomers and the pubic alike as NASA's New Horizons probe gives humanity its first close-up view of the icy world.
For astronomer Mike Brown, the self-proclaimed "man who killed Pluto" and the discoverer of rival Eris and several other large trans-Neptunian worlds, Pluto is not a planet now and never was. And neither are any of its currently known Kuiper belt relatives, including the ones he discovered.
"The debate is basically over amongst astronomers and even amongst the public," he said in an interview. "If you talk to kids, for example, they all now learn in school that Pluto's not a planet, and they don't feel any remorse for it. It's harder for adults than it is for kids."
And, he added, "there are a small number of very vocal people who will never let it go. In particular, people who study Pluto, I think, feel the worst about it no longer being a planet. I understand how they feel that way, but this is not an emotional decision, it's really looking at the solar system as it really is."
But to Alan Stern, the principal investigator of the New Horizons mission, Pluto may be a new type of planet -- a dwarf planet -- but it's still a planet by any common sense definition of the word. He says the IAU's definition -- that a planet orbits the sun, that it's generally spherical and that its gravity has cleared out the path around it -- fails what he calls "the Star Trek test."
"When Kirk and Spock show up orbiting an object, just by looking at the picture of it, they know it's a planet," he told astronomer Neil deGrasse Tyson in an episode of NOVA on U.S. public television. "In an IAU world, Spock would have to come back and say, 'Captain, let me survey the entire solar system, determine whatever objects are there. I'll integrate the orbits overnight. I'll get back to you.' It's not that hard."
As for the IAU's requirement that a planet must clear its path over astronomically long periods -- a test Pluto fails -- Stern said "that's all about location, and location is for realtors not scientists. I don't think it counts at all in terms of what it means to be a planet."
For Stern, Pluto, Eris and similar bodies actually represent the dominant class of planets in the solar system. But they're all planets of one type or another.
"Most planetary scientists call Pluto a planet and don't (care) what the astronomers say," Stern said in an interview with CBS News. "We are the experts in this field, not the astronomers. We wouldn't pretend to classify galaxies to them because we're not expert in it. I tell public audiences, don't go to a podiatrist for brain surgery, don't go to an astronomer for planetary science.
"If you go to a planetary science meeting, any type of technical meeting, go sit in the audience, don't say who you are, just sit there and you'll hear people calling Pluto a planet and Eris a planet and the other small planets in the Kuiper belt planets, and Ceres, and for that matter, the really big satellites like Europa and Ganymede and our moon get called planets all the time. Because that's what they are."
Not so fast, says Brown.
He, too, takes issue with the IAU definition. And he would agree that any definition that makes Pluto a planet would have to also include Eris and other large Kuiper belt objects, if not any body with a radius of more than about 200 kilometers (124 miles) or so -- roughly the point at which gravity creates a generally spherical body.
But to Brown, the eight IAU-sanctioned planets -- Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune -- are clearly in a class by themselves, a class Pluto, Eris and similar bodies just as clearly do not belong to.
"We all know what galaxies are, we all know what stars are," he said. "In the solar system, it is really obvious that these eight things are distinct from all the other small bodies floating around, and so conceptually it's really clear."
The argument is analogous to one put forth by the late U.S. Supreme Court Justice Potter Stewart, who once wrote in a decision that he could not define pornography "but I know it when I see it." To Brown, the big eight are clearly planets. Or at least something Pluto is not.
Brown believes it was a mistake for the IAU to even attempt coming up with a definition in the first place.
"If you write down a lawyerly definition like the IAU did, then you get lawyers starting to argue about your definition and saying silly things like, oh, you know, there are near Earth asteroids so the Earth hasn't cleared it's orbit so I guess the Earth is not a planet," he said.
"No, no. No. The goal, I hope, is that we get away from nitpicking the definition, because that's actually totally meaningless." More important, he said, is "understanding the concept. That's where I wish the IAU had got, here's the concept, here's why. These aren't planets."
Astronomers faced a similar dilemma at the turn of the 19th Century when what became known as astroids were first discovered. Searching for a planet between Mars and Jupiter, predicted by a mathematical relationship known as Bode's Law, Giuseppe Piazzi discovered the asteroid Ceres on New Year's Day, 1801.
Over the next eight years, three more new bodies -- Pallas, Juno and Vesta -- were discovered in the zone between Mars and Jupiter. Many more eventually were found and Sir William Herschel eventually proposed the term "asteroid," or star-like, to describe them. The name stuck.
In the middle of the 19th century, Neptune was discovered, boosting the accepted number of planets to eight. Detailed analyses of the orbits of Uranus and Neptune led some astronomers to believe there could be a ninth planet even farther out.
The American astronomer Percival Lowell began a search for what he called "Planet X," but by the time of his death in 1916 no such world had been found. Finally, in 1930, Clyde Tombaugh, assigned the task of continuing the search, struck pay dirt, spotting a tiny, moving point of light an average of 3.7 billion miles (5.9 billion kilometers) from the sun: Pluto.
Lowell had calculated Planet X should have a mass of seven to eight times that of Earth to explain the perturbations believed to be seen in the orbit of Uranus. As it turned out, the perturbations were not real, and Pluto turned out to be much, much smaller than expected.
Based on the latest observations, Pluto has a radius of about 715 miles, making it much smaller than Earth's moon. It also features a large moon of its own, Charon, and at least four smaller satellites.
Despite its small size, Pluto retained its ranking as the ninth planet for three quarters of a century, until astronomers using ever more powerful telescopes realized Pluto was simply the largest and brightest body in what became known as the Kuiper Belt, a vast realm of icy debris discovered in the early 1990s beyond the orbit of Neptune.
The Kuiper Belt begins at roughly 30 astronomical units from the sun -- 2.8 billion miles and extends to about 50 AU, or 4.6 billion miles.
The problem for Pluto was the discovery of other large bodies in the Kuiper Belt that rivaled or exceeded Pluto in size or mass. Brown led the discovery of several such bodies, including Haumea, with one third Pluto's mass, and Makemake, with a diameter two thirds that of Pluto. A later discovery, subsequently named Eris, is almost exactly Pluto's size, and more massive to boot.
The discoveries triggered a wide-ranging debate over the nature of planethood and eventually led to the IAU's famous, or infamous, 2006 conclusion that 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 neighborhood around its orbit."
The definition triggered a broader debate that continues to this day, with many focusing on the third clause, the requirement that a planet "clears" debris out of its orbit. By that standard, one could argue, Earth isn't a planet. Or Jupiter. And what about the bodies referred to as "planets" orbiting other stars?
"I don't particularly like the IAU's definition because of the clause about clearing out its zone," Hal Weaver, the New Horizons project scientist, said in an interview. "That was a little too complicated. If they took that one away, then Pluto would qualify. And then ... it seems silly to call something a 'dwarf planet' and then say it's not a planet."
Weaver helped organize a 2009 workshop at the Advanced Physics Laboratory at Johns Hopkins University where the New Horizons probe was built "to try and gather a bunch of planetary scientists together to see if we could come to some consensus" as to a better definition.
"I worked really hard to try to build a consensus, but I got disgusted at the end because some people on both extremes ... were refusing to budge their personal perspective on things," he said. "So I just gave up."
Asked his personal opinion, Weaver said he preferred "the geophysical definition that once an object gets to be a certain mass so internal gravitational attraction causes it to be almost a perfect sphere, that distinguishes it pretty clearly from the smaller objects, the asteroids and the comets."
By that standard, Eris and the rest would also have to be classified as planets.
"Yeah, yeah, definitely," Weaver said.
The latest tally, according to Brown, is 86 objects that are probably dwarf planets and nearly 370 that are possibly dwarf planets. So far.
In his book "How I Killed Pluto and Why It Had It Coming," Brown writes that the only classification most people will ever apply to the solar system is the concept of the planet. "The definition of the word 'planet,' then, had better carry with it the most profound description of the solar system possible for a single word."
"If you think of the solar system as a place consisting of eight planets ... and then a swarm of asteroids and a swarm of Kuiper belt objects, you have a profound description of the local universe around us," he writes. "Understanding how such a solar system came to be is one of the major tasks of a wide range of modern astronomers."
If, on the other hand, your definition simply requires a spherical shape and you "think of the solar system as a place with large things that are round and smaller things that are not quite round, you have a relatively trivial description of the universe around us. There is nothing important to study here: We've known for hundreds of years that gravity pulls things in space into the shape of a sphere."
But for now, the IAU definition stands. The solar system is officially made up of the sun, eight planets, an unknown number of dwarf planets, comets, gas and dust. Lamenting -- or celebrating? -- Pluto's expected demotion in the late 1990s, American folk singer Christine Lavin captured the debate in a song titled "Planet X." It ends like this:
St. Christopher is looking down on all this
and he says, 'Pluto, I can relate.
When I was demoted from sainthood
I gotta tell you little buddy,
it didn't feel real great.'
And Scorpios look up in dismay
because Pluto rules their sign.
Is now reading their daily Horoscope
just a futile waste of time?
At the turn of (the 20th) century
astromathematician Percival Lowell
in his quest for "Planet X"
started this ball to roll.
At the end of the 20th Century
we think he may have been way off base.
Now we look at the sky
and wonder what new surprises
await us in outer space.
Stern would agree about the wonder and new surprises. But not about being off base.
"We've really gone through a revolution, a paradigm shift in planetary science," he said before New Horizons was launched. "We really just didn't realize the diversity of planetary types in our solar system. Pluto looked like a misfit because it was the only one we saw. And just as a Chihuahua is still a dog, these ice dwarfs are still planetary bodies. They're large enough to make themselves round by self gravity and they surely pass the test of planethood."
But that depends on who's doing the grading.
An earlier version of this story ran in Astronomy Now magazine.