To quote another great space adventurer: “Almost there!”
The New Horizons probe, launched in 2006, will finally reach Pluto next Tuesday, providing the first close-up view of the tiny, icy world since we discovered it in 1930. We’re already seeing features never glimpsed before. It’s a truly historic occasion, right up there with the Dawn mission to the giant asteroid Ceres, known since 1801 but never seen clearly before this year.
But what is Pluto?
Here are the facts: Pluto is a small world of rock and ice, a little less dense than Earth or the Moon. It’s about two-thirds the diameter of the Moon, and about 18 percent of the Moon’s mass, or 0.2 percent of Earth’s mass. Pluto has four really tiny moons—Nix, Hydra, Kerberos, and Styx—and it is locked in mutual orbit with Charon, a much larger moon roughly 10 percent of Pluto’s mass.
I suspect everyone is impatient with me now, because I’ve carefully avoided calling Pluto a planet, a dwarf planet, or anything else. According to the International Astronomical Union (IAU), a body of professional astronomers that meets every three years, Pluto is not a planet.
But I don’t care if it’s called a “planet” or not.
The real point is that Pluto is really awesome, whatever we call it: it’s a different sort of world than any we’ve yet visited. It resembles some of the outer Solar System moons of Neptune and Saturn, but has big differences too. Pluto’s uniqueness is independent of what we decide to call a “planet.” (I do admit the “Pluto is obviously a planet!” crowd sometimes makes me want to say it ain’t just out of spite.)
The controversial decision to change Pluto’s status was made by the IAU in 2006, based on a definition of “planet” that frankly sucks. The definition (simplified!) says a planet must:
1. Orbit the Sun directly2. Be big enough to be shaped by the balance of gravity and internal forces to be spherical (or roughly so)3. Be the gravitationally dominant object in its orbit, clearing out other smaller bodies
The first and third points are the ones that bug a lot of scientists in the pro-Planet Pluto camp. Exoplanets orbit other stars, so by this definition they aren’t “planets,” and “clearing its orbit” is a criterion that supposedly indisputable planets like Earth or Neptune sometimes pass and sometimes fail.
The problem ultimately with the IAU’s definition (in this grouch’s opinion) is that it’s designed to exclude. The first criterion is intended to exclude the planet-like moons: Titan, Ganymede, the Moon, Charon, and so forth. The third is intended specifically to exclude Pluto and other largish icy worlds at the edge of the Solar System, which have orbits regularly crossing other orbits. But both of these end up feeling more artificial than intuitive: If you have to do a sophisticated calculation to determine if something is actually a planet, then it’s not much value for classification.
But it’s also equally clear that the old “nine planets” view of the Solar System is dead. Eris (discovered in 2005) is almost exactly the same size as Pluto, but slightly more massive. Makemake, Quaoar, Haumea, Orcus, and several more objects are smaller but fall into the same general category as Pluto: worlds of ice and rock orbiting beyond Neptune. Any definition of “planet” including Pluto but excluding these other worlds would be at least as crappy as the IAU’s crap definition.
It made sense to include Pluto back in 1930—astronomers initially thought it might be nearly as big as Earth, because we didn’t have a good gauge of how bright it was. Over the decades, various measurements refined both its size and mass downward, to the point where a joke paper in 1980 predicted Pluto would vanish completely by 1984, then reappear. (Yeah, scientists have odd senses of humor.)
Measuring a small world’s size from a distance is challenging, though: We have to wait for it to pass in front of a star and time how long it takes for the star’s light to reappear. The discovery of the big moon Charon in 1978 gave scientists the first good estimate of Pluto’s mass, but we’re still refining size measurements.
Over time, it became clear Pluto is different than the eight planets. It is neither gaseous like Jupiter nor quite as rocky as Earth, and it is much smaller than Mercury. It orbits on a different plane than the big planets, and has an elongated orbit (though not as stretched out as a comet’s). The discovery of Eris, Quaoar, and the like was actually good in that sense: Pluto ceased being an oddball and became the first of a new group of objects. Whether you want to call them planets or not, Pluto and its kin are an important population out beyond Neptune. Pluto is the first among equals; each of these worlds has unique things that make them interesting. If one is a planet, they all are planets.
And whether Pluto is a planet or not, it’s very interesting. Astronomers are already examining the New Horizons images to figure out why Pluto’s surface is so dark near the equator, and what the various splotches could be. Pluto has a thin atmosphere of nitrogen, methane, and other gases that might sometimes freeze entirely onto the surface, so that might be part of the answer.
Next Tuesday, we’ll get the data to help us know for sure. Like many of us, I’ll be watching NASA TV around 9 p.m. U.S. Eastern time to see the first reports from Pluto. (If you’re near Cleveland, you can even watch with me and other local scientists! I’m the guy in the bowler hat.)
Every new world we study teaches us something new about the Solar System, and Pluto will be no different—whatever name we give it.