Most of us grew up believing that tiny, distant Pluto was the outermost planet in our solar system. Then, one day, the scientific powers that be decreed that it wasn't. But it seems the matter is far from settled.
"The International Astronomical Union of wants to hide from the issue," David Weintraub of Vanderbilt University told me in an email. "They have refused to allow the issue to appear on the agenda at the 2009 or 2012 or 2015 meetings. They hope it will go away."
Weintraub—who describes Pluto's exile as a stunt organized by a "very small clique of Pluto-haters"—would have the dwarf world rejoin the ranks of our Solar System's fully-fledged planets today. But solid evidence that Pluto deserves the title may come in July, when NASA's New Horizons spacecraft slingshots around the icy rock and sends us back a detailed picture of its composition.
Pluto's planethood was revoked by majority vote on the final day of the 2006 IAU conference. Over 2,500 astronomers attended the meeting throughout the week, but only 394 votes ultimately decided Pluto's fate: 237 in favor of demoting the planet and 157 against.
"About 2,000 astronomers who attended the opening sessions of the meeting didn't get to vote because they were gone," Weintraub told me. "And another 8,000 members were never consulted. And many of the rest of us, who are not IAU members, had no voice at all."
Resolution B5, "Definition of a Planet in the Solar System" declared that, to be considered a planet, a celestial body must have "cleared the neighborhood around its orbit"—that is, become the dominant gravitational force in its orbital zone. A large celestial body that meets other criteria but can't keep its backyard clean is considered a dwarf planet. But as Weintraub points out, no rigorous definition of a planet's "neighborhood" yet exists. What's more, it isn't certain that other bona fide planets, like Earth, would be able to clear Pluto's neighborhood, either.
To Weintraub, the semantic issues with clearing the neighborhood are symptomatic of a larger problem: The current definition of a planet is just plain crude. There are many considerations that make a celestial body more or less planet-like, which Weintraub enumerates in his recent book, the aptly titled "Is Pluto A Planet?" But most of the qualities we think of when we think of planets aren't acknowledged in the formal definition.
"Beyond orbiting a star and being big enough to have a spherical shape and not being so big that the object is a star, we have no easily agreed upon criteria for deciding what is (or isn't) a planet," Weintraub told me. "And that's the problem."
Attributes we often associate with planets include: An atmosphere, one or more moons, a solid surface, a magnetic field, rings, plate tectonics, and volcanic activity. But every planet in our solar system lacks at least one of these features. The question, it seems, is whether having some of these traits should count toward a celestial body's planetary status.
Pluto, at 3.6 billion miles from our sun and only 2,300 kilometers in diameter, is a cold and lonely runt. But it might have enough planetary attributes to put it on par with other small worlds, like Mercury. It has at least five moons, and an atmosphere that expands and contracts during its 248-year orbit around the Sun. Astronomers believe Pluto's surface is rich in ice that may contain methane, nitrogen, and even complex organic molecules.
The New Horizons mission is expected to unlock many of the dwarf world's secrets, including its atmospheric composition, geologic activity, and magnetic properties. Once the new information starts flooding back to Earth, Weintraub believes the astronomical community will be forced to see Pluto as a complex, evolving, honest-to-god planet.
As a prelude to Pluto's summer of reckoning, NASA's Dawn spacecraft is slated to arrive at Ceres, our asteroid belt's largest celestial object, in exactly one week. Ceres was briefly awarded planetary status over 200 years ago when it was first discovered. The 1,000 kilometer dwarf planet may harbor subsurface oceans that contain as much water as Earth, as well as a thin atmosphere. As with Pluto, these attributes might cause astronomers to question whether this little rock, too, is complex enough to deserve a planetary title.
"We only get to visit Ceres and Pluto for the very first time, once," Weintraub wrote. "This year. March 6 and July 15. In your lifetime. In this incredible year of the dwarf planet. Get ready to party. Ceres and Pluto are coming home."