— Never again can Pluto be the ninth planet. Or the littlest planet. Or the most distant planet. But does that make Pluto a non-planet?
Even before Pluto was discovered, the solar system was divided into two classes of planets: the rocky worlds like Earth, and the gas giants beyond. Pluto has pointed the way to the solar system’s third great class of planets, no less important than the other two.
Pluto isn’t the ninth of nine; it’s the first of many. Thanks to the discoveries of the past couple of decades, we’ve gained a whole new tribe of worlds to watch in the Kuiper Belt, and the vast, diffuse Oort Cloud represents an even farther frontier we haven’t yet begun to explore.
These discoveries will shift our view of the universe the way Galileo and Copernicus shifted it four centuries ago. In the 17th century, the world came to understand that Earth was not the center of the universe. In the 21st century, we will come to understand that Earth provides just one template for the way the cosmos builds planets — and not even the most common template.
"The original view, until 10 or 15 years ago, was that we had four Earthlike terrestrial planets, four gas giants and the misfit Pluto. But the new view is four terrestrial planets, four gas giants and hundreds of Plutos," planetary scientist Alan Stern said. “It's jarring, because Pluto’s no longer the misfit. It's the Earthlike planets that are the misfits.”
Some people may find it difficult to handle a planetary paradigm shift, but shift happens, whether we like it or not.
For example, the conventional wisdom once held that very few planets existed in the universe. At the time that Pluto was discovered, Sir Arthur Eddington estimated that only one star system out of 100 million had the right conditions to produce planets. Arthur Compton, one of the foremost American physicists of the day, declared that "a planet is a very rare occurrence."
Eighty years later, the paradigm shift couldn’t be more dramatic. The Geneva Observatory’s Michel Mayor, a pioneer in the quest to find alien worlds, estimates that a third of all stars like the sun have planets ranging in size between Earth and Neptune. What’s more, planets are being found in formerly unthinkable places. One star harbors a super-Earth within a fraction of Mercury’s distance from our sun. Another star has a Jupiter-scale planet that is three times farther away than Pluto is from the sun.
The evidence emerging from the hunt for extrasolar planets would argue for going with a wide-ranging definition of planethood, based on physical properties. After all, that’s how scientists define stars, ranging all the way from dwarfs to giants. Anything that’s massive enough to fire up an internal fusion furnace is considered a star or a brown dwarf. Similarly, anything that’s massive enough to build up layers of material into a gravitationally bound ball should be considered a planet.
A planet’s ability to sweep out other objects in its vicinity is important, to be sure. When astronomers study how planetary systems are sculpted, they might find it useful to ignore the gravitational effects (or non-effects) of the individual smaller objects and think of them merely as parts of a larger population — say, a belt of asteroids, or a ring of comets. But that’s no reason to draw an X through the legions of Planet X’s.
An overly narrow classification system is just asking to be rendered obsolete by future additions to the planetary list. Caltech astronomer Mike Brown, for instance, once suggested that Pluto and the dwarf planets that he and his team discovered should not be considered true planets because they could be grouped with similar things. He said they should be seen merely as parts of a bigger population. “If you don’t understand that, you get a misguided impression of the architecture of the solar system and how things formed and where things are,” Brown said.
But to paraphrase Shakespeare, there are more things in the heavens and on super-Earths than are dreamt of in that philosophy. The possibilities for planetary architecture are likely to be much more varied than we think. Even in our own solar system, we’re discovering moons that lurk right in the middle of a planet’s rings. If that’s so, then why can’t planets lurk in the wide rings of material that surround our sun and other stars?
"What will Mike Brown say when we find a system with 10 Saturns orbiting as a group?" Stern asked. "Or an Earth in the Oort Cloud? Or two Mercurys in close to one another? And what about when our own solar system was young, and Jupiter and Saturn crossed paths? Were they temporarily not planets during that era? Ridiculous, huh?"
To be sure, Stern isn’t a dispassionate participant in this debate. He’s a longtime Plutophile, as well as the principal investigator for New Horizons, NASA’s mission to the dwarf planet and the Kuiper Belt. But a good number of planetary scientists who are watching from the sidelines agree with him that the IAU’s hastily written definition just won’t cut it. And they’d like to see something done about that.
“If the IAU adopts a clearly flawed definition, nobody is under any obligation to accept it,” David Grinspoon, curator of astrobiology at the Denver Museum of Nature and Science, wrote in Sky & Telescope. “But I’m getting sick of this. Do planetary scientists really want to be known as the community that can’t stop fighting about what a planet is, during a decade when we are actually finding more planets every year than in all of human history, and launching spacecraft to solve mysteries of planetary climate, landscapes, and habitability?”
Grinspoon said it’s time for a compromise that includes both a physical and a dynamical perspective. "A planet is a round object orbiting a star," he declared. "If we learn that it has not gravitationally dominated its surroundings, then it goes in a sub-class called dwarfs. Dwarf planets join terrestrial planets such as Earth and Jovian planets such as Jupiter or Gliese 581b as full-fledged citizens of their planetary systems, with all the rights and privileges thus implied."
Strangely enough, it was just such a compromise that was voted down during the Battle of Prague.