Planet politics

NAMING HAS ALWAYS been a tricky business in astronomy. When Galileo discovered four ``planets" accompanying Jupiter, he intended to name them ``Medicean stars," after his hoped-for patron. He got the job, but lost on the names. Kepler introduced the word ``satellites," and Medicean never stuck.

William Herschel tried to name his new planet ``Georgium Sidus" for King George III, but the international astronomy community would have none of that, calling it Uranus instead.

Lowell Observatory in Flagstaff, Ariz., was luckier with Pluto, which was discovered there in 1930. The name, suggested by an 11-year-old Oxford school girl, incorporated the initials of Boston Brahmin Percival Lowell, who had initiated the search for the object decades earlier.

Yet controversy finally caught up with Pluto in 2006. Pluto, it had turned out, was not a massive object influencing Uranus and Neptune in their courses, as Lowell and his contemporaries had assumed. It was a dwarf, smaller than our moon, and 400 times less massive than the earth. The supposed wobbles in Neptune's positions resulted from using an incorrect mass for Neptune itself when calculating its orbit. Had these facts been known in 1930, Pluto's planetary status would have been disputed at the outset.

The International Astronomical Union has for 80-odd years been naming comets and asteroids without controversy, but when Mike Brown of Cal Tech and his colleagues recently found an object even bigger than Pluto plying the frozen reaches beyond Neptune, it ignited a crisis of nomenclature. Had he found a 10th planet? And were there more candidates waiting to be discovered among the numerous icy chunks in the so-called Kuiper belt?

In a quandary, the IAU decided to appoint a small but broadly based committee to look into the matter. As both an astrophysicist and historian of science, I was tapped to chair it.

There are two distinct scientific ways to approach the problem of defining planethood. One is extrinsic, to define a planet in terms of its neighbors and its interactions with its environment. This route would select the dominant bodies in the solar system, the ones whose gravity perturbs one another.

It was the flash of insight that not only the sun holds the planets in orbit, but that each planet attracts each other one, that led Isaac Newton to the concept of universal gravitation. Neptune attracts Pluto, locking it into a resonant orbit so that in the time Neptune takes to round the sun three times, Pluto revolves exactly twice. But Pluto is too lightweight to have an observable effect on giant Neptune. The eight dominant heavyweights, from Mercury to Neptune, are big enough to rule their zones and swallow up most of the smaller bodies or kick them out of the way.

Choosing such dominance is a comfortable way to go when defining what constitutes a planet: While it would dismiss Pluto, it would forever place the other eight planets in an exclusive club.

An alternative way to define a planet, however, is intrinsic-that is, by the properties of the body itself, more or less independent of its environment. This is the way planetary geologists look at the problem, and pretty much the way astronomers looking at the hundred-plus planets circling other stars do. The idea of using the basic physics of the object appealed to our committee as the forward-looking way to define planets, for it could apply not only to the far stretches of the solar system, but to the objects being found around distant stars as well.

Rather than arbitrarily drawing a line in the sand that would either include or exclude Pluto, we opted to let nature pick a dividing line between planets and the hundreds of thousands of lumpy asteroids now known-nearly 140,000 numbered objects and an equal number awaiting their catalog numbers. If a body is massive enough, with enough self-gravity to pull itself into a ball, let it be planet. If it's just a lump, let it be an asteroid or a comet.

Of course, the roundness definition could open the gates to a dozen or more snowballs in the Kuiper belt, and we were quite aware that these bodies would be minor lightweights in a system of traditional heavyweights. So we decided to distinguish the eight classical planets from the Kuiper belt planets. As a group, these Kuiper belt planets needed a name, something that gave a tip of the hat to the longstanding eminence of Pluto. We eventually backed the name ``plutonian." Thus Pluto, while being demoted, would be cast in the position of being a group leader and would still be a planet, though of a different sort.

In August our committee took this recommendation to Prague, to the triennial IAU congress, where it became the most-talked-about issue of the meeting. (I recently put ``Pluto" and ``Gingerich" into Google and got 41,000 hits!) The media there had two questions: Was Pluto still a planet? and how many planets would there be?

In the committee it had never occurred to us to count the current number, because we knew the tally of plutonians would rapidly grow. We also knew that the Hubble Space Telescope had recently shown that the earliest known asteroid, Ceres, was round. Ceres would thus fit our definition of a planet, though it would be in a category of its own, being neither a heavyweight nor a plutonian-a term we reserved for the lightweight planets beyond Neptune.

All this was more complexity than could be accounted for in any handy mnemonic phrase. We felt, however, that our definition reflected the new-found diversity in our solar system, which modern science shows is far more complex than dreamt of by Percival Lowell. Our definition suggested the growing richness of our environment. But it also proved to be the Achilles' heel of our proposal.

The media, eager to promote controversy, quickly found vocal critics to say that the whole business was too complicated. In the final days of the congress, a group of astronomers persuaded enough of their colleagues that a simpler result was desirable (even if it required a more complicated definition), and the intrinsic approach was voted down.

The IAU thereafter defined a planet in our solar system as an object large enough to clear the smaller bodies from its orbit, a definition just murky enough to give teachers a considerable challenge to explain precisely what this means. Taking the exclusive club approach for the heavyweights, the IAU went on to create a class of ``dwarf planets," including Pluto, that by definition are not planets. To me this is a linguistic absurdity, a contradiction that could have been avoided if they had chosen to define only the eight classical planets as the basic type of planets, allowing dwarf planets to be considered planets too, albeit of a different kind. But this cultural compromise was specifically rejected.

In their muddled wisdom, the IAU members did at least vote to make Pluto the prototype object for a new category. Ceres will stand alone as a dwarf planet in the asteroid belt while Pluto will be the charter member of a new class of dwarf planets beyond Neptune, much as our committee originally proposed. But, by a virtual tie vote, the IAU decided not to adopt the obvious name ``plutonians" for that category. The name awaits in the wings.

Owen Gingerich is professor emeritus of astronomy and history of science at the Harvard-Smithsonian Center for Astrophysics and the author of ``God's Universe," to be published this month.