Too Many Planets Numb the Mind

Editorial - New York Times: "When a Caltech astronomer, Michael Brown, announced last year that his team had found a distant object three-fourths the size of Pluto orbiting the Sun, he declined to call it a planet, and he even suggested that Pluto should not be considered a planet either. There was, he said, just no good scientific rationale for considering either of those distant bodies in the same league as the eight indisputable planets that circle the Sun at closer range.

Now Dr. Brown has found something orbiting the Sun that's bigger than Pluto and even farther away. He's changed his mind and proposed that Pluto keep its designation, and that the new object, an extremely big lump of ice and rock, should also be deemed a planet. There is still no good scientific rationale for the judgment, he admitted, but this is a case where habit - 75 years of calling Pluto a planet - should trump any scientific definition.

There is no real debate that the four terrestrial planets - Mercury, Venus, Earth and Mars - and the four gaseous giants - Jupiter, Saturn, Uranus and Neptune - deserve their status as planets. But scientists have long been uneasy about including Pluto, an icy ball smaller than our Moon, whose orbit is more eccentric than the others and tilts in a different plane.

Try as they might, scientists could not come up with a definition that would retain Pluto as a planet without requiring that scads of other objects be deemed planets as well. Nor could they satisfy the legions of space enthusiasts who remain certain from their grade school lessons that there are nine planets - no more, no less.

So now Dr. Brown proposes that scientists give up the battle and accept a cultural definition of what a planet is. It's either the nine planets we learned about in grade school, or those nine plus any new-found object orbiting the Sun that turns out to be bigger than Pluto. He opts for the latter approach on the theory that most people, deep down, accept that definition. This definition would also, of course, qualify Dr. Brown for the historical footnotes as the discoverer of a new planet.

Our own preference is to take a cleaner way out by dropping Pluto from the planetary ranks. Scientists may well discover many more ice balls bigger than Pluto, and it's a safe bet that few in our culture want to memorize the names of 20 or more planets. Far better to downgrade Pluto to the status of an icy sphere that was once mistakenly deemed a planet because we had not yet discovered its compatriots on the dark fringes of the solar system."