Pluto's Honor

Natural History, Feb, 1999 by Neil de Grasse Tyson

Hailed as the newest member of the solar system in 1930, the smallest planet now rices expulsion.

According to the latest orbital data, on Tuesday, February 11, 1999, at 11:29 A.M., eastern standard time, the planet Pluto will regain its distinction as the most far-out planet in the solar system. For the last twenty years of its 248-year orbit, it had been closer than Neptune to the Sun.

In an informal poll of ten thousand junior-high-school children, Pluto was the overwhelming favorite among the nine planets. The poll was simply a measure of how much noise the children made during a tour of the solar system in a live planetarium show I presented to five hundred children at a time. They consistently cheered the loudest for Pluto, especially when I recited the planets in sequence, aided by the time-honored mnemonic My Very Educated Mother Just Served Us Nine Pizzas.

But Pluto has "peculiar" written all over it. The planet was discovered by Lowell Observatory astronomer Clyde W. Tombaugh in 1930, the same year Walt Disney created the lovable, slow-witted bloodhound that shares its name. Of all the planet names, "Pluto" sounds the most like a punch line to a hilarious joke: "... he thought he was on Pluto!" And while all the other planets are named for Roman gods who represent various aspects of human life--such as commerce, war, love, harvest, and fertility--Pluto is named for the god of Hades, a dark and dank residence for the dead.

Curious about moon names? Those of Uranus are named for Shakespearean characters, while the lone moon of Pluto, Charon, is named for the boatman who ferries your unfortunate soul across the river Styx into Hades.

Peculiar enough for you? I'm not finished.

Pluto's orbit has a tilt that is seventeen degrees out of the plane of the solar system--two and a half times that of Mercury's, which has the next most tilted orbit among the nine planets. And Pluto moves in the most eccentric ellipse. To picture the orbit, just take a perfect circle (such as a hula hoop) and gently sit on it: the shape formed under your butt greatly resembles an ellipse. Pluto is the only planet whose orbit crosses that of another planet. Pluto's tidal forces have locked the rotation of its moon, Charon, forcing it to forever show the same face to Plutonians. Actually, Pluto is in good company here. Earth has tidally locked the rotation of its moon (the Moon), so that it always shows the same face to Earthlings. The embarrassing part is that Charon is so large compared with Pluto that its tidal forces have locked Pluto's rotation as well. So both moon and planet show the same side to one another as they waltz forever in space. In fact, Pluto and Charon's barycenter--the point around which planet and moon revolve---falls outside the body of Pluto. (The Earth-Moon barycenter lies 1,000 miles beneath Earth's surface.) With a diameter of 1,400 miles, Pluto is by far the smallest planet. The solar system even has seven moons that are larger: Jupiter's Io, Europa, Ganymede, and Callisto; Saturn's Titan; Neptune's Triton; and, of course, Earth's Moon. Finally, neither rocky nor gaseous, Pluto is the only planet made primarily of ices.

Maybe Pluto isn't really a planet.

I mean no disrespect to Clyde Tombaugh, who died in 1997 at the age of ninety, seemingly secure in his status as the third person ever to discover a planet in our solar system. But there is no question that if Pluto were discovered today, it would not be classified as a planet.

Is no discovery sacred? What's the definition of a planet, anyway? The ancient Greeks called anything that wandered against the background stars a planet (from the Greek word for "wanderer"). They recognized seven of them: the Sun, the Moon, Mars, Mercury, Jupiter, Venus, and Saturn. All would ultimately become assigned to days of the week (in the sequence given) in various Western languages. When the Copernican, suncentered universe was installed and confirmed, Earth was demoted to the rank of planet, the Moon became a satellite, and the Sun joined the ranks of the stars.

Perhaps a planet is simply anything other than a comet that orbits the Sun. William Herschel discovered Uranus in 1781, and Neptune was discovered by Johann Galle, of the Berlin Observatory, in 1846. Few people know, however, that in 1801 Giuseppe Piazzi discovered the planet Ceres, orbiting the Sun between Mars and Jupiter. The suspiciously large gap between the two planets had finally been filled. But astronomers rapidly determined that Ceres was much, much smaller than any other planet: at six hundred miles in diameter, it was dwarfed by Mercury, the reigning smallest planet. Maybe size does matter. Maybe an object can be too small to be defined as a planet. Shortly after 1801, other small objects were found in orbits similar to that of Ceres. A new class of object was identified: the rocky asteroids. And a new swath of real estate in the solar system was settled: the asteroid belt.

Ceres was discovered first because it is the brightest and largest. At twice the mass of all the other asteroids combined--of which there are thousands known and millions that await discovery--Ceres swiftly went from being the smallest in the class of planet to the largest in the class of asteroid.