Number ten? A new object, bigger and farther than Pluto, is orbiting the Sun. But is it a planet?

Natural History, Oct, 2005 by Charles Liu

Pluto--perhaps for reasons having as much to do with Walt Disney's animated dog as with either the Roman god of the underworld or the body's status as the "little guy" of the solar system--seems to be our sentimental favorite among the nine planets. Discovered in 1930 by the American astronomer Clyde W. Tombaugh, Pluto has been an object of curiosity in the three-quarters of a century since then. It is less than 1,500 miles across; in composition it is more like a comet than a gas giant or a terrestrial planet; it has an eccentric elliptical orbit, whose plane is tilted at seventeen degrees from the orbits of the rest of the planets; and it is just one among the uncounted hordes of similar small, icy and rocky bodies in a bagel-shaped region of the solar system known as the Kuiper Belt.

Faced with overwhelming scientific evidence, many of my colleagues have called for Pluto's reclassification from "major planet" to "minor planet." That action, they argue, would lead to more detailed study of Pluto as a representative of an entire class of objects in the solar system. Many other people, however--including, apparently, much of the American public--passionately oppose reclassification. Why should their beloved ninth planet have to endure the indignity of a "de motion"? Some who defend the status quo have even proposed defining "planet" to exclude every Kuiper Belt object (KBO) except Pluto.

Until recently, the commonest way to include Pluto among the planets has been to insist that size matters. The argument goes like this: even though it's smaller than Earth's Moon (or, for that matter, the moons Io, Europa, Ganymede, Callisto, Titan, and Triton), Pluto orbits the Sun. And because it's the largest object to do so--other than Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune--it belongs to the planetary pantheon.

Well, Pluto-bashers can now smugly rejoice. Michael E. Brown, an astronomer at Caltech, together with Chadwick A. Trujillo of the Gemini North Observatory in Hawai'i and David L. Rabinowitz of Yale University, have discovered a new KBO, nine billion miles from the Sun, whose diameter exceeds that of Pluto-possibly by half again as much. At its greatest distance from the Sun, 2003 UB313 (the body's provisional name) is twice as far from the Sun as Pluto ever gets. It takes 557 Earth years for it to make just one solar orbit, compared to "just" 248 years for Pluto.

Most astronomers have figured that one day another KBO would outdo Pluto in size. The basic planet-finding technique hasn't changed much since Tombaugh's time: basically, you look at a series of images of the same area of the sky made at various times, and you scan for objects that have moved between one frame and the next. What has changed is astronomical technology--so dramatically that astronomers such as Brown, Trujillo, and Rabinowitz can readily detect and study solar system objects more than a million times fainter than Pluto.

There's a world of difference, though, between having the ability to detect a KBO and actually discovering one. Discovery is hampered most of all by the simple fact that the sky is really, really big. Planet hunters can search only a small patch of sky at a time, and so they've tended to focus on parts of the sky in roughly the same orbital plane as that occupied by the planets Mercury through Neptune. With literally billions of background stars potentially confusing the field of view, it's hard enough to pick a planet out of a restricted part of the sky--much less the entire celestial sphere.

Nevertheless, undaunted by such an expanded search, planet pioneers have had enough successes in the past half decade to make steady inroads on Pluto's size supremacy among KBOs. Varuna, discovered in November 2000, measures nearly 600 miles across--a little less than half the size of Pluto. Less than two years later, Brown and Trujillo discovered Quaoar (pronounced KWAH-o-wahr), which is about 800 miles wide. In March 2004, Brown, Trujillo, and Rabinowitz announced the discovery of Sedna--probably not a KBO, but rather a member of the even more distant Oort Cloud of cometlike bodies--thereby pushing the maximum size of "nonplanetary" objects in the outer solar system up to about 1,000 miles in diameter.

The new Pluto-plus-size object, announced this past July, came to light the same way Varuna, Quaoar, and Sedna did. Brown, Trujillo, and Rabinowitz actually first recorded it in images made in 2003. Following up with spectroscopic observations, the investigators found that its surface, like Pluto's, is covered mostly with frozen methane. Even at its smallest estimated size, 2003 UB313 has dethroned Pluto, the erstwhile king of KBOs.

With one less argument in their favor, will the Pluto-is-a-(major)-planet crowd abandon Pluto to minor planethood? Nah. Larger bodies notwithstanding, many Pluto lovers still think their favorite should retain its status as a "major." By that logic, though, the new KBO must be hailed as the solar system's tenth major planet. If the pace of KBO discoveries keeps up, there may soon be a dozen or more planets--and with them, new names for schoolchildren to learn, new scripts for planetarium shows, and revisions galore in astronomy textbooks. After seventy-five years of nine planets, is the world ready for such a radical reordering of the cosmos?