WHAT IS A PLANET? ●➔ By Steven Soter The controversial new official definition Most of us grew up with the convention- al definition of a planet as a body that of “planet,” which banished Pluto, orbits a star, shines by reflecting the star’s light and is larger than an aster- has its flaws but by and large captures oid. Although the definition may not essential scientific principles have been very precise, it clearly catego- rized the bodies we knew at the time. In the 1990s, however, a remarkable series of discoveries made it untenable. Be- yond the orbit of Neptune, astronomers found hundreds of icy worlds, some quite large, occupying a doughnut- shaped region called the Kuiper belt. Around scores of other stars, they found other planets, many of whose orbits DAVID EMMITE PHOTOGRAPHY 34 SCIENTIFIC AMERICAN JANUARY 2007 COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC. look nothing like those in our solar sys- bit.” Controversially, the new definition reflects advances in our understanding tem. They discovered brown dwarfs, removes Pluto from the list of planets. of the architecture of our solar system which blur the distinction between Some astronomers said they would re- and others. These systems formed by ac- planet and star. And they found planet- fuse to use it and organized a protest cretion within rotating disks: small like objects drifting alone in the dark- petition. grains clump together to form bigger ness of interstellar space. This is not just a debate about words. ones, which draw themselves together These findings ignited a debate about The question is an important one scien- to form still bigger ones, and so on. This what a planet really is and led to the de- tifically. The new definition of a planet process eventually produces a small cision last August by the International Astronomical Union (IAU), astrono- STEVEN SOTER is a research associate in the department of astrophysics at the American mers’ main professional society, to de- Museum of Natural History in New York City and Scientist-in-Residence at the Center for fine a planet as an object that orbits a Ancient Studies at New York University, where he teaches undergraduate seminars on star, is large enough to have settled into topics ranging from “Scientific Thinking and Speculation” to “Geology and Antiquity in a round shape and, crucially, “has the Mediterranean.” He collaborated with Carl Sagan and Ann Druyan to create the cleared the neighborhood around its or- T H E A U T H O R Cosmos television series. www.sciam.com SCIENTIFIC AMERICAN 35 COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC. HISTORICAL COUNT OF PLANETS DATE PLANETS Planets come, planets go as a result of new discoveries and changing conceptions Pre- Mercury, Venus, Mars, of what a “planet” is. The decision to recategorize Pluto is simply another step 1543 Jupiter, Saturn, sun, moon in this historical progression. Earth added 24 - 1543 sun, moon deleted 22 - 20 - 1781 Uranus 18 - 1801 Ceres 16 - Neptune, 1846 1802 Pallas t 14 - un 12 - 1804 Juno Delete sun and moon Delete asteroids, 1852 et Co et 10 - Add Earth, 1543 Uranus, 1781 an 1807 Vesta Pl 8 - Pluto, 1930–2006 6 - 1845 Astraea Ceres, 1801 4 - 1846 Neptune 2 - 0 - 1847 Hebe, Iris, Flora 1500 1600 1700 1800 1900 2000 Year 1848 Metis Saturn Saturn Uranus 1849 Hygiea Jupiter Jupiter Mars Sun Mars Venus Earth 1850 Parthenope, Victoria, Egeria Mercury Venus Moon Mercury 1851 Irene, Eunomia SUN EARTH Ceres, Pallas, 1852 Asteroids deleted Juno, Vesta 1930 Pluto Classical (pre-1543) 1543 1825 2006 Pluto deleted number of massive bodies—the plan- When Earth Became a Planet Mars, Jupiter and Saturn. They called ets—and a large number of much small- astronomers’ reevaluation of the them planetes, or wanderers. Note that er bodies—the asteroids and comets, nature of planets hasMars deep historical Earth is not on this list. For most of hu- which represent the debris left over from roots. The ancient Greeks recognized man history, Earth was regarded not as planet formation. In short, “planet” is seven lights in the sky that moved a planet but as the center—or founda- not an arbitrary category but an objec- against the background pattern of stars: tion—of the universe. After Nicolaus tive class of celestial bodies. the sun, the moon, Mercury, Venus, Copernicus persuaded astronomers that the sun rather than Earth lies at the cen- Overview/Planet Definition ter, they redefined planets as objects or- biting the sun, thereby putting Earth on ■ Last August members of the International Astronomical Union voted to the list and deleting the sun and moon. define a planet as a body that orbits a star, is large enough to be round, Telescope observers added Uranus in and has cleared other bodies out of its neighborhood. The definition was 1781 and Neptune in 1846. intended to bring closure to a long-standing debate but instead seems Ceres, discovered in 1801, was ini- to have poured fuel on the fire. tially welcomed as the missing planet SaS turn ■ Critics have called the definition arbitrary and imprecise, but the charge that filled the gap between Mars and Ju- is unfounded. The solar system dividesJupite cleanlyerr into eight bodies massive piter. But astronomers began to have enough to dominate their orbital zones and swarms of smaller ones that doubts when they found Pallas in a sim- MaMarsr occupy intersecting orbits. This patternVenuus appears to reflect the way the ilar orbit the following year. Unlike the Mercurcuurryy solar system formed and evolved. EaEEarth classical planets, which telescopes re- vealed as little disks, both these bodies LUCY READING-IKKANDA 36 SCIENTIFIC AMERICAN JANUARY 2007 COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC. appeared as mere pinpricks of light. with scores of other large KBOs; con- English astronomer William Herschel versely, if Pluto is not a planet, neither proposed naming them “asteroids.” By are the other KBOs. On what objective FREQUENTLY 1851 their number had increased to 15, grounds could astronomers decide? ASKED QUESTIONS and it was becoming unwieldy to con- sider them all planets. Astronomers then Clearing the Air Isn’t the definition of a planet decided to list asteroids by their order of to avoi d an open-ended prolifera- ●Q really arbitrary? discovery rather than by distance from tion of planets, Alan Stern and Harold the sun, as for planets—the de facto ac- Levison of the Southwest Research In- A Scientists need precise definitions ceptance of the asteroids as members of stitute suggested in 2000 that a planet ● to communicate effectively. a distinct population. If we still counted could be defined as a body less massive Careful definitions reflect our asteroids as planets, schoolchildren than a star but large enough for its grav- understanding of basic studying the solar system would now ity to overcome its structural rigidity relationships in nature. If new have to cope with more than 135,000 and pull it into a round shape. Most discoveries render an old definition planets. bodies larger than several hundred kilo- misleading or obsolete, we need to Pluto has a similar story. When meters in radius satisfy the latter crite- revise it accordingly. Clyde Tombaugh discovered it in 1930, rion. Smaller ones often have a craggy astronomers welcomed Pluto as the shape; many of them are basically giant ●Q What’s wrong with the old definition long-sought “Planet X” whose gravity boulders. of a planet as a nonluminous body would account for unexplained pecu- This definition was the one advocat- orbiting a star and larger than liarities in the orbit of Neptune. Pluto ed in early August by the IAU Planet an asteroid? turned out to be smaller not only than Definition Committee, chaired by Owen the other eight planets but also than Gingerich of Harvard University. It ●A It makes no distinction between seven of their satellites, including Earth’s would have retained Pluto as a planet, planets and Kuiper belt objects, even moon. Further analysis showed the pe- but at the expense of admitting poten- though they are clearly different. culiarities in Neptune’s orbit to be illu- tially dozens of KBOs and restoring the sory. For six decades, Pluto was a unique planetary status of Ceres, the largest as- ●Q The definition approved by the anomaly at the outer edge of the plane- teroid and the only one known to be International Astronomical Union tary system. spherical. says that a planet “has cleared the Just as Ceres began to make sense Many astronomers argued that the neighborhood around its orbit.” only when it was recognized as one of a roundness criterion is inadequate. In But many asteroids and comets vast population of asteroids, Pluto fell practical terms, it is very difficult to ob- cross Earth’s orbit, so why is it still into place only when astronomers found serve the shapes of distant KBOs, so called a planet? Or, for that matter, why is even Jupiter a planet? The it was one of a vast population of Kui- their status would remain ambiguous. Trojan asteroids share Jupiter’s per belt objects (KBOs) [see “The Kui- Furthermore, asteroids and KBOs span orbit, so Jupiter hasn’t “cleared” per Belt,” by Jane X. Luu and David C. an almost continuous spectrum of sizes its neighborhood. Jewitt; Scientific American, May and shapes. How are we to quantify the 1996, and “Migrating Planets,” by degree of roundness that distinguishes a ●A The clearing is never perfect Renu Malhotra; Scientific Ameri- planet? Does gravity dominate such a because asteroids, comets and can, September 1999]. Astronomers body if its shape deviates from a spher- meteoroids continue to stray into began to reconsider whether it should oid by 10 percent or by 1 percent? Na- the neighborhoods of the planets.