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Home , Astronomy, Planet, Solar System, Steven Soter

WHAT IS A ? ●➔ By

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 , a , shines by reflecting the star’s 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 . In the , however, a remarkable series of discoveries made it untenable. Be- yond the of , found hundreds of icy , some quite large, occupying a doughnut- shaped region called the . Around scores of other , they found

other , many of whose orbits 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 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 . 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 at the American mers’ main professional society, to de- of Natural in New York City and -in-Residence at the Center for fine a planet as an object that orbits a Ancient Studies at New York , where he teaches undergraduate seminars on star, is large enough to have settled into topics ranging from “Scientific Thinking and Speculation” to “ and Antiquity in a round shape and, crucially, “has the Mediterranean.” He collaborated with and 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- , , , of what a “planet” is. The decision to recategorize Pluto is simply another step 1543 , , , in this historical progression. added 24 - 1543 sun, moon deleted 22 - 20 - 1781 18 - 1801 16 - Neptune, 1846 1802 Pallas t 14 - un 12 - 1804 Delete sun and moon Delete , 1852 et Co et 10 - Add Earth, 1543 Uranus, 1781 an 1807 Vesta Pl 8 - Pluto, 1930–2006 6 - 1845 Ceres, 1801 4 - 1846 Neptune 2 - 0 - 1847 Hebe, Iris, Flora 1500 1600 1700 1800 1900 2000 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 , or wanderers. Note that er bodies—the asteroids and , 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 in the that moved a planet but as the center—or founda- not an arbitrary category but an objec- against the pattern of stars: tion—of the . 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, 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 . 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- MMaarsr occupy intersecting orbits. This patternVenuus appears to reflect the way the ilar orbit the following year. Unlike the Mercurcuurryy solar system formed and evolved. EartEaE h classical planets, which re-

vealed as little disks, both these bodies 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 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, and Harold the sun, as for planets—the de facto ac- Levison of the Southwest Research In- A 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. 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 ” whose 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 ? 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 , 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 - 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 , why is even Jupiter a planet? The it was one of a vast population of Kui- their status would remain ambiguous. 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 its neighborhood. Jewitt; Scientific American, May and shapes. How are we to quantify the 1996, and “Migrating Planets,” by 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- continue to stray into began to reconsider whether it should oid by 10 percent or by 1 percent? Na- the neighborhoods of the planets. still be called a planet. Historically, re- ture provides no unoccupied gap be- Yet the amount of this debris is voking the planetary status of Pluto tween round and nonround shapes, so negligible compared with each would not be unprecedented; the ranks any boundary would be an arbitrary planet’s . A more precise of ex-planets include the sun, moon and choice. definition would say that a planet asteroids. Nevertheless, many people Stern and Levison proposed another “dominates” its orbital zone. have argued for continuing to call Pluto criterion that does, however, to a Jupiter’s gravity controls the orbits a planet, because almost everyone has nonarbitrary way to classify objects. of the Trojan asteroids. The IAU grown quite accustomed to thinking of They remarked that some bodies in the definition has the right idea, but its it as one. solar system are massive enough to have unqualified use of the word The discovery in 2005 of (for- swept up or scattered away most of their “cleared” has inadvertently caused merly known as 2003 UB313 or Xena), immediate neighbors. Lesser bodies, some confusion. a KBO even larger than Pluto, brought unable to do so, occupy transient, un- Questions continued on page 40 the issue to a head. If Pluto is a planet, stable orbits or have a heavyweight then Eris must also be one, together guardian that stabilizes their orbits. For www.sciam.com SCIENTIFIC AMERICAN 37 COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC. instance, Earth is big enough that it eventually sweeps up or flings away any body that strays too close, such as a THE “NEW” near-Earth asteroid. At the same time, SOLAR SYSTEM MERCURY Main Earth protects its moon from being asteroid swept up or scattered away. Each of the The planet definition approved by the PLUTO belt EARTH four giant planets rules over a sizable International Astronomical Union is based brood of orbiting satellites. Jupiter and on the observed architecture of the solar Scattered Kuiper disk belt MARS SUN Neptune also maintain their own fami- system, in which a small number of VENUS lies of asteroids and KBOs (called Tro- dominant bodies, the eight planets, have JUPITER well-separated orbits, in contrast to the jans and , respectively) in spe- NEPTUNE CERES swarms of smaller asteroids, comets and cial orbits known as stable resonances, Kuiper belt objects. Ceres and Pluto, once Trojan where an orbital synchrony prevents URANUS asteroids considered planets, are (along with Eris) ERIS SATURN collisions with the planets. swarm dwellers. Trojan asteroids share the These dynamical effects suggest a orbit of Jupiter and are dynamically practical way to define a planet. That is, controlled by it. Centaurs are comets Centaurs a planet is a body massive enough to orbiting between Jupiter and Neptune. dominate its orbital zone by flinging smaller bodies away, sweeping them up in direct collisions, or holding them in stable orbits. According to basic orbital , the likelihood that a massive body will deflect a smaller one from its neighborhood within the age of the so- lar system is roughly proportional to the square of its mass (which determines the gravitational reach of the massive body for a given amount of deflection) and inversely proportional to its orbital pe- TAXONOMY OF riod (which governs the rate at which the encounters occur). CELESTIAL BODIES The eight planets from Mercury ●➔ OBJECTS: Stars, through Neptune are thousands of brown dwarfs, sub-brown dwarfs ➔ more likely to sweep up or deflect small They form when an interstellar ● DEBRIS: Asteroids, neighboring bodies than are even the collapses under its own gravity. Those with comets, Kuiper belt objects largest asteroids and KBOs, which in- at least 80 times the mass of Jupiter They form like secondary objects, but their clude Ceres, Pluto and Eris. Mercury undergo stable of growth is arrested. They do not dynamically and Mars by themselves are not massive and are called stars. Those in the range of control their orbital zones. Asteroids are enough to scatter away all the bodies in ) 13 to 80 Jupiter undergo a brief small rocky worlds, most of which reside in period of nuclear fusion of , a rare a belt between the orbits of Mars and their vicinities. But Mercury is still large orbits of hydrogen, and are called brown Jupiter. Kuiper belt objects are small icy enough to sweep up most of the nearby and dwarfs. Less massive ones may be termed bodies that orbit in a belt beyond Neptune; small objects that cross its orbit, and

graph sub-brown dwarfs. the belt appears to be the source of most Mars has sufficient gravitational influ- periodic comets. The distinction between ence to deflect passing objects into near- ●➔ SECONDARY OBJECTS: Planets asteroids and comets is sometimes by unstable orbits, including some with They form when grains clump together ambiguous: comets are typically more periods exactly one-third or one-quar- in the rotating disk of material around a volatile-rich and form farther from the sun. primary object. They undergo a period of ter that of Jupiter. The gravity of the gi- ➔ ROGUE PLANETS ant planet then completes the task of runaway growth in which the larger ones ● sweep up most of the rest of the material. They form as secondary objects but have ejecting those objects from the vicinity ); LUCY READING-IKKANDA ( been ejected to interstellar space. of Mars. A planet that reaches a certain critical can also pull in a thick envelope of gas. Simulations suggest that such objects may The ability of a body to clear its outnumber the stars in our . neighborhood depends on its dynamical solar system ●➔ TERTIARY OBJECTS: Satellites Observationally, though, they will be context; it is not an intrinsic property of They orbit secondary objects, either having difficult to detect, let alone distinguish the body. Nevertheless, the large gap in been formed in place or captured from from free-floating sub-brown dwarfs that

dynamical power provides a clear way to DON DIXON ( independent orbits. formed as primary objects.

38 SCIENTIFIC AMERICAN COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC. COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC. MERCURY Main asteroid PLUTO belt EARTH Scattered Kuiper MARS disk belt VENUS SUN JUPITER NEPTUNE CERES Trojan ERIS URANUS SATURN asteroids

Centaurs

107 106 105 PLANETS ELSEWHERE IN THE GALAXY 4

e 10 ORBITAL DOMINANCE by a few bodies appears to be a property of other known

alu 3 planetary systems, too. In most, the planets’ orbits do not overlap (left), so they

V 10

� 2 are unable to collide. Even in those few cases where the orbits do overlap (right), o 10 ti an orbital synchrony prevents them from colliding. Ra 10

ss ss 1 Ma 10–1 10–2 10–3 r s o h s e us te re ars Eris nu un turn M an Plut Eart Ce rcury Ve Sa Jupi Ur Nept Me

CLEAR DIVISION between planets () and lesser bodies (cream) is evident in their mass ratio —the mass of a body divided by the total mass of all other bodies that share its orbital zone. All eight planets have a  value of at least 5,000, whereas Pluto’s is less than 1. A  value of 100 serves as a convenient system HD 82943 system dividing line between planets and nonplanets in our solar system.

SCIENTIFIC AMERICAN 39 COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC. distinguish the planets from other bod- mass in its orbital zone. But the exact ies. We do not need to make an arbitrary value of this cutoff is not critical. Any MORE QUESTIONS distinction because, at least in our own value between about 10 and 1,000 solar system, nature does it for us. would have the same effect. ●Q Pluto’s orbit crosses that of A planet is thus a body that has Neptune, so why is Neptune called Kings of Their Kingdoms swept up or scattered away most of the a planet but not Pluto? a closely related criterion was mass from its orbital zone. The clean di- proposed by astronomer Michael vision of bodies into planets and non- ●A Neptune is more than 8,000 times Brown of the California Institute of planets reveals important aspects of the as massive as Pluto and dominates Technology in 2004. He defined a plan- process that formed the solar system. its neighborhood gravitationally. et as “any body in the solar system that All these bodies grew from a flattened Neptune long ago locked Pluto’s is more massive than the total mass of disk of gas and dust orbiting the primor- orbit into a resonance with its own, all of the other bodies in a similar or- dial sun. In the competition for the lim- making a collision between the two bit.” To make this more precise, I have ited amount of raw material, some bod- bodies impossible. Pluto is too small suggested replacing “similar orbit” ies won out. Their growth became self- to dominate anything beyond its with the concept of an orbital zone. reinforcing, so instead of a continuous own satellites (one of which, Two bodies share such a zone if their spectrum of bodies of all sizes, the result , is almost half as big as it is). orbits ever cross each other, if their or- was a single large body that dominated bital periods differ by less than a factor each orbital zone. The smaller bodies Doesn’t having a qualify ●Q of 10, and if they are not in a stable res- were swept up by the larger ones, eject- a celestial body to be a planet? onance. To apply this definition, I un- ed from the solar system or swallowed dertook a census of the known small by the sun, and the survivors became the A No. Many asteroids and Kuiper belt ● bodies that orbit the sun. planets we see today. The asteroids and objects have satellites, but Earth, for example, shares its orbital comets, including the KBOs, are the Mercury and Venus do not, and zone with an estimated 1,000 asteroids leftover debris. no one would deny that they are planets. larger than one kilometer in diameter, Our solar system is now in the final most of which are relatively recent arriv- cleanup phase of . The aster- als from the main between oids have intersecting orbits that allow ●Q If we discovered a Mars-size body in the outer Kuiper belt or even a Mars and Jupiter. They add up to less them to collide with one another and Neptune-size body in the distant than 0.0001 percent of the mass of our with the planets. The Kuiper belt is a , would we call it a planet? planet. The ratio between the mass of a remnant of the outer part of the original body and the mass of all other bodies , where the material was ●A Not according to the new definition, that share its orbital zone can be abbre- too sparse to form another planet. The because such a body would not viated µ. For Earth, µ is about 1.7 mil- planets of our solar system have orbits dominate its neighborhood. We lion. In fact, Earth appears to have the that do not intersect and so are unable might need to coin a new term for it. highest µ value in the solar system. Jupi- to collide. As the dynamically dominant ter is 318 times more massive but shares bodies, they must be few in number. If ●Q Isn’t it more practical to classify its orbital zone with a larger swarm of another planet tried to squeeze in be- celestial bodies based only on their bodies. Mars has the lowest µ value for tween the existing ones, gravitational intrinsic features rather than on any of the planets (5,100), but even that perturbations would eventually destabi- their orbital context? is far greater than the value for Ceres lize its orbit. (0.33) or Pluto (0.07) [see box on pre- A similar situation appears to be ●A Not necessarily. We already classify ceding page]. The result is striking: the true of other planetary systems as well. many objects as “” based on planets are in a different league from the So far observers have found about 20 their orbital context. Some are as asteroids and KBOs, and Pluto is clearly systems with more than one planet. In large as planets, and others are a KBO. most, the planets have orbits that do not simply captured asteroids or Such arguments persuaded the IAU intersect, and in the three exceptions, comets, but we classify them by the to define a planet in terms of “clearing” the overlapping orbits appear to be reso- shared dynamical characteristic of its orbital neighborhood. The IAU may nant, allowing the planets to survive orbiting a planet. need to amend the definition to specify without colliding. All the known non- what degree of clearing qualifies a body stellar companions of sunlike stars are Ask more questions at as a planet. I have suggested setting the massive enough to deflect nearby small www.sciam.com/ontheweb cutoff at a µ value of 100. That is, a body bodies. They would probably qualify as sa in our solar system is a planet if it ac- planets by the criterion of dynamical counts for more than 99 percent of the dominance.

40 SCIENTIFIC AMERICAN JANUARY 2007 COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC. Endgame a pl a net is, in effect, the end product of accretion from a disk around a star. This definition applies only to mature systems, such as ours, in which accre- tion has run effectively to completion. For younger systems, where accretion is still important, the largest bodies are not strictly planets but are called plan- etary embryos, and the smaller bodies are called . The IAU definition still includes roundness as a criterion for a planet, though strictly speaking, that is unnec- essary. The orbital-clearance criterion already distinguishes planets from aster- oids and comets. The definition also re- moves the need for an upper mass limit to distinguish planets from stars and brown dwarfs. The relatively rare companions orbiting close to stars can be classified as planets; unlike brown dwarfs in wider orbits, they are thought to have formed by disk accretion. In short, the difference between planets and nonplanets is quantifiable, both in and by observation. All the planets in our solar system have enough mass to have swept up or scat- tered away most of the original plane- from asteroids, KBOs and ejected plan- Pluto was a planet. Some argue that cul- tesimals from their orbital zones. Today etary embryos, which do not. The eight ture and tradition are sufficient grounds each planet contains at least 5,000 times planets are the dominant end products to leave it that way. But cannot more mass than all the debris in its vi- of disk accretion and differ recognizably remain bound by the misconceptions of cinity. In contrast, the asteroids, comets from the vast populations of asteroids the past. To be useful, a scientific defini- and KBOs, including Pluto, live amid and KBOs. tion should be derived from, and draw swarms of comparable bodies. The historical definition of nine plan- attention to, the structure of the natural A prominent objection to any defini- ets no doubt retains a strong sentimental . We can revise our definitions tion of this kind is the contention that attraction. But ad hoc definitions devised when necessary to reflect the better un- astronomical objects should be classified to grandfather in Pluto tend to conceal derstanding that arises from new discov- only by their intrinsic properties, such as from the public the profound changes eries. The debate on the definition of a size, shape or composition, and not by that have occurred since the early 1990s planet will provide educators with a their location or dynamical context. in our understanding of the origin and textbook example to show how scien- This argument overlooks the fact that architecture of the solar system. tific concepts are not graven in stone but astronomers classify all objects that or- For 76 , our schools taught that continue to evolve. bit planets as “moons,” although two of them are larger than the planet Mercury MORE TO EXPLORE and many are captured asteroids and Regarding the Criteria for Planethood and Proposed Planetary Classification Schemes. comets. Context and location are clearly Presented at the XXIVth General Assembly of the IAU, Manchester, U.K., August 7–18, 2000. important. In fact, distance from the sun S. Alan Stern and Harold F. Levison in Highlights of , Vol. 12, pages 205–213; 2002. www.boulder.swri.edu/ hal/PDF/planet def.pdf determined that close-in bodies became ˜ – Planetesimals to Brown Dwarfs: What Is a Planet? Gibor Basri and Michael E. Brown in small rocky planets and farther ones be- Annual Review of Earth and Planetary , Vol. 34, pages 193–216; 2006. came giant planets rich in volatile What Is a Planet? Steven Soter in Astronomical Journal, Vol. 132, No. 6, pages 2513–2519; and gases. The new definition distin- December 2006. Preprint available at .org/abs/astro-ph/0608359 guishes planets, which dynamically A World on the Edge. Michael Brown. Available online at

DAVID EMMITE PHOTOGRAPHY dominate a large volume of orbital space, http://solarsystem.nasa.gov/scitech/display.cfm?ST–ID=105

www.sciam.com SCIENTIFIC AMERICAN 41 COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC.