Migrating Planets

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Did the solar system always look

the way it does now? New

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outer planets may have

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PLANETESIMAL

NEPTUNE

Migrating Planets

NEWLY FORMED NEPTUNE traveled amid a swarm of small rocky andicy bodies called planetesimals (opposite page). Some hit the planet but mostwere scattered by Neptune’s gravity toward Jupiter, which ejected themfrom the solar system (above). In a typical scattering, Neptune gained ener-gy, and its orbit spiraled outward very slightly. Billions of such encountersmay have caused the planet to migrate to its current orbit.

56 Scientific American September 1999

by Renu Malhotra

Copyright 1999 Scientific American, Inc.

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In the familiar visual renditions of the solarsystem, each planet moves around the sunin its own well-defined orbit, maintaining a

respectful distance from its neighbors. Theplanets have maintained this celestial merry-go-round since astronomers began recording theirmotions, and mathematical models show thatthis very stable orbital configuration has existedfor almost the entire 4.5-billion-year history ofthe solar system. It is tempting, then, to assume

that the planets were “born” in the orbits thatwe now observe.

Certainly it is the simplest hypothesis. Mod-ern-day astronomers have generally presumedthat the observed distances of the planets fromthe sun indicate their birthplaces in the solarnebula, the primordial disk of dust and gas thatgave rise to the solar system. The orbital radiiof the planets have been used to infer the massdistribution within the solar nebula. With this


basic information, theorists have derivedconstraints on the nature and timescalesof planetary formation. Consequently,much of our understanding of the earlyhistory of the solar system is based onthe assumption that the planets formedin their current orbits.

It is widely accepted, however, thatmany of the smaller bodies in the solarsystem—asteroids, comets and the plan-ets’ moons—have altered their orbits overthe past 4.5 billion years, some more dra-matically than others. The demise ofComet Shoemaker-Levy 9 when it col-lided with Jupiter in 1994 was strikingevidence of the dynamic nature of someobjects in the solar system. Still smallerobjects—micron- and millimeter-size in-terplanetary particles shaken loose from

comets and asteroids—undergo a moregradual orbital evolution, gently spiral-ing in toward the sun and raining downon the planets in their path.

Furthermore, the orbits of many plan-etary satellites have changed significant-ly since their formation. For example,Earth’s moon is believed to have formedwithin 30,000 kilometers (18,600 miles)of Earth—but it now orbits at a distanceof 384,000 kilometers. The moon hasreceded by nearly 100,000 kilometers injust the past billion years because oftidal forces (small gravitational torques)exerted by our planet. Also, many satel-lites of the outer planets orbit in lock-step with one another: for instance, theorbital period of Ganymede, Jupiter’slargest moon, is twice that of Europa,

which in turn has a period twice that ofIo. This precise synchronization is be-lieved to be the result of a gradual evo-lution of the satellites’ orbits by meansof tidal forces exerted by the planet theyare circling.

Until recently, little provoked the ideathat the orbital configuration of theplanets has altered significantly sincetheir formation. But some remarkabledevelopments during the past five yearsindicate that the planets may indeedhave migrated from their original or-bits. The discovery of the Kuiper belt hasshown that our solar system does notend at Pluto. Approximately 100,000icy “minor planets” (ranging between100 and 1,000 kilometers in diameter)and an even greater number of smaller

58 Scientific American September 1999 Migrating Planets

SUNJUPITER SATURN

URANUS NEPTUNE PLUTO

SUNJUPITER SATURN

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bodies occupy a region extending fromNeptune’s orbit—about 4.5 billion kilo-meters from the sun—to at least twicethat distance. The distribution of theseobjects exhibits prominent nonrandomfeatures that cannot be readily ex-plained by the current model of the so-lar system. Theoretical models for theorigin of these peculiarities suggest theintriguing possibility that the Kuiperbelt bears traces of the orbital history ofthe gas-giant planets—specifically, evi-dence of a slow spreading of these plan-ets’ orbits subsequent to their formation.

What is more, the recent discovery ofseveral Jupiter-size companions orbitingnearby sunlike stars in peculiarly smallorbits has also focused attention onplanetary migration. It is difficult to un-

derstand the formation of these putativeplanets at such small distances fromtheir parent stars. Hypotheses for theirorigin have proposed that they accretedat more comfortable distances fromtheir parent stars—similar to the dis-tance between Jupiter and the sun—andthen migrated to their present positions.

Pluto: Outcast or Smoking Gun?

Until just a few years ago, the onlyplanetary objects known beyond

Neptune were Pluto and its satellite,Charon. Pluto has long been a misfit inthe prevailing theories of the solar sys-tem’s origin: it is thousands of times lessmassive than the four gas-giant outerplanets, and its orbit is very different

from the well-separated, nearly circularand co-planar orbits of the eight othermajor planets. Pluto’s is eccentric: dur-ing one complete revolution, the plan-et’s distance from the sun varies from29.7 to 49.5 astronomical units (one as-tronomical unit, or AU, is the distancebetween Earth and the sun, about 150million kilometers). Pluto also travels 8AU above and 13 AU below the meanplane of the other planets’ orbits [see il-lustration at left]. For approximatelytwo decades in its orbital period of 248years, Pluto is closer to the sun thanNeptune is.

In the decades since Pluto’s discoveryin 1930, the planet’s enigma has deep-ened. Astronomers have found thatmost Neptune-crossing orbits are unsta-ble—a body in such an orbit will eithercollide with Neptune or be ejected fromthe outer solar system in a relativelyshort time, typically less than 1 percentof the age of the solar system. But theparticular Neptune-crossing orbit inwhich Pluto travels is protected fromclose approaches to the gas giant by aphenomenon called resonance libration.Pluto makes two revolutions around thesun during the time that Neptune makesthree; Pluto’s orbit is therefore said to bein 3:2 resonance with Neptune’s. Therelative motions of the two planets en-sure that when Pluto crosses Neptune’sorbit, it is far away from the larger plan-et. In fact, the distance between Plutoand Neptune never drops below 17 AU.

In addition, Pluto’s perihelion—itsclosest approach to the sun—always oc-curs high above the plane of Neptune’sorbit, thus maintaining Pluto’s long-term orbital stability. Computer simula-tions of the orbital motions of the outerplanets, including the effects of theirmutual perturbations, indicate that therelationship between the orbits of Plutoand Neptune is billions of years old andwill persist for billions of years into thefuture. Pluto is engaged in an elegantcosmic dance with Neptune, dodgingcollisions with the gas giant over the en-tire age of the solar system.

How did Pluto come to have such apeculiar orbit? In the past, this questionhas stimulated several speculative and adhoc explanations, typically involvingplanetary encounters. Recently, however,significant advances have been made inunderstanding the complex dynamics oforbital resonances and in identifyingtheir Jekyll-and-Hyde role in producingboth chaos and exceptional stability inthe solar system. Drawing on this body

Migrating Planets Scientific American September 1999 59

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PLANETARY MIGRATION is shown in illustrations of the solar system atthe time when the planets formed (top left) and in the present (bottom left).The orbit of Jupiter is believed to have shrunk slightly, while the orbits ofSaturn, Uranus and Neptune expanded. (The inner planetary region was notsignificantly affected by this process.) According to this theory, Pluto wasoriginally in a circular orbit. As Neptune migrated outward, it swept Plutointo a 3:2 resonant orbit, which has a period proportional to Neptune’s(above). Neptune’s gravity forced Pluto’s orbit to become more eccentric andinclined to the plane of the other planets’ orbits.


of knowledge, I proposed in 1993 thatPluto was born somewhat beyond Nep-tune and initially traveled in a nearly cir-cular, low-inclination orbit similar tothose of the other planets but that it wastransported to its current orbit by reso-nant gravitational interactions with Nep-tune. A key feature of this theory is thatit abandons the assumption that the gas-giant planets formed at their present dis-tances from the sun. Instead it proposesan epoch of planetary orbital migrationearly in the history of the solar system,with Pluto’s unusual orbit as evidence ofthat migration.

The story begins at a stage when theprocess of planetary formation was al-most but not quite complete. The gasgiants—Jupiter, Saturn, Uranus andNeptune—had nearly finished coalesc-ing from the solar nebula, but a residu-al population of small planetesimals—

rocky and icy bodies, most no largerthan a few tens of kilometers in diame-ter—remained in their midst. The rela-tively slower subsequent evolution ofthe solar system consisted of the scat-tering or accretion of the planetesimalsby the major planets [see illustration onpage 56]. Because the planetary scatter-ing ejected most of the planetesimal de-bris to distant or unbound orbits—es-sentially throwing the bodies out of thesolar system—there was a net loss of or-bital energy and angular momentum

from the giant planets’ orbits. But be-cause of their different masses and dis-tances from the sun, this loss was notevenly shared by the four giant planets.

In particular, consider the orbital evo-lution of the outermost giant planet,Neptune, as it scattered the swarm ofplanetesimals in its vicinity. At first, themean specific orbital energy of the plan-etesimals (the orbital energy per unit ofmass) was equal to that of Neptune it-self, so Neptune did not gain or lose en-ergy from its gravitational interactionswith the bodies. At later times, howev-er, the planetesimal swarm near Nep-tune was depleted of the lower-energyobjects, which had moved into thegravitational reach of the other giantplanets. Most of these planetesimalswere eventually ejected from the solarsystem by Jupiter, the heavyweight ofthe planets.

Thus, as time went on, the specificorbital energy of the planetesimals thatNeptune encountered grew larger thanthat of Neptune itself. During subse-quent scatterings, Neptune gained or-bital energy and migrated outward.Saturn and Uranus also gained orbitalenergy and spiraled outward. In con-trast, Jupiter lost orbital energy; its lossbalanced the gains of the other planetsand planetesimals, hence conserving thetotal energy of the system. But becauseJupiter is so massive and had so much

orbital energy and angular momentumto begin with, its orbit decayed onlyslightly.

The possibility of such subtle adjust-ments of the giant planets’ orbits wasfirst described in a little-noticed paperpublished in 1984 by Julio A. Fernan-dez and Wing-Huen Ip, a Uruguayanand Taiwanese astronomer duo work-ing at the Max Planck Institute in Ger-many. Their work remained a curiosityand escaped any comment among plan-et formation theorists, possibly becauseno supporting observations or theoreti-cal consequences had been identified.

In 1993 I theorized that as Neptune’sorbit slowly expanded, the orbits thatwould be resonant with Neptune’s alsoexpanded. In fact, these resonant orbitswould have swept by Pluto, assumingthat the planet was originally in a near-ly circular, low-inclination orbit beyondNeptune. I calculated that any such ob-jects would have had a high probabilityof being “captured” and pushed out-ward along the resonant orbits as Nep-tune migrated. As these bodies movedoutward, their orbital eccentricities andinclinations would have been driven tolarger values by the resonant gravita-tional torque from Neptune. (This ef-fect is analogous to the pumping-up ofthe amplitude of a playground swingby means of small periodic pushes atthe swing’s natural frequency.) The final


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maximum eccentricity would thereforeprovide a direct measure of the magni-tude of Neptune’s migration. Accordingto this theory, Pluto’s orbital eccentrici-ty of 0.25 suggests that Neptune hasmigrated outward by at least 5 AU.Later, with the help of computer simu-lations, I revised this to 8 AU and alsoestimated that the timescale of migra-tion had to be a few tens of millions ofyears to account for the inclination ofPluto’s orbit.

Of course, if Pluto were the only ob-ject beyond Neptune, this explanationof its orbit, though compelling in manyof its details, would have remained un-verifiable. The theory makes specificpredictions, however, about the orbitaldistribution of bodies in the Kuiperbelt, which is the remnant of the pri-mordial disk of planetesimals beyondNeptune [see “The Kuiper Belt,” byJane X. Luu and David C. Jewitt; Sci-entific American, May 1996]. Pro-vided that the largest bodies in the pri-mordial Kuiper belt were sufficientlysmall that their perturbations on theother objects in the belt would be negli-gible, the dynamical mechanism of res-onance sweeping would work not onlyon Pluto but on all the trans-Neptunianobjects, perturbing them from theiroriginal orbits. As a result, prominentconcentrations of objects in eccentricorbits would be found at Neptune’s

two strongest resonances, the 3:2 andthe 2:1. Such orbits are ellipses withsemimajor axes of 39.5 AU and 47.8AU, respectively. (The length of thesemimajor axis is equal to the object’saverage distance from the sun.)

More modest concentrations of trans-Neptunian bodies would be found atother resonances, such as the 5:3. Thepopulation of objects closer to Neptunethan the 3:2 resonant orbit would beseverely depleted because of the thor-ough resonance sweeping of that regionand because perturbations caused byNeptune would destabilize the orbits ofany bodies that remained. On the otherhand, planetesimals that accreted be-yond 50 AU from the sun would be ex-pected to be largely unperturbed and stillorbiting in their primordial distribution.

Fortunately, recent observations ofKuiper belt objects, or KBOs, have pro-vided a means of testing this theory.More than 174 KBOs have been dis-covered as of mid-1999. Most have or-bital periods in excess of 250 years andthus have been tracked for less than 1percent of their orbits. Nevertheless,reasonably reliable orbital parametershave been determined for about 45 ofthe known KBOs [see illustration onopposite page]. Their orbital distribu-tion is not a pattern of uniform, nearlycircular, low-inclination orbits, as wouldbe expected for a pristine, unperturbed

planetesimal population. Instead onefinds strong evidence of gaps and con-centrations in the distribution. A largefraction of these KBOs travel in eccen-tric 3:2 resonant orbits similar to Plu-to’s, and KBOs in orbits interior to the3:2 orbit are nearly absent—which isconsistent with the predictions of theresonance sweeping theory.

Still, one outstanding question re-mains: Are there KBOs in the 2:1 reso-nance comparable in number to thosefound in the 3:2, as the planet migrationtheory would suggest? And what is theorbital distribution at even greater dis-tances from the sun? At present, thecensus of the Kuiper belt is too incom-plete to answer this question fully. Buton Christmas Eve 1998 the Minor PlanetCenter in Cambridge, Mass., announcedthe identification of the first KBO orbit-ing in 2:1 resonance with Neptune. Twodays later the center revealed that an-other KBO was traveling in a 2:1 reso-nant orbit. Both these objects have largeorbital eccentricities, and they may turnout to be members of a substantial pop-ulation of KBOs in similar orbits. Theyhad previously been identified as orbit-ing in the 3:2 and 5:3 resonances, re-spectively, but new observations madelast year strongly indicated that theoriginal identifications were incorrect.This episode underscored the need forcontinued tracking of known KBOs in



order to map their orbital distributioncorrectly. We must also acknowledgethe dangers of overinterpreting a stillsmall data set of KBO orbits.

In short, although other explanationscannot be ruled out yet, the orbital dis-tribution of KBOs provides increasinglystrong evidence for planetary migra-tion. The data suggest that Neptune wasborn about 3.3 billion kilometers fromthe sun and then moved about 1.2 bil-lion kilometers outward—a journey ofalmost 30 percent of its present orbitalradius. For Uranus, Saturn and Jupiter,the magnitude of migration was small-er, perhaps 15, 10 and 2 percent, re-spectively; the estimates are less certainfor these planets because, unlike Nep-tune, they could not leave a direct im-print on the Kuiper belt population.

Most of this migration took placeover a period shorter than 100 millionyears. That is long compared with thetimescale for the formation of the plan-ets—which most likely took less than10 million years—but short comparedwith the 4.5-billion-year age of the so-

lar system. In other words, the planetarymigration occurred in the early historyof the solar system but during the laterstages of planet formation. The totalmass of the scattered planetesimals wasabout three times Neptune’s mass. Thequestion arises whether even more dras-tic orbital changes might occur in plane-tary systems at earlier times, when theprimordial disk of dust and gas containsmore matter and perhaps many proto-planets in nearby orbits competing inthe accretion process.

Other Planetary Systems?

In the early 1980s theoretical studiesby Peter Goldreich and Scott Tre-

maine, both then at the California Insti-tute of Technology, and others conclud-ed that the gravitational forces betweena protoplanet and the surrounding diskof gas, as well as the energy lossescaused by viscous forces in a gaseousmedium, could lead to very large ex-changes of energy and angular momen-tum between the protoplanet and the

disk. If the torques exerted on the pro-toplanet by the disk matter just insidethe planet’s orbit and by the matter justbeyond it were slightly unbalanced,rapid and drastic changes in the planet’sorbit could happen. But again, this the-oretical possibility received little atten-tion from other astronomers at thetime. Having only our solar system asan example, planet formation theoristscontinued to assume that the planetswere born in their currently observedorbits.

In the past five years, however, thesearch for extrasolar planets has yieldedpossible signs of planetary migration. Bymeasuring the telltale wobbles of nearbystars—within 50 light-years of our solarsystem—astronomers have found evi-dence of more than a dozen Jupiter-masscompanions in surprisingly small orbitsaround main-sequence stars. The firstputative planet was detected orbiting thestar 51 Pegasi in 1995 by two Swiss as-tronomers, Michel Mayor and DidierQueloz of the Geneva Observatory, whowere actually surveying for binary stars.

In April 1999 astronomer R. PaulButler of the Anglo-Australian Ob-

servatory and his colleagues an-nounced the discovery of what isapparently the first known caseof a planetary system with severalJupiter-mass objects orbiting a sun-like star. (Previously, only systemswith one Jupiter-mass companionhad been detected.) The star is Up-silon Andromedae; it is approxi-mately 40 light-years from our so-lar system and is slightly more mas-sive and about three times moreluminous than our sun.

The astronomers say their analy-sis of the observations shows that Upsilon Andromedae harborsthree companions.The innermost object is at least 70 percent asmassive as Jupiter and is moving in a nearly circular orbit only 0.06AU—or about nine million kilometers—from the star. The outer-most companion object is at least four times as massive as Jupiterand travels in a very eccentric orbit with a mean radius of 2.5 AU—half the radius of Jupiter’s orbit.The intermediate object is at leasttwice as massive as Jupiter and has a moderately eccentric orbitwith a mean radius of 0.8 AU.

If confirmed, the architecture of this system would pose someinteresting challenges and opportunities for theoretical modelsof the formation and evolution of planetary systems.A number of

dynamicists (including myself) havealready determined that the orbitalconfiguration of this putative sys-tem is at best marginally stable.Thesystem’s dynamical stability wouldimprove greatly if there were nomiddle companion. This is note-worthy, as the observational evi-dence for the middle companion isweaker than that for the other two.

The Upsilon Andromedae systemappears to contradict all the theo-rized mechanisms that would causegiant planets to migrate inwardfrom distant birthplace orbits.If disk-protoplanet interactions caused theorbits to decay, the more massiveplanet would most likely be the ear-liest born and hence found at theshortest distance from the star—

contrary to the pattern in the Upsilon Andromedae system. If onlythe innermost and outermost companions are real, the systemcould represent an example of the planet-planet scattering modelin which two massive planets migrate to nearby orbits,then gravi-tationally scatter each other,eventually yielding one in a close,near-ly circular orbit and the other in a distant,eccentric orbit.A difficul-ty with this scenario is that the more massive companion would beexpected to evolve to the small orbit and the less massive one tothe distant orbit—again,contrary to the characteristics of the Up-silon Andromedae system.

Could this system represent a hybrid case of these two scenar-ios—that is,orbital decay caused by disk-protoplanet interactions


A Planetary

System at Last?

UPSILON ANDROMEDAE SYSTEM is believed toinclude three Jupiter-mass companions orbiting thestar (top). Their theorized orbits are much tighterthan Jupiter’s orbit in our solar system (bottom).

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Their observations were quickly con-firmed by Geoffrey W. Marcy and R.Paul Butler, two American astronomersworking at Lick Observatory near SanJose, Calif. As of June 1999, 20 extraso-lar planetary candidates have been iden-tified, most by Marcy and Butler, insearch programs that have surveyed al-most 500 nearby sunlike stars over thepast 10 years. The technique used inthese searches—measuring the Dopplershifts in the stars’ spectral lines to deter-mine periodic variations in stellar veloci-ties—yields only a lower limit on the

masses of the stars’ companions. Mostof the candidate planets have minimummasses of about one Jupiter-mass andorbital radii shorter than 0.5 AU.

What is the relationship between theseobjects and the planets in our solar sys-tem? According to the prevailing modelof planet formation, the giant planets inour solar system coalesced in a two-stepprocess. In the first step, solid planetesi-mals clumped together to form a proto-planetary core. Then this core gravita-tionally attracted a massive gaseous en-velope from the surrounding nebula.This process must have been completedwithin about 10 million years of the for-mation of the solar nebula itself, as in-ferred from astronomical observationsof the lifetime of protoplanetary disksaround young sunlike stars.

At distances of less than 0.5 AU froma star, there is insufficient mass in theprimordial disk for solid protoplane-tary cores to condense. Furthermore, itis questionable whether a protoplanetin a close orbit could attract enoughambient gas to provide the massive en-velope of a Jupiter-like planet. One rea-son is simple geometry: an object in atight orbit travels through a smallervolume of space than one in a large or-bit does. Also, the gas disk is hotterclose to the star and hence less likely tocondense onto a protoplanetary core.These considerations have arguedagainst the formation of giant planetsin very short-period orbits.

Instead several theorists have suggest-ed that the putative extrasolar giantplanets may have formed at distancesof several AU from the star and subse-quently migrated inward. Three mecha-nisms for planetary orbital migrationare under discussion. Two involve disk-protoplanet interactions that allowplanets to move long distances fromtheir birthplaces as long as a massivedisk remains.

With the disk-protoplanet interactionstheorized by Goldreich and Tremaine,

the planet would be virtually locked tothe inward flow of gas accreting ontothe protostar and might either plungeinto the star or decouple from the gaswhen it drew close to the star. The sec-ond mechanism is interaction with aplanetesimal disk rather than a gas disk:a giant planet embedded in a very mas-sive planetesimal disk would exchangeenergy and angular momentum with thedisk through gravitational scattering andresonant interactions, and its orbit wouldshrink all the way to the disk’s inneredge, just a few stellar radii from the star.

The third mechanism is the scatteringof large planets that either formed in ormoved into orbits too close to one an-other for long-term stability. In this pro-cess, the outcomes would be quite un-predictable but generally would yieldvery eccentric orbits for both planets. Insome fortuitous cases, one of the scat-tered planets would move to an eccentricorbit that would come so near the star atits closest approach that tidal frictionwould eventually circularize its orbit; theother planet, meanwhile, would bescattered to a distant eccentric orbit. Allthe mechanisms accommodate a broadrange of final orbital radii and orbitaleccentricities for the surviving planets.

These ideas are more than a simpletweak of the standard model of planetformation. They challenge the widelyheld expectation that protoplanetarydisks around sunlike stars commonlyevolve into regular planetary systems likeour own. It is possible that most planetsare born in unstable configurations andthat subsequent planet migration canlead to quite different results in each sys-tem, depending sensitively on initialdisk properties. An elucidation of therelation between the newly discoveredextrasolar companions and the planetsin our solar system awaits further theo-retical and observational developments.Nevertheless, one thing is certain: theidea that planets can change their orbitsdramatically is here to stay.


The Author

RENU MALHOTRA did her undergraduate studies at the Indi-an Institute of Technology in Delhi and received a Ph.D. in physicsfrom Cornell University in 1988. After completing postdoctoral re-search at the California Institute of Technology, she moved to hercurrent position as a staff scientist at the Lunar and Planetary Insti-tute in Houston. In her research, she has followed her passionate in-terest in the dynamics and evolution of the solar system and otherplanetary systems. She also immensely enjoys playing with her four-year-old daughter, Mira.

Further Reading

Newton’s Clock: Chaos in the Solar System. Ivars Peterson.W. H. Freeman and Company, 1993.

Detection of Extrasolar Giant Planets. Geoffrey W. Marcyand R. Paul Butler in Annual Review of Astronomy and Astro-physics, Vol. 36, pages 57–98; 1998.

Dynamics of the Kuiper Belt. Renu Malhotra et al. in Proto-stars and Planets IV. Edited by V. Mannings et al. University ofArizona Press (in press). Available at http://astro.caltech.edu/~vgm/ppiv/preprints.html on the World Wide Web.

in the case of the innermost object andmutual gravitational scattering for the oth-er two companions? Perhaps entirely differ-ent formation and evolution processes arealso involved, such as the fragmentation ofthe protostellar gas cloud that is thought toproduce multiple-star systems and browndwarf companions.

If only the innermost and outermostcompanions are real, the system would bearchitecturally similar to classic triple-stellarsystems consisting of a tight binary with adistant third star in an eccentric orbit. Atpresent, we have only speculations for theUpsilon Andromedae system. More obser-vations and further analysis should helpfirm up the evidence for the number ofcompanions and for their masses and or-bital parameters.

The discovery methods employed so farare unable to detect planetary systems likeour own because the stellar wobble fromEarth-size planets in close orbits—or fromJupiter-size planets in more distant orbits—is below the observable threshold. There-fore, it would be premature to leap to con-clusions about the astronomical frequencyof Earth-like planets. Our understanding ofthe origin of the recently identified com-panions to sunlike stars is sure to evolveand thereby expand our understanding ofour own solar system. —R.M.

SA


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Migrating Planets