Less than a decade ago, planetary scien-tists were working with a tiny data set:the nine members of our Solar System.

But the past few years have been a boomtime for planet hunters — more than 100planets orbiting other stars have now beenlogged. As new detection methods comeinto use, this tally is certain to climb higher.

Not everyone is celebrating, however.Extrasolar planets have peculiar properties,and our understanding of how planets form,which was incomplete even before the newdata became available, now looks even shakier.The newly discovered bodies have strange,highly elliptical orbits. They are also far closer to their stars than equivalent planets inour Solar System. Amid the thrill of discov-ery, planetary scientists are wondering howto make sense of the processes that shapedthese strange new worlds.

The main method of planet detection is theradial-velocity survey. As a planet orbits a star,its gravity makes the star wobble, causing aperiodic change in the spectrum of the star’semitted light from which astronomers inferthe planet’s presence. This technique will soonbe joined by two more: astrometry, in whichthe star’s wobble is measured by tracking itsposition, and the transit method, in which thepassage of the planet in front of the star isobserved. Both are still in their infancy — onlyone planet has been spotted in transit studies,and none has yet been definitively identified byastrometry.

In terms of mass, the new planets are simi-lar to Jupiter, weighing between one-tenth andten times as much — the majority fall between0.75 and 3.0 jovian masses. Measuring size ismore difficult, as only transit studies can pro-vide information on the object’s radius. Theplanet observed using the transit method —an object orbiting a star in the constellation ofPegasus1 — is slightly larger than Jupiter.

But that’s where the similarities end. Theorbits of most extrasolar planets follow ellip-tical paths, in contrast to the near-circularorbits of our Solar System’s giant planets.They also orbit much closer to their parentstars, most at a distance of less than 2 astro-nomical units (1 AU being the distancebetween Earth and the Sun), compared withmore than 5 AU for Jupiter.

It is these properties that seem to defy pop-

ular models of planetary formation. The twomain theories each start with a slowly spin-ning ball of gas. The hot, central part becomesa star, while the material farther out is flat-tened by its rotation into a cloud known as aprotoplanetary accretion disk. This providesthe raw materials from which planets form.

Rocky startFrom there on, the process is open todebate, with the answer partly dependingon the size of the disk. The core-accretionmodel, which dates from the 1960s, arguesthat planets start life as small chunks ofrock, dust and sand-grain-sized debris thatcome together through collisions. As therocky core grows, its gravitational pullscoops up more dust and gas from the disk.If the core is heavier than a few Earth masses,it accretes enough gas over a few millionyears to become a gas giant like Jupiter andSaturn. Less-massive cores result in rockyplanets like Earth.

This model ran into problems even beforeextrasolar planets were identified. For onething, it seems to take too long. Accretion disksare thought to evaporate within a million yearsor so, probably as a result of the stream of elec-trically charged particles that all stars emit, orof bombardment from high-energy ultravio-

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let photons from other nearby stars. The main rival theory, which also surfaced

in the 1960s, avoids this problem. Known asthe disk-instability model, it proposes that, inlarger disks, patches of denser gas can form andpull in more gas — leading, in some cases, to asudden collapse that forms one or more plan-ets. Such collapses do not occur in the core-accretion model, either because the disk is notlarge enough to produce them, or because anysmall instability that forms will tend to spreadthroughout the disk, restoring stability.

Planets are thought to form more rapidlyin the disk-instability scenario. Last autumn,Lucio Mayer, a theoretical astronomer then atthe University of Washington in Seattle,described high-resolution computer simula-tions of protoplanetary disks using the disk-instability model2. Together with colleagueselsewhere in North America, Mayer showedthat giant planets could form in as little as1,000 years. The difference in planet-formingrates is probably the most important distin-guishing characteristic between the two mod-els, and is a boost for the disk-instability idea,says Alan Boss, a theoretical astrophysicist atthe Carnegie Institution of Washington.

Others urge caution. Jack Lissauer, a planetary scientist at NASA’s Ames ResearchCenter in Moffett Field, California, says that

Our knowledge of planets outside our Solar System has been transformedin the past few years. But these new-found worlds don’t look much like ourplanetary neighbours, and no one is quite sure why. Dan Falk investigates.

Worlds apart

Cloudy picture: computer simulations have yet to nail down the finer points of planetary evolution.

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the resolution of the computer models is stilltoo poor to give conclusive results. Perhapsmore importantly, the new data on extrasolarplanets do not sit happily with either theory.The models have trouble explaining, forexample, why Jupiter-sized planets are createdrather than brown dwarfs — objects that areintermediate in size between planets and stars.“You would expect the mass of planets torange from Jupiter mass up to stellar masses,”says Douglas Lin, an astrophysicist at the Uni-versity of California, Santa Cruz. There oughtto be just as many brown dwarfs as Jupitersorbiting Sun-like stars — something thatobservations have not turned up.

Inner workingsOther aspects of the new data are causingproblems for both models. Neither, for exam-ple, accounts for the proximity of the extra-solar planets to their stars. There isn’t muchmaterial in the inner region of the disk, andthe particles there should have enough energyto resist clumping. The solution, astronomerssuggest, is that giant planets form farther out and then migrate inwards3 as a result of interactions between the disk and the planet.The mechanism differs in the two models,but the end result is that young planets sailthrough the disk towards the star.

But this raises another question: whatstops the planet from ploughing into its parent star? Several mechanisms have beensuggested. One option is that the migrationends when the disk evaporates — but it’s notclear whether this can happen quicklyenough, as migration occurs on a roughlymillion-year time scale. Another option isthat the planet’s gravitational pull distortsthe shape of the star, and that this in turnaffects the pull of the star on the planet insuch a way as to balance the planet’s inwardmovement. Finally, it could be that the star’smagnetic field clears out the inner disk byrepelling electrically charged particles. In

this situation, says Boss, the inner 0.5 AU ofthe disk would be empty — and few extra-solar planets have orbital radii much smallerthan this. “It’s attractively simple,” says Boss.

Such explanations are plausible, but thereis no way of knowing which is correct. Even if this issue is resolved, it is still unclearwhether planets form by disk instability or bycore accretion before they begin their migra-tion. And on top of that, astronomers arestruggling to explain why so many extrasolarplanets follow elliptical paths, as both forma-tion models predict roughly circular orbits.The best explanation so far proposed is basedon the gravitational tug-of-war between dif-ferent planets in a multi-planet system.

The data needed to eradicate this andother uncertainties could come from a seriesof new observatories that will allowresearchers to study protoplanetary disks,perhaps catching planetary systems in the actof formation. Telescopes that are sensitive tomillimetre-range electromagnetic radiationare crucial, as these wavelengths passthrough the gas and dust surrounding such

systems, allowing astronomers to see into theheart of the disk. And although a host star isbrighter than its disk across the entire spec-trum, the star’s radiation is less obscuring atmillimetre wavelengths than at optical ones.

Two new telescope arrays operating atthese wavelengths will soon start takingreadings. The Sub-Millimeter Array onMauna Kea in Hawaii should get to worklater this year, and the Combined Array forResearch in Millimeter-wave Astronomy, tobe located at a high-altitude site in Califor-nia, could come online by 2005. But of all thedetectors being planned, the most ambitiousis the Atacama Large Millimeter Array(ALMA), a joint effort by Europe, Japan, theUnited States and Canada.

Array of hopeALMA will use 64 receiving dishes, each 12metres across, perched high in the Andeanplateau in Chile — this array will be equivalent to a single dish 14 kilometresacross. Water vapour in Earth’s atmosphereis the main enemy of such observations, butthe Atacama desert is one of the driest locations on the planet. “It’s arguably thebest site one can send humans to on a rou-tine basis,” says Lee Mundy, an astronomerat the University of Maryland in CollegePark, and a member of ALMA’s scientificadvisory committee.

At one-millimetre wavelengths, ALMAwill have a resolution of 0.1 arc-seconds orbetter, equivalent to the Hubble Space Tele-scope’s performance at visible wavelengths.This will allow astronomers to see featuresjust a few AU across in nearby planetary sys-tems, and might be enough to observe theeffects of young planets ploughing throughtheir accretion disks. The array could begintaking readings in 2007.

Once data from these arrays start to flow,our understanding of protoplanetary disks islikely to be transformed. Together with thenew planet-detection techniques, which arepoised to reveal the true diversity of extra-solar worlds, they should provide the data toeliminate some theories, and constrain others. “We’re getting the census done rightnow,” says Boss. “In another ten years we’llreally know what’s out there.” ■

Dan Falk is a freelance science writer in Toronto.

1. Cody, A. M. & Sasselov, D. D. Astrophys. J. 569, 451–458 (2002).

2. Mayer, L., Quinn, T., Wadsley, J. & Stadel, J. Science 298,

1756–1759 (2002).

3. Trilling, D. E. et al. Astron. Astrophys. 394, 241–251 (2002).

The Extrasolar Planets Encyclopaedia➧ www.obspm.fr/planetsSub-Millimeter Array➧ http://sma-www.harvard.eduCombined Array for Research in Millimeter-waveAstronomy➧ www.mmarray.orgAtacama Large Millimeter Array ➧ www.eso.org/projects/alma➧ www.alma.nrao.edu

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Starry eyes: the Sub-Millimeter Array in Hawaii will peer into the hearts of distant planetary systems.

Alan Boss: eagerly awaiting new planetary data.

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