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e x t r a s o l a r p l a n e t s special r e p o r t How do you make a

A YOUNG gas-giant clears a swath giant through the that formed it in this artist’s concept. In 2004, astrono- mers using NASA’s Spitzer Space Telescope detected such a clearing around the million- year-old CoKu Tau 4, which lies about ? 420 light-years away. NASA/JPL-Caltech/Robert Hurt

or several decades, theorists This leaves scientists with no widely what astronomers think they know Scientists endlessly debate theories have worked to understand the accepted mechanism for formation of the about how terrestrial form. ’s origin, with mixed planets found beyond the solar system. of how giant planets form, but results. All agree that , In a field just over a decade old, Building rocky planets only observations will settle the , , and — explaining why we see what we see is an Earthlike planets grow as micrometer- question. ⁄ ⁄ ⁄ BY Alan P. Boss the so-called terrestrial planets — important first step, but the goal is to size grains collide and stick together, Fformed as progressively larger rocky make predictions future observations forming pebbles. The pebbles collide to bodies banged together. But theories now can test. Yet, even as theorists go back make boulders, which smash together in vogue have trouble accounting for the and forth over giant-planet formation, to build kilometer-size . KNOTS OF GAS appear in the disk of matter solar system’s massive gas giants, astronomers have discovered evidence Planetesimals are massive enough that around a young star in this illustration. Some of and . That’s a problem because for the existence of rocky extrasolar their own helps them grow fur- these knots will give rise to gas-giant planets like most of the more than 200 planets. These objects, with masses sev- ther. That is, two bodies that would Jupiter. Dana Berry, SkyWorks Digital astronomers know about are also giants. eral times Earth’s, appear to validate otherwise miss one another collide

© 2010 Kalmbach Publishing Co. This material may not be reproduced in any form without permission from the publisher. www..comwww.astronomy.com 39 up and temperature, tells the tale.temperature, the and tells up make- chemical revealsits centralstar’swhich the atspectrum, looking directly.But planets. formed has star young a that indication first the center.missing be maya It with disk a indicates star’sthis low-temperaturespectrum in A bump disk’spresence. wavelengths, (infrared)revealingthe long at energy its of most emits materialcooler The contribution. spectral own its makes star the surrounding gas and dust of disk warm The wavelengths.longer at energy less wavelengthsand short at energy its of most star’sthe lawstemperature.and emits star This physicalon based pattern specific followsa wavelengthgivenany at light of distribution The Disks Deducing THE DEBRIS DISKS DEBRIS THE that produce planets are often too small for telescopes to image totelescopes for small too often are planets producethat Star withadiskgap Star withafulldisk Star withadiskgap Star withafulldisk Star withnodisk Star withnodisk Astronomy : R oen Kell oen y

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Brightness Light distribution Light distribution Light distribution Wavelength Wavelength Wavelength Wavelength Wavelength Wavelength s (Wiley, 1998). System(Wiley, s for Looking of author Washingtonthe of and Institution P.Alan Carnegie the at astrophysicist an is Boss picture. basic this The raceis on to find Recent support discoveries lend further to giant? ice or Super-Earth a young star. comparativelythe dust benign disk around made scientists optimistic it could work in could work insuch environment ahostile from stellar the explosion. That process the ets appear to have formed out of debris process. In of case the pulsar, the plan the - first confirmationthe of accumulative this ets orbiting pulsar the PSR B1257+12was planets are on stable orbits. major collision and occurs surviving the outcome until uncertain is last highly the otic process, one final the inwhich building appears an to be intrinsically cha- of how planets terrestrial formed. Planet- astronomers areasonably complete picture that later accreted to form the . ated a spray of debris trapped in Earth orbit struck the early Earth off-center. This cre else. Closer to home, a Mars-size embryo left Mercury with an iron-rich core and little that became Mercury. The crash stripped most of the rocky material from the speeds up to 22,000 mph (36,000 km/h). between planet-size bodies colliding at punctuated by incredibly energetic impacts which takes tens of millions of years, is larger bodies. This final phase of growth, embryosetary collide and merge into even these orbits grow eccentric enough, plan orbits become increasingly elliptical. Once over many orbits, their initially circular objects interact with one another’s gravity in which embryos “compete.” As these moving on nearly circular orbits. hundreds of lunar-mass embryos planetary solar system might contain aswarm of ies. In as little as 100,000years, anascent stronger , gobble- up bod smaller The most massive planetesimals, with their large as Moon the is arunaway process. of planetesimals into embryos as planetary together.them gravitational their when attraction pulls The 1992 discovery of planThe 1992discovery Earth-mass - models of processesDetailed these give In our solar system, such a collision After this comes a longer-lasting phase Astronomers growth further the think arths: : T he Race to Find New Solar Solar FindNew to Race he - - like our own are frequent denizens of our own ourare frequentdenizens like habitable worlds astronomers , expect commonly accompany disks such young Because protoplanetarydisk. a in occurring processes result of the inescapable almost operatesets also elsewhere inour galaxy. creating solar the system’s innermost plan - sional accumulation process responsible for starstheir to seems prove that colli the - multi-Earth-mass planets orbiting to close an icegiant. Either way, presence the of of water, ammonia, or carbon dioxide —it’s world the then may contain alarge fraction transiting exoplanet is Earth’s half density, they’ve found asuper-Earth. If, however, a solar system —astronomers sure be will densest the planetsimilar to Earth, inthe planet’sthe density. If density the — is high Thus, astronomers able be to estimate will that on depends planet’s the diameter. transits star’s dimthe light by an amount infront passes perspective, of its star. Such multi-Earth-mass planet that, from our Earth’s —right ice-giant inthe range. have 10and masses between 20times or Venus. of newfound the Several planets planets and than to Earth a composition to closer that of ice-giant the “super-Earths”the made couldof be ice— are probably composed of rock and metal, normal stars. But pulsar the planets while masses as low circling as 5.5Earths around extrasolaridentified planets with inferred far,planets So like Earth. astronomers have highest-density regions, where planet-forming clumps exist. exist. clumps planet-forming where regions, highest-density the show orange and Blue disk’smidplane. the in dust and gas of density the sent from the the from times 20 about equaling protostar the around region a show Both evolution. of years 300 about after disks the show images computer-simulation these of Both (right). star single a orbiting disk a within than (left) system star binary A MAKING JUPITER-MASS MAKING Moreover,an appearbe to planets Eventually, astronomers a find will lan P. Bo ss , C , S arn un. The young protostar lies unseen at the center of each disk. Colors repre Colors disk. each of center the at unseen lies protostar young The un. egie egie Ins titutio n o f W clumps in a protoplanetary disk happens more readily in a in readily more happens disk protoplanetary a in clumps as hi n gto n of directly detecting and studying earthlike planets. Earth-mass of multi-presence by stars the in induced tiny wobbles the detect ableto be will for 2014, launch after scheduled currently PlanetQuest), (SIM MissionPlanetQuest are. NASA’s really Interferometry Space estimateof how common worlds such direct providefirst the andwill planets of earthlike of transits dozens the detect to able be will for 2008, launch in scheduled galaxy.NASA’s Kepler Mission,currently of the same star using the using star same the of study a 2005, worlds.In rocky enormous or giants gas either be could 69830. planets These May, In tune. ASTRONOMERS NOW NOW ASTRONOMERS Astronomers will need telescopes capable

E S arth’s average distance arth’sdistance average wiss researchers announced three such worlds orbit the orbit worlds such three announcedresearchers wiss are finding planets with masses between those of Uranus and Uranus of those between masses with planets finding are Spitzer Spacebelt. Telescopepossible a up turned

- continuum of intermediate possibilities, exist. Two entirely different theories, as well as a roughly 150 gas-giant exoplanets formed. little agreement how Jupiter, Saturn, and the with gas giants is more contentious. There’s about exo-Earth formation, the situation While astronomers largely agree on ideas giants gas Making solid rock/ice core buried beneath a massive of gas and dust, yielding a final planet with a quickly accretes hundreds of Earth-masses gas from the disk. The solid core then ger stable; the embryos rapidly attract more that their gaseous atmospheres are no lon roughly 5 to 10 Earths, they’re so massive inner disk’s final growth phase. 10 million years — much faster than the Earth-masses could grow in less than about believe planetary embryos with several area, too. Because of these effects, theorists Orbits in the outer disk enclose a larger planetary building blocks by 2 or 3 times. the disk boosts the number of potential metal. The addition of the icy particles in coexist as solid particles with rock and volatile substances like water and ammonia etary disk, where cooler temperatures let tom up” in the outer part of a protoplan ects will survive tightening budgets. scopes, but it’s unclear whether these proj Agency (ESA) have planned such space tele planets. Both NASA and the European Space One group believes gas giants form “bot Once the embryos reach masses of www.astronomy.com un-like star HD star -like Nep E - S O - - 41 - - - Saturn Neptune

ing drag forces on solid particles. This pulls the particles toward the arms’ centers, where they’re more likely to collide and grow. The test’s the thing Astronomers developed the core- mechanism several decades ago. At that time, theorists believed Jupiter, Saturn, Jupiter Uranus, and Neptune contained solid cores of about 15 Earth-masses. New models of Jupiter’s interior suggest it possesses a core smaller than 3 Earth-masses. If correct, Jupiter couldn’t have formed by core accre- tion unless it formed with a much bigger core than it now has. Can the cores of giant planets erode over time? If so, then core masses will lose much of their importance in discriminating between Uranus formation scenarios. Saturn appears to have a core mass of 15 to 20 Earths. Why didn’t Sat- urn’s core erode? And why, with its larger JUPITER, the solar system’s most massive planet, may have a core and Neptune best Jupiter’s, too. Did Jupiter’s core erode after it core, didn’t Saturn become the solar system’s only 3 times Earth’s mass. Instead, Saturn has the largest core (15 formed? Or did proto-Jupiter suddenly lose core mass even as it most massive planet? to 20 Earths), and planetary scientists think the cores of Uranus gathered gas? Planetary scientists want to know. Astronomy: Roen Kelly The disk-instability theory tries to explain the core masses of Saturn, Uranus, and Neptune. In this model, each planet Core accretion seems to require several disks, which led to more solids that could and other young stars found by the Spitzer PROTOSTARS appear in RCW 49, which is one of the ’s busiest birthing grounds. began with a mass of around 3 , million years or more to form a gas giant. serve as building blocks for giant-planet Space Telescope. Core accretion doesn’t In this false-color Spitzer Space Telescope image, RCW 49’s older stars appear at the which led to cores of less than 18 Earth- Yet, observations of young stars suggest a cores. However, higher metal content is also seem able to form planets this quickly. cloud’s center in blue, gas filaments in green, and tendrils of dust in pink. Speckled masses. Astronomers think the solar system disk’s gas disappears in a few million years associated with faster inward migration of throughout the dust clouds are more than 300 stars not previously seen. The nebula lies formed in a crowded stellar nursery similar or less. The core-accretion method may be giant planets after they open gaps in the Choices, choices 13,700 light-years away in Centaurus. NASA/JPL-Caltech/E. Churchwell, University of Wisconsin to what we see today in the Orion Nebula. too slow a process to form gas-giant planets disk gas. This effect could explain part of Neither core accretion nor disk instability is The ultraviolet light of nearby massive stars in large numbers. the metal-content correlation. We would a completely developed theoretical mecha- boiled away any excess gas from Saturn, On the other hand, disk instability is fast expect to find giant planets on short-period nism. Both approaches leave major ques- envelope of and gas. Such where they intersect, transient clumps of Uranus, and Neptune. This prevented them enough to form a gas giant in even the orbits around metal-rich stars. tions unanswered. For instance, how does “core accretion” is the most popular mecha- high-density gas appear within 1,000 years. from outgrowing Jupiter. shortest-lived protoplanetary disk. Most Disk instability seems able to form gas affect core accretion? nism for giant-planet formation, by far. One If these clumps are dense and cool enough, Astronomers lack much in the way of stars form in clusters containing high-mass, giants in both metal-poor and metal-rich How fast can disk instability cool a proto- reason: It uses the same collisional accumu- they’ll contract to higher densities. They’re limits for extrasolar gas-giant cores. One luminous stars. Energetic radiation and planetary disk? Ongoing investigations are lation process astronomers agree must on the path to becoming gas-giant proto- transiting planet (HD 149026b) appears to strong stellar winds from these stars likely addressing these problems. occur in the disk’s inner regions. planets. Astronomers refer to this top-down have a 70-Earth-mass core surrounded by a evaporate disk gas rapidly. The fact that we Disk instability is Perhaps both mechanisms occur in the A “top-down” approach to giant-planet approach as the disk-instability mechanism. 20-Earth-mass gaseous envelope. On the detect so many gas giants suggests disk galaxy, but some environments favor one formation lies at the other extreme. In this Dust particles within a dense clump other hand, the first observed transiting exo- instability is necessary. the dark horse in the over the other. While the impetus for extra- view, the disk’s gas itself begins the process, begin to coagulate and fall toward the pro- planet, HD 209458b, may not have a solid The lower the mass of the host star, the solar-planet studies remains the search for without requiring a solid core. Most of the toplanet’s center. This process takes place in core at all; similar models can explain the more acute core accretion’s time-scale prob- gas-giant-formation habitable worlds, this search also will help protoplanetary disk’s mass resides in hydro- 100,000 years or less — much faster than it sizes of other transiting planets. For the lem becomes. M-type dwarf stars rarely sweepstakes. us learn more about extrasolar gas giants. gen and helium gas — solid particles make will take the clump itself to contract to moment, such observations are little help in form gas giants by core accretion. Disk Ground-based facilities like Chile’s up only 1 percent. In recent core-accretion planetary densities. A Jupiter-mass proto- narrowing down how giant planets form. instability, however, is rapid enough that Atacama Large Millimeter Array and the models, astronomers assume the disk has planet could then end up with a rock/ice Spectroscopic surveys of extrasolar plan- M dwarfs can build gas giants in abun- disks. A key test will be to see if metal-poor planned Giant Segmented Mirror Tele- about 10 percent of the star’s mass; this core of up to about 6 Earth-masses without ets indicate that about 10 percent of Sun-like dance. While the frequency of gas giants stars host long-period gas giants. At least scope will work with space-based instru- allows gas giants to form quickly. accreting any solids. Disk instability is the stars have gas-giant planets between 0.1 to around M dwarfs appears to be less than one such system exists: a Jupiter-mass world ments like NASA’s Spitzer, Kepler, and the But such a massive disk is likely to be on dark-horse candidate in the gas-giant- 10 Jupiter-masses and orbital periods of a that around G dwarfs like the Sun, it isn’t orbits a pulsar and its white-dwarf compan- future SIM PlanetQuest and James Webb the verge of gravitational instability. This formation sweepstakes. It’s championed by few years or less. Another 10 percent or so of zero, and these planets may require the ion in the M4. The dwarf telescopes. These studies will provide the means any lumps in the gas can grow by only a few wild-eyed theorists who like these stars may host longer-period giant disk-instability mechanism. has 20 to 30 times less metal than the Sun. observational tests needed to determine pulling more gas onto themselves through long odds — including myself. planets with orbit sizes similar to Jupiter’s. Spectroscopic surveys have focused on Some giant planets seem to form in less whether theoreticians’ amusements have a their own gravitational forces. In a few Between these two extremes lie hybrid While the census is not yet complete, it metal-rich stars, as studies show these stars than a million years. Two examples: the place in the real . orbital periods, this runaway process leads mechanisms. They make giant planets by seems many nearby stars harbor gas-giant harbor more short-period planets than do imaging of a possible Jupiter-mass proto- Come to www.astronomy.com/toc to the formation of spiral arms much like combining collisional accumulation of sol- planets. To explain the prevalence of giant metal-poor stars. This is commonly taken planet around the young star GQ Lupi, and ONLINE those in spiral galaxies. Multiple spiral arms ids and spiral arms in an unstable gas disk. planets, astronomers need at least one robust as proof of core accretion’s dominance: the spectroscopic evidence for gaps caused EXTRA to see simulations of planetary migration and disk instability. form and collide with each other, and, Spiral arms have the desirable trait of creat- formation mechanism. Metal-rich stars presumably had metal-rich by a in the disks of CoKu Tau 4

42 astronomy ⁄ ⁄ ⁄ October 06 www.astronomy.com 43