Downloaded by guest on September 27, 2021 www.pnas.org/cgi/doi/10.1073/pnas.1305211110 a carbon-richplanet(Bottom). mature terrestrial planetsorbitingBetaPictoris.Awater-rich planetsimilartotheEarth( Top Artist’s conceptionofthestarBeta Pictoris.Intheinsetpanels,compositionoftwopossible like our cometsbodies, and asteroids. Now, some stars are ringed by belts of rocky small like home: In addition to multiple planets, standards, systems these are much very dents intriguingly look familiar. By many systems resia fewsmaller planetary - whose mos holds many exotic and diverse objects. ers that orbit star their backward—the cos- yearwhose is shorter than an Earthday, oth- mightthey have diamond rinds, worlds double-sunned worlds. Planets dense so and casting set, double shadows over their skiesarealien home to two suns that rise other’s inthe rises periodically sky. Other two planets live together close so that each 210 light-years away. Around another star, A planet than Mercury smaller circles astar Science Writer Nadia Drake System’s originsthatisexcitingastronomers. around otherstarsare, it’s theirpotentialtoshed lightonourSolar Amazing asthediscoveriesofplanets,comets,andasteroid belts on our own systems light shed Extrasolar And yet, among are oddities eclectic the NEWS FEATURE

toward surprisingly the mundane. “In a ing an almost inexorable march Copernican from realm the of unique, the and advanc- System—even Solar the Earth—and further about our own System,” Solar she says. features and to them what link we know “Our challenge now is to common identify of University the of Arizona, Tucson, AZ. stellar systems,” says astronomer Kate Su System’s past and future other by observing System.Solar events that gave to birth and shaped our systems to more learn about celestial the systems, inthese objects andseen the use chitecture to predict presence the of un- astronomers System’s Solar can the use ar These commonThese features are moving “We definitely learn more Solaraboutthe PNAS|April9,2013 |vol.110no.155735–5737 | ); Image courtesy of NASA/FUSE/Lynette Cook. - systems. observing otherstellar past andfuture by about theSolarSystem’s We more definitelylearn Earth and Sun). the Earth But aside from Pluto, (1 AU equals meanthe distance between Neptune, at 30astronomical units (AU) Kuiperthe beyond orbit begins the Belt of dwarf planet Pluto. In our System, Solar ets and frozen worlds like five-mooned the sion of our own Kuiper home Belt, to com- ered in 1984 (1), is like an extrasolar ver dust and debris rocks. field, This icy discov- is from sun—is the Earth belt acold, of icy 90 and from farther 120times star the than aroundserved ahandful of other stars. belted configurationthat has been ob- only of debris separated by aclean gap, adouble- version of our own, with two rocky bands tions. night and imagination the fueled of genera - centuries has helped travelers navigate the already as massive twice as sun, the and for a bulging equator. Though young, Vega is butical, squashed, with flattenedpoles and away, star the spins rapidly so it’s not spher stars Deneb and Altair. Just 25light-years where it forms with triangle asummertime iar star burns brightly sky,in northern the kinship to ours is that of Vega. This famil- One stellar system with arecently identified Chasing Comets extraordinary way, abnormal.” Congress inWashington, “or, DC, insome Chair of Astrobiology at of US Library the tem—is ‘normal,’” says David Grinspoon, if our own world—and our own Sys Solar - broader we’re sense, to understand trying Looming on Vega’s outskirts—between Vega’s extrasolar system is like alarger - -

NEWS FEATURE objects populating the Kuiper Belt weren’t have been thought,” says astronomer Bar- change as more sensitive instruments come observed until 1992 (2), though the belt ry Welsh of the University of California, online. Finding these inner debris disks is had been a hypothetical reservoir for short- Berkeley, CA. “Two or 3 years ago, we had a trickier than detecting cooler, more dis- period comets. “It can fairly be said that we thousand exoplanets and only four comets. tant disks, because the inner disks radiate knew about Kuiper Belts around other stars So we tried to come up with a formula for at a wavelength that’s obscured by the star’s 8 years before we began to know that the which stars might harbor exocomets.” glare. Sun had one, too,” says Karl Stapelfeldt, an Welsh and Sharon Montgomery, from To spy on Vega’s inner disk, Su and her astronomer at National Aeronautics and Clarion University, Clarion, PA, selected a colleagues used two space telescopes— Space Administration (NASA)’s Goddard group of young, rapidly rotating stars—the NASA’s Spitzer and the European Space Space Flight Center in Greenbelt, MD. Agency’s Herschel Space Observatory. A Like the Kuiper Belt, Vega’s disk is a likely “Wherever there are band of warm dust radiating in the infrared reservoir of comets, which would streak planets, there are going revealed the disk, the probable result of col- through the young exosystem more fre- liding asteroids. The team also took a closer quently than in our own. “It takes time for to be comets. Or look at Fomalhaut and found a system with the population of comets to get used up by asteroids. Or dust.” similar proportions—two belts separated collisional grinding and gas evaporation,” by a clean gap. Stapelfeldt says. same type as Vega—that have known debris The sculpted gap between the two rings is Vega and the sun are not the only two disks (8). Then, from the McDonald Ob- the likely handiwork of at least one planet, stars hosting icy debris rings: Cold, dusty servatory in Texas, they searched for star- even though no planets have been observed ribbons have been found around several grazing comets passing between their host around Vega. Yet. stars, including Beta-Pictoris, Fomalhaut, stars and Earth. “We look for tiny signatures “Planets are needed in between the two and HR 8799 (3–5). Another stellar sys- in the spectrum of the star,” says Montgom- belts to keep the gap clear of dust,” says Sta- tem, Eta Corvi, was observed being pelted ery. Those signatures are produced as the pelfeldt, a member of the asteroid-detect- by what scientists surmise was a hailstorm gas in an evaporating comet’s tail alters the ing team. Several planets, Jupiter-mass or of comets, flung inward from one of these star’s light that reaches Earth. At the end of smaller, might be gravitationally sweeping more distant reservoirs (6). NASA’s Spitzer the observing run, Welsh and Montgomery up small bodies from the gap—a familiar Space Telescope studied the system in the had found evidence for six new exocom- architecture that is also found around the infrared and found evidence for water, ice, ets—about one for every four stars they star HR 8799, where four giant planets orbit organics, and shattered rock near the star— searched. between two dusty belts. perhaps the result of comets colliding with Now, says Welsh, it appears that exocom- But identifying planets around Vega is a planet. ets are probably extremely common—at a project for the future—perhaps for the Eta Corvi is now about one billion years least as common as exoplanets. “Wher- James Webb Space Telescope, scheduled to old—the same age the Sun was when a sim- ever there are planets, there are going to launch in 2018. The telescope’s enormous, ilar cataclysm happened in our Solar Sys- be comets. Or asteroids. Or dust,” Welsh 6.5-m primary mirror, with about seven tem. During this period of adolescent rebel- says, noting that planetary systems are like times more collecting area than the Hub- lion, the giant planets migrated outward, a construction sites; even after the planets ble Space Telescope, could image the giant march that sent small, rocky bodies hurtling are built, there’s material left over. “Bits of planets directly with the help of a light- toward the inner Solar System. Scars from wood, nails—that’s basically the debris left blocking star shade. Or, an instrument such this period, called the Late Heavy Bom- over that can form comets,” he says. as the Gemini Planet Imager, scheduled to bardment, are still carried on objects like go online in the Southern Hemisphere in the Moon, whose surface contains the cra- Alien Asteroids late 2013, should be able to see Vega’s plan- tered record of these early growth spasms. Closer to Vega is a second debris disk, ets if it moves north, says Franck Marchis, a Now, astronomers estimate that as many warmer and more recently discovered than planetary astronomer at the SETI Institute as 20% of nearby stars come with cold, the icy belt. These two belts are situated in Mountain View, CA. comet-bearing belts, reservoirs for the type much like the Sun’s, with the colder disk of objects thought to have delivered water being roughly 10 times farther out than its Moon Hunt to Earth. warmer counterpart. Giant planets, whether in the Vega system But the evidence for exocomets doesn’t Though astronomers haven’t actually seen or elsewhere, are tantalizing candidates to simply lie in cold debris disks. Individual them, Vega’s asteroids are probably dense search for the presence of exomoons. A comets have been detected, the first in 1987 and rocky, similar to ours. This inner, warm large, Earth-sized moon, for example, could around the star Beta-Pictoris (7). A half- belt, announced in January 2013*, is one of provide a suitable niche for life. dozen new exocomets, announced in Janu- only a few that have been directly identified. “We have no idea whether big moons are ary 2013*, more than double the previous Others have been detected around Fomal- out there,” says David Kipping, an astrono- total. haut, HR 8799, and a star called Epsilon Er- mer at the Harvard-Smithsonian Center for “They’re certainly in more places than idani (8). So far, scientists estimate that per- Astrophysics in Cambridge, MA. “We’re haps 10% of large stars have such asteroid searching for an object we’ve never seen

*221st meeting of the American Astronomical Society, January belts; in smaller Sun-like stars, the numbers b e fore .” 6–10, 2013, Long Beach, CA. drop to about 1%. Those figures are likely to Kipping and his colleagues are looking

5736 | www.pnas.org/cgi/doi/10.1073/pnas.1305211110 Drake Downloaded by guest on September 27, 2021 Downloaded by guest on September 27, 2021 Drake white dwarfs. After death,white a dwarf’s sic astronomers are already studying other sphere size of the Earth. with asolar mass of material packed into a a densestellar corpse,awhite dwarf star shedit its will outer layers and shrivel into and expands, becoming ared giant. Then, reddenlight with age, will as star the cools ping says. most beautifulthings Ican imagine,” Kip- that’s filling your sky—it’s just one theof moon looking up [and agas giant seeing] pipeline. the craft stays healthy,data those shouldbe in detection; but as long as Kepler the space- many transits for needed exomoon asolid far from stars their to have produced the candidates bestThe planetary are too still year,this an be exomoon,” there will he says. Marchis says. “I’m betting that at end the of any exomoons. It’s only amatter of time, (11). But far, so team the hasn’t found starstheir to retain moon an Earth-sized that are big enough and far enough from Kipping and his team identified 35 planets blocking asmidgen of starlight the (10). detectable, moonbe asthe could itself, be perturbations on planet’s the orbit should itses between moon’s star the and Earth, duce inits planet’s orbit. As planet the pass- will beanexomoon.” end ofthisyear, there “I’m bettingthatatthe and wobbleswiggles amoon would pro- with Kepler, is searching for telltale the theThe project,called Hunt forExomoons on 150,000faraway iswhich focused stars. from Keplerto the Earth spacetelescope, for large moons using data back beamed While Vega’s is demise yet to come, foren- In another half-billion years or so, Vega’s idea of standing“The theon surface of a In first the nine quarters of Keplerdata, Pacific 124(920):1042–1056. A-type stars.PublicationsoftheAstronomical Societyofthe gaseous absorptionfeatures inthedebrisdisksaround young 8 MontgomerySL,Welsh BY(2012)Detectionofvariable Astrophys 173(2):289–292. Pictoris circumstellar diskIV. RedshiftedUVlines.Astron 7 Lagrange AM,Ferlet R,Vidal-MadjarA(1987)TheBeta Gyr. arXiv:1110.4172. bombardment and theformationofurelites inηCorviat~1 6 LisseCM,etal.(2011)Spitzerevidenceforalateheavy and Fomalhaut. ApJ 763(2):118. 5 SuKYL, etal.(2013) Asteroid beltsindebrisdisktwins: Vega 705(1):314–327. 4 SuKYL, etal.(2009) Thedebrisdiskaround HR8799.ApJ Pictoris. Science226(4681):1421–1424. 3 SmithBA,Terrile RJ(1984)Acircumstellar diskaround β object 1992QB 2 JewittD,Luu J(1993) DiscoveryofthecandidateKuiper Belt excess ofVega. Nature 307:441–442. 1 HarveyPM,WilkingBA,JoyM(1984)Onthefar-infrared signatures planet of Earth-like formation withobjects afamiliar composition: “The silicon, and magnesium point all toward The ratios of elements like iron, oxygen, lessonthe is profound, if not surprising. tronomers about learn rocky planets—and planetssmall (13). dwarfs are destroying chunks sizeof the ies have suggested that some of white these tons second, per Farihi says. stud Other - consumed being bodies at rate the of 110 stars studied they bore marks the of rocky 80 nearby white dwarfs. Roughly 50%of the Hubble the ed Space Telescope at more than uphas set afavorable flytrap for us.” around instellar graveyards. universe “The Cambridge, United Kingdom, digs who astronomer at University of Cambridge, write it down all for us,” says Jay Farihi, an planets,in-falling asteroids, or comets (12). chemicalthe elements that once comprised star’s pristine atmosphere otherwise with The remnantspollute bodies the these of ies that are inits orbit and rip apart. them intense gravity- attract can still bod small Even indeath, it stars seems can help as- Recently, Farihi and his colleagues point - “Like awhite piece of paper, white dwarfs 1 . Nature 362:730–732. PNAS|April9,2013 |vol.110no.155737 | disrupted minorplanetsatwhitedwarfs.ApJ694(2):805–819. 13 Farihi J, Jura M,ZuckermanB(2009)Infrared signatures of planet byawhitedwarf. ApJ732(2):90–99. 12 MelisC.etal.(2011)Accretion ofaterrestrial-like minor candidates. arXiv:1301.18523. pler (HEK):II.Analysisofsevenviablesatellite-hostingplanet 11 KippingDM,etal.(2013)TheHuntforExomoonswithKe- 10.1088/0004-637X/750/2/115. Description ofanewobservationalproject. ApJ750(2):115, A (2012)TheHuntforExomoonswithKepler (HEK):I. 10 KippingDM,BakosGÁ,BuchhaveL,NesvornyD,Schmitt 1538. Submillimeter Observatoryobservations.ApJ690(2):1522– disk: Structure anddynamicsbasedonSpitzerCaltech 9 BackmanD,etal.(2009)EpsilonEridani’splanetarydebris on Earth as well.on Earth ter of universe the could include soon life tion that displaced our planet from cen- the worlds, alien revoluthese Copernican the - if astronomers dofind signatures of life on oids inour System Solar aren’t unique. And ous than stars. Even comets the and aster a population that is perhaps more numer universe) the ishood teeming with planets, systems. (and likeli galaxy - Our inall etary of light; many are powering own their plan- have stars Other are fallen. not mere points makes our corner of cosmosthe unique it into column.” that observed umn. But we’re our hardest trying to move says. “Life inthat is still speculation col- turnedthen into observations,” Grinspoon tions about how normal we are that have breathing biosphere. with unmistakable the signs of aliving, form of an exoplanet or exomoon pulsing fore signs of exolife emerge, maybeinthe itthen should just be amatter of- be time are common,” Farihi says. One by one, our assumptions about what “There’sthis been progression of assump - If rocky worlds Earth-like are common, - -

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