NEWS FEATURE NEWS FEATURE Extrasolar systems shed light on our own Amazing as the discoveries of planets, comets, and asteroid belts around other stars are, it’s their potential to shed light on our Solar System’s origins that is exciting astronomers. Nadia Drake broader sense, we’re trying to understand Science Writer if our own world—and our own Solar Sys- tem—is ‘normal,’” says David Grinspoon, Chair of Astrobiology at the US Library of A planet smaller than Mercury circles a star astronomers can use the Solar System’s ar- Congress in Washington, DC, “or, in some 210 light-years away. Around another star, chitecture to predict the presence of un- extraordinary way, abnormal.” two planets live so close together that each seen objects in these systems, and use the periodically rises in the other’s sky. Other systems to learn more about the celestial Chasing Comets alien skies are home to two suns that rise events that gave birth to and shaped our One stellar system with a recently identified and set, casting double shadows over their Solar System. kinship to ours is that of Vega. This famil- double-sunned worlds. Planets so dense “We definitely learn more about the Solar iar star burns brightly in the northern sky, they might have diamond rinds, worlds System’s past and future by observing other where it forms a summertime triangle with whose year is shorter than an Earthday, oth- stellar systems,” says astronomer Kate Su stars Deneb and Altair. Just 25 light-years ers that orbit their star backward—the cos- of the University of Arizona, Tucson, AZ. away, the star spins so rapidly it’s not spher- mos holds many exotic and diverse objects. “Our challenge now is to identify common ical, but squashed, with flattened poles and And yet, among the eclectic oddities are features and link them to what we know a few planetary systems whose smaller resi- about our own Solar System,” she says. We definitely learn more dents look intriguingly familiar. By many These common features are moving about the Solar System’s standards, these systems are very much Earth—and the Solar System—even further past and future by like home: In addition to multiple planets, from the realm of the unique, and advanc- some stars are ringed by belts of small rocky ing an almost inexorable Copernican march observing other stellar bodies, like our comets and asteroids. Now, toward the surprisingly mundane. “In a systems. a bulging equator. Though young, Vega is already twice as massive as the sun, and for centuries has helped travelers navigate the night and fueled the imagination of genera- tions. Vega’s extrasolar system is like a larger version of our own, with two rocky bands of debris separated by a clean gap, a double- belted configuration that has only been ob- served around a handful of other stars. Looming on Vega’s outskirts—between 90 and 120 times farther from the star than Earth is from the sun—is a cold, icy belt of dust and rocks. This icy debris field, discov- ered in 1984 (1), is like an extrasolar ver- sion of our own Kuiper Belt, home to com- ets and frozen worlds like the five-mooned dwarf planet Pluto. In our Solar System, Image courtesy of NASA/FUSE/Lynette Cook. Image courtesy of NASA/FUSE/Lynette the Kuiper Belt begins beyond the orbit of Artist’s conception of the star Beta Pictoris. In the inset panels, composition of two possible Neptune, at 30 astronomical units (AU) mature terrestrial planets orbiting Beta Pictoris. A water-rich planet similar to the Earth (Top); (1 AU equals the mean distance between a carbon-rich planet (Bottom). Earth and the Sun). But aside from Pluto, www.pnas.org/cgi/doi/10.1073/pnas.1305211110 PNAS | April 9, 2013 | vol. 110 | no. 15 | 5735–5737 Downloaded by guest on September 27, 2021 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.