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NEWS FEATURE

A planet smaller than Mercury circles a star 210 light-years away. Around another star, two planets live so close together that each periodically rises in the other’s sky. Other alien skies are home to two suns that rise and set, casting double shadows over their double-sunned worlds. Planets so dense they might have diamond rinds, worlds whose year is shorter than an Earthday, oth-ers that orbit their star backward—the cos-mos holds many exotic and diverse objects.

And yet, among the eclectic oddities are a few planetary systems whose smaller resi-dents look intriguingly familiar. By many standards, these systems are very much like home: In addition to multiple planets, some stars are ringed by belts of small rocky bodies, like our comets and asteroids. Now,

astronomers can use the Solar System’s ar-chitecture to predict the presence of un-seen objects in these systems, and use the systems to learn more about the celestial events that gave birth to and shaped our Solar System.

“We definitely learn more about the Solar System’s past and future by observing other stellar systems,” says astronomer Kate Su of the University of Arizona, Tucson, AZ. “Our challenge now is to identify common features and link them to what we know about our own Solar System,” she says.

These common features are moving Earth—and the Solar System—even further from the realm of the unique, and advanc-ing an almost inexorable Copernican march toward the surprisingly mundane. “In a

broader sense, we’re trying to understand if our own world—and our own Solar Sys-tem—is ‘normal,’” says David Grinspoon, Chair of Astrobiology at the US Library of Congress in Washington, DC, “or, in some extraordinary way, abnormal.”

Chasing CometsOne stellar system with a recently identified kinship to ours is that of Vega. This famil-iar star burns brightly in the northern sky, where it forms a summertime triangle with stars Deneb and Altair. Just 25 light-years away, the star spins so rapidly it’s not spher-ical, but squashed, with flattened poles and

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, the Kuiper Belt begins beyond the orbit of Neptune, at 30 astronomical units (AU) (1 AU equals the mean distance between Earth and the Sun). But aside from Pluto,

Nadia DrakeScience Writer

Extrasolar systems shed light on our ownAmazing 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.

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Artist’s conception of the star Beta Pictoris. In the inset panels, composition of two possible mature terrestrial planets orbiting Beta Pictoris. A water-rich planet similar to the Earth (Top); a carbon-rich planet (Bottom).

We definitely learn more about the Solar System’s past and future by observing other stellar systems.

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objects populating the Kuiper Belt weren’t observed until 1992 (2), though the belt had been a hypothetical reservoir for short-period comets. “It can fairly be said that we knew about Kuiper Belts around other stars 8 years before we began to know that the Sun had one, too,” says Karl Stapelfeldt, an astronomer at National Aeronautics and Space Administration (NASA)’s Goddard Space Flight Center in Greenbelt, MD.

Like the Kuiper Belt, Vega’s disk is a likely reservoir of comets, which would streak through the young exosystem more fre-quently than in our own. “It takes time for the population of comets to get used up by collisional grinding and gas evaporation,” Stapelfeldt says.

Vega and the sun are not the only two stars hosting icy debris rings: Cold, dusty ribbons have been found around several stars, including Beta-Pictoris, Fomalhaut, and HR 8799 (3–5). Another stellar sys-tem, Eta Corvi, was observed being pelted by what scientists surmise was a hailstorm of comets, flung inward from one of these more distant reservoirs (6). NASA’s Spitzer Space Telescope studied the system in the infrared and found evidence for water, ice, organics, and shattered rock near the star—perhaps the result of comets colliding with a planet.

Eta Corvi is now about one billion years old—the same age the Sun was when a sim-ilar cataclysm happened in our Solar Sys-tem. During this period of adolescent rebel-lion, the giant planets migrated outward, a march that sent small, rocky bodies hurtling toward the inner Solar System. Scars from this period, called the Late Heavy Bom-bardment, are still carried on objects like the Moon, whose surface contains the cra-tered record of these early growth spasms.

Now, astronomers estimate that as many as 20% of nearby stars come with cold, comet-bearing belts, reservoirs for the type of objects thought to have delivered water to Earth.

But the evidence for exocomets doesn’t simply lie in cold debris disks. Individual comets have been detected, the first in 1987 around the star Beta-Pictoris (7). A half-dozen new exocomets, announced in Janu-ary 2013*, more than double the previous total.

“They’re certainly in more places than

have been thought,” says astronomer Bar-ry Welsh of the University of California, Berkeley, CA. “Two or 3 years ago, we had a thousand exoplanets and only four comets. So we tried to come up with a formula for which stars might harbor exocomets.”

Welsh and Sharon Montgomery, from Clarion University, Clarion, PA, selected a group of young, rapidly rotating stars—the

same type as Vega—that have known debris disks (8). Then, from the McDonald Ob-servatory in Texas, they searched for star- grazing comets passing between their host stars and Earth. “We look for tiny signatures in the spectrum of the star,” says Montgom-ery. Those signatures are produced as the gas in an evaporating comet’s tail alters the star’s light that reaches Earth. At the end of the observing run, Welsh and Montgomery had found evidence for six new exocom-ets—about one for every four stars they searched.

Now, says Welsh, it appears that exocom-ets are probably extremely common—at least as common as exoplanets. “Wher-ever there are planets, there are going to be comets. Or asteroids. Or dust,” Welsh says, noting that planetary systems are like construction sites; even after the planets are built, there’s material left over. “Bits of wood, nails—that’s basically the debris left over that can form comets,” he says.

Alien Asteroids Closer to Vega is a second debris disk, warmer and more recently discovered than the icy belt. These two belts are situated much like the Sun’s, with the colder disk being roughly 10 times farther out than its warmer counterpart.

Though astronomers haven’t actually seen them, Vega’s asteroids are probably dense and rocky, similar to ours. This inner, warm belt, announced in January 2013*, is one of only a few that have been directly identified. Others have been detected around Fomal-haut, HR 8799, and a star called Epsilon Er-idani (8). So far, scientists estimate that per-haps 10% of large stars have such asteroid belts; in smaller Sun-like stars, the numbers drop to about 1%. Those figures are likely to

change as more sensitive instruments come online. Finding these inner debris disks is trickier than detecting cooler, more dis-tant disks, because the inner disks radiate at a wavelength that’s obscured by the star’s glare.

To spy on Vega’s inner disk, Su and her colleagues used two space telescopes—NASA’s Spitzer and the European Space Agency’s Herschel Space Observatory. A band of warm dust radiating in the infrared revealed the disk, the probable result of col-liding asteroids. The team also took a closer look at Fomalhaut and found a system with similar proportions—two belts separated by a clean gap.

The sculpted gap between the two rings is the likely handiwork of at least one planet, even though no planets have been observed around Vega. Yet.

“Planets are needed in between the two belts to keep the gap clear of dust,” says Sta-pelfeldt, a member of the asteroid-detect-ing team. Several planets, Jupiter-mass or smaller, might be gravitationally sweeping up small bodies from the gap—a familiar architecture that is also found around the star HR 8799, where four giant planets orbit between two dusty belts.

But identifying planets around Vega is a project for the future—perhaps for the James Webb Space Telescope, scheduled to launch in 2018. The telescope’s enormous, 6.5-m primary mirror, with about seven times more collecting area than the Hub-ble Space Telescope, could image the giant planets directly with the help of a light-blocking star shade. Or, an instrument such as the Gemini Planet Imager, scheduled to go online in the Southern Hemisphere in late 2013, should be able to see Vega’s plan-ets if it moves north, says Franck Marchis, a planetary astronomer at the SETI Institute in Mountain View, CA.

Moon HuntGiant planets, whether in the Vega system or elsewhere, are tantalizing candidates to search for the presence of exomoons. A large, Earth-sized moon, for example, could provide a suitable niche for life.

“We have no idea whether big moons are out there,” says David Kipping, an astrono-mer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA. “We’re searching for an object we’ve never seen before.”

Kipping and his colleagues are looking

“Wherever there are planets, there are going to be comets. Or asteroids. Or dust.”

*221st meeting of the American Astronomical Society, January 6–10, 2013, Long Beach, CA.

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for large moons using data beamed back to Earth from the Kepler space telescope, which is focused on 150,000 faraway stars. The project, called the Hunt for Exomoons with Kepler, is searching for the telltale wiggles and wobbles a moon would pro-

duce in its planet’s orbit. As the planet pass-es between its star and Earth, the moon’s perturbations on the planet’s orbit should be detectable, as could be the moon itself, blocking a smidgen of the starlight (10).

In the first nine quarters of Kepler data, Kipping and his team identified 35 planets that are big enough and far enough from their stars to retain an Earth-sized moon (11). But so far, the team hasn’t found any exomoons. It’s only a matter of time, Marchis says. “I’m betting that at the end of this year, there will be an exomoon,” he says. The best planetary candidates are still too far from their stars to have produced the many transits needed for a solid exomoon detection; but as long as the Kepler space-craft stays healthy, those data should be in the pipeline.

“The idea of standing on the surface of a moon looking up [and seeing] a gas giant that’s filling your sky—it’s just one of the most beautiful things I can imagine,” Kip-ping says.

In another half-billion years or so, Vega’s light will redden with age, as the star cools and expands, becoming a red giant. Then, it will shed its outer layers and shrivel into a dense stellar corpse, a white dwarf star with a solar mass of material packed into a sphere the size of Earth.

While Vega’s demise is yet to come, foren-sic astronomers are already studying other white dwarfs. After death, a white dwarf ’s

intense gravity can still attract small bod-ies that are in its orbit and rip them apart. The remnants of these bodies pollute the star’s otherwise pristine atmosphere with the chemical elements that once comprised in-falling planets, asteroids, or comets (12).

“Like a white piece of paper, white dwarfs write it all down for us,” says Jay Farihi, an astronomer at University of Cambridge, Cambridge, United Kingdom, who digs around in stellar graveyards. “The universe has set up a favorable fly trap for us.”

Recently, Farihi and his colleagues point-ed the Hubble Space Telescope at more than 80 nearby white dwarfs. Roughly 50% of the stars they studied bore the marks of rocky bodies being consumed at the rate of 110 tons per second, Farihi says. Other stud-ies have suggested that some of these white dwarfs are destroying chunks the size of small planets (13).

Even in death, it seems stars can help as-tronomers learn about rocky planets—and the lesson is profound, if not surprising. The ratios of elements like iron, oxygen, silicon, and magnesium all point toward objects with a familiar composition: “The signatures of Earth-like planet formation

are common,” Farihi says.If Earth-like rocky worlds are common,

then it should be just a matter of time be-fore signs of exolife emerge, maybe in the form of an exoplanet or exomoon pulsing with the unmistakable signs of a living, breathing biosphere.

“There’s been this progression of assump-tions about how normal we are that have then turned into observations,” Grinspoon says. “Life is still in that speculation col-umn. But we’re trying our hardest to move it into that observed column.”

One by one, our assumptions about what makes our corner of the cosmos unique have fallen. Other stars are not mere points of light; many are powering their own plan-etary systems. Our galaxy (and in all likeli-hood the universe) is teeming with planets, a population that is perhaps more numer-ous than stars. Even the comets and aster-oids in our Solar System aren’t unique. And if astronomers do find signatures of life on these alien worlds, the Copernican revolu-tion that displaced our planet from the cen-ter of the universe could soon include life on Earth as well.

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“I’m betting that at the end of this year, there will be an exomoon.”

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