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The The Planet Planet Hunters Hunters S 51 Pegasi was the first star similar to our sun found to have a planet orbiting around it. 14 imagine ince the ancient past, humanity has looked up at the night sky and wondered if those faint stars are in fact bright suns shining on distant worlds. From the ancient Greeks to Enlightenment philosophers to Hollywood producers, it seems like everyone has had their own speculations about what other planets may be like. But only in the last few years has it become possible to move beyond speculation to true knowledge. It doesn’t take the starship Enterprise to seek out new worlds: Armed with powerful telescopes and the latest digital detectors, astronomers are now capable not only of detecting the presence of planets orbiting other stars, but of measuring their physical properties and even in some cases chemical makeups, all from right here on Earth. In fact, since the first distant planets were discovered in the mid-1990s, the study of such “exoplanets” has blossomed into one of the hottest and most active areas of modern astronomy. The total number of planets known around other stars is now about 300—and that number increases by one or two new worlds every week. It’s a very exciting time to be an astronomer! Wobble Wobble, Little Star The challenge of detecting distant planets is that they are tiny and dim, almost impossibly faint compared to their parent stars. Recall that we see our own solar system’s planets in reflected sunlight. If we try to do the same for distant worlds, we find that the starlight they reflect is literally millions or even billions of times fainter than the direct glare of their parent stars. To see such faint pinpricks of light right next to a blazing star is a daunting challenge indeed. Thus the first, and still far most successful, method of detecting exoplanets relies on a clever indirect route. As a planet orbits a star, its gravitational pull tugs on the star and causes it to “wobble” back and forth slightly. That slight wobble causes an almost imperceptible Doppler shift in the star’s light, a tiny effect but one that can be measured using careful techniques and special instruments on some of the world’s largest telescopes. These measurements can reveal the masses, distances from the star, and orbital periods of planets that remain hidden from direct sight. November/December 2008 Image: ©Lynette Cook by Marshall Perrin, Ph.D. This “radial velocity” method was pioneered in the 1990s by two teams of astronomers, one led by Michel Mayor and Didier Queloz in Switzerland, and one led by Geoff Marcy and Paul Butler in California. Mayor and Queloz stunned the scientific world with their first discovery in October 1995, of a planet now called 51 Pegasi b, but Marcy and Butler’s team quickly followed with discoveries of their own. In the years since then, the mostly good-natured competition among these teams and others has led to science at its finest. Each success spurs another team to improve techniques still further, allowing ever smaller or more distant worlds to be found. Strange Neighbors All these discoveries have allowed us to piece together an increasingly detailed tourist’s guide to the solar neighborhood. Perhaps the biggest surprise of the exoplanet revolution has been the tremendous diversity of our neighboring systems. With only our own solar system as a model, scientists once assumed that most solar systems would consist of small rocky planets near the star and massive gas giants at greater distances, which took decades to round the star in their ponderous orbits. Yet the very first discovery was of a Jupiter-mass planet in a four-day orbit, whipping around at a rocket’s pace just barely above the stellar surface, ten times closer than Mercury is to our own sun. To the naked eye, it would glow like an ember, shrouded in clouds of molten metal and rock vapor. The surprising 51 Peg b proved to be just the first of many such “hot Jupiters,” and today dozens of such worlds are known. Detecting worlds through the radial velocity technique required watching at least one full orbit, so naturally these fast-orbiting worlds were the quickest to find. It took patient years of searching to find more distant planets on their slow Jupiter-like orbits, but eventually such worlds started to be found. Today they dominate the catalogs, and hot Jupiters seem to be the exception, not the rule. The majority of giant planets are located far from their parent stars, just as in our own solar system. Yet there were other surprises in store: Unlike the stately circular orbits of our own planets, the vast majority of exoplanets have highly elliptical orbits. Some worlds swing from beyond the Earth’s orbital distance all the way in to hot Jupiter territory, and back out again, in a never-ending bake/freeze cycle. Others come in pairs locked in the gravitational dance called orbital resonance. Only a few percent of systems contain massive planets in circular orbits relatively November/December 2008 Marshall Perrin in front of the Gemini North observatory on Mauna Kea in Hawaii. far from their stars, like our own solar system. Recent surveys have also taught us a lot about where planets aren’t, indicating that no more than one in five stars has planets as massive as those in our solar system. Theoretical models are still struggling to explain all this diversity. It seems likely that the formation of solar systems is a complicated and chaotic process, and one that depends very sensitively on the initial conditions around young stars. Like a galactic Goldilocks tale, stars that begin their lives too heavily surrounded by dust and gas probably give birth to hot Jupiters or planets forced into crazy orbits, while those with too little dust are barren and planetless. Most likely, only a very narrow range of conditions is just right for producing solar systems like our own—but this question remains far from settled. The Tiniest Eclipses While the tried-and-true radial velocity technique has revealed hundreds of planets, there is a limit to what those measurements can tell us. For one thing, we still can’t detect planets as small as the Earth; the wobbles are just too small. For another, radial velocities tell us only planets’ masses and orbits, but nothing about their sizes or compositions. For those, we must turn instead to another method called planetary transits. A small fraction of exoplanets have orbital planes that line up precisely with the Earth so that every so often the planets pass in front of their parent stars, causing tiny partial eclipses. Because planets are so much smaller ➜ 42 Hunters imagine 15 ©Lynette Cook ➜ 15 Hunters ➜ than stars, typically only one optics coronagraph is under ten-thousandth of a star’s light is construction right now by a team blocked. Measuring the amount of astronomers, of which I am a of this slight dimming lets us part, from California, New York, directly see a planet’s size. Similar and Canada. Our instrument, the measurements in the infrared Gemini Planet Imager, should can reveal a planet’s temperature. be operational by early 2011. Better yet, measuring transits Its improved sensitivity will let in multiple wavelengths of light us observe Jupiter-mass planets lets us detect gases in planets’ much closer to their parent stars; atmospheres based on how we will be able to see planets with different gases absorb different masses and orbits comparable to wavelengths. When NASA’s Gliese 581 c, shown from the perspective of a possible moon, is thought to be a rocky world Jupiter and Saturn in our own Hubble and Spitzer Space about 1.5 times the radius of Earth. Its temperature is uncertain, yet it resides near the star’s solar system. Just as with radial Telescopes were designed, such habitable zone, making the existence of oceans and lakes a possibility. velocities, there is a scientific race measurements were beyond spurring us onwards, with a team astronomers’ wildest dreams—yet those two telescopes have proven of our European colleagues and friends building a similar instrument champions at the extraordinarily precise measurements needed to turn of their own. If all goes well, in a few years these instruments will be distant eclipses into newfound knowledge. snapping photos and measuring the atmospheres of planets by the This coming spring, those two spacecraft will be joined by a new hundred. space telescope called Kepler, a special-purpose craft optimized Beyond that, the future gets hazier. NASA is today planning a series precisely for measuring planetary transits for many more stars far more of Exoplanet Probe missions over the next few years, building toward carefully than ever before. If all goes according to plan, Kepler will an ambitious Terrestrial Planet Finder program around perhaps 2020 be able to detect the one-part-in-a-million microeclipses caused by or beyond. That hypothetical craft would use techniques similar to an Earth-like planet passing in front of a star. By surveying hundreds coronagraphs to directly image Earth-like planets in search of alien of thousands of stars over the next few years, Kepler boldly aims life signs, but the precise details of how to actually build such a thing to produce a galactic census of Earth-mass planets orbiting in the remain far from clear. We seek no less than to answer at long last those “habitable zone” where temperatures are right for life like ours. age old questions: How common are worlds like our own? Do any distant suns shine down on worlds teeming with alien life, perhaps Toward Pale Blue Dots even intelligent life? The chances are greater than ever that these These and other indirect methods have unmasked for us the secrets questions will be answered within our lifetimes. Doing so will take the of several hundred worlds, but astronomers have never given up the combined efforts of many hard-working scientists and brilliant young dream of seeing such worlds directly. Such observations would let planet hunters. Maybe they’ll even be answered by some of you! i us find new planets with a single snapshot, instead of the long years of waiting required for radial velocity or transit work, and would Marshall Perrin is an astronomer working at UCLA. Originally from open the way to much more detailed studies of atmospheres and Annapolis, MD, he attended CTY Lancaster from 1993–1995. After gradcompositions. Over the past decade, we have nurtured adaptive optics uating from Harvard, he headed west to California and earned a Ph.D. technology, which can correct for the blurring and twinkling caused in astronomy from UC Berkeley in 2006. His other interests include sciby the Earth’s turbulent atmosphere, and developed instruments called ence education, biking and backpacking, environmental conservation, coronagraphs that can block out almost 100 percent of a star’s light and science fiction. For more of his writing, check out his bimonthly while still letting an orbiting planet be seen. For several years, such column in the online magazine Strange Horizons (http://strange instruments have surveyed nearby stars to no avail—until now. horizons.com ). On the very day that I am writing this, a team of Canadian astronomers using the Gemini Telescope’s adaptive optics system has just presented their very first direct image of a probable planet Word Wise Solution orbiting a young sunlike star. It’s a weird object, “Seen for the first time through eight times more massive than Jupiter but a backyard telescope, ringed orbiting shockingly far from its parent star, ten Saturn, icon of the otherworldly, times Neptune’s distance from our sun, where is the vision most likely to turn the greater separation from the star made it an unsuspecting viewer into an easier to detect. astronomer forever.” This pale dot is but the tip of the —Dava Sobel, The Planets iceberg. A much more advanced adaptive 42 imagine November/December 2008