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  • 14 imagine November/December 2008

    Since 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 fi rst 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 refl ected sunlight. If we try to do the same for distant worlds, we fi nd that the starlight they refl ect 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 fi rst, 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.

    51 Pegasi was the fi rst star similar to our sun found to have a planet orbiting around it.

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  • 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 scientifi c world with their fi rst 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 fi nest. 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 fi rst 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 fi rst 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 fi nd. It took patient years of searching to fi nd 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

    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 fi ve 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

    November/December 2008 imagine 15

    Marshall Perrin in front of the Gemini North observatory on Mauna Kea in Hawaii.

    ➜ 42 Hunters

  • 42 imagine November/December 2008

    than stars, typically only one ten-thousandth of a star’s light is blocked. Measuring the amount of this slight dimming lets us directly see a planet’s size. Similar measurements in the infrared can reveal a planet’s temperature. Better yet, measuring transits in multiple wavelengths of light lets us detect gases in planets’ atmospheres based on how different gases absorb different wavelengths. When NASA’s Hubble and Spitzer Space Telescopes were designed, such measurements were beyond astronomers’ wildest dreams—yet those two telescopes have proven champions at the extraordinarily precise measurements needed to turn distant eclipses into newfound knowledge.

    This coming spring, those two spacecraft will be joined by a new space telescope called Kepler, a special-purpose craft optimized precisely for measuring planetary tra