Planetary science: The search for Earth's twin

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Sara Seager enjoys a frank and vivid account of planet hunting.

Mirror Earth: The Search for Our Planet's Twin

Walker: 2012. 304 pp. $26, £19.99 9780802779007 | ISBN: 978-0-8027-7900-7

People have pondered the existence of Earth-like worlds beyond our Solar System for millennia. In 1995, the first exoplanet was spotted, orbiting a Sun-like star. Fewer than two decades later, hundreds of planets and thousands of candidates are known. Yet the sheer variety of exoplanet masses, sizes and orbits indicates that close copies of the Solar System are rare.

In Mirror Earth, science writer Michael Lemonick relates astronomers' abiding interest in sister worlds, from ancient Greece — where Aristotle surmised that there is only one world, Earth — to today, and beyond. Spotting a small, light and faint planet near its massive, bright host star is difficult and requires abstruse techniques. Lemonick sets out these methods — radial velocity, transits, microlensing, direct imaging, astrometry and pulsar timing — with impressive clarity.

NASA's Kepler space telescope is rightly a focus of the book. The Kepler mission's goal is to establish how common Earth-size planets in Earth-like orbits about Sun-like stars are. But, as Lemonick is careful to spell out, these will not necessarily be Earth 'twins' — planets with water oceans, continents and thin atmospheres conducive to life.

Knowing a planet's size doesn't tell us whether it is habitable. Venus, for example, is about the same size as Earth but not at all Earth-like. A thick carbon dioxide atmosphere causes greenhouse heating of the surface to temperatures exceeding 700 kelvin, hot enough to melt lead.

The Kepler telescope, launched in March 2009 and trailing Earth as the planet orbits the Sun, is making astonishing discoveries. One is that 'mini-Neptunes' — planets a few times Earth's diameter and a little smaller than Neptune — are many times more common than larger 'Jupiters', planets about 11 times the size of Earth. There are hints that smaller planets may be even more numerous. This tantalizing finding could imply that rocky planets (thought to reach no more than 1.75 times Earth's size) outnumber their gas-rich cousins.

Kepler has also vastly increased our knowledge of the variety of planetary systems. These include single planets orbiting two suns and dozens of multiple-planet systems, some in surprisingly compact orbits. The book covers the zoo of planetary types thought to exist, as well as theories of planet formation, conditions for habitability and the origin of life.

Lemonick describes Kepler's rocky road from conception to launch, including how its technical feasibility was repeatedly challenged, leading to the proposal being rejected by NASA several times. And he discusses the implication of the recent realization that most Sun-like stars are more variable than our Sun. That means Kepler must scour them for twice as long to pick out the signature dimming of passing planets, doubling the time it will take to reach the census goal.

“Knowing a planet's size doesn't tell us whether it is habitable.”

Finding exoplanets is an art as well as a science, and Lemonick gives examples to demonstrate how hard it can be to tease a weak signal out of data. “It's my Rembrandt,” declared exoplanet hunter Paul Butler of his computer code — six years in the making — that cracked the puzzle of how to measure the precise motion of a star caused by an orbiting planet. Controversy invariably follows about which detections are robust, and Lemonick captures some of those lively exchanges with quotes and stories.

Lemonick provides a brutally honest character exposition of well-known exoplaneteers. He details Geoff Marcy's feelings of being, in Marcy's words, “an impostor” early in his career owing to a severe lack of confidence that plagues many young scientists, and Debra Fischer's unusual route into astrophysics via nursing.

One intriguing thread in the book is the tension between scientists and journalists. Exoplanet discoveries are ripe for misinterpretation and exaggeration. The media and the public want an exciting story even when there is none, and it is easy for a scientist to get caught up in the hype. Lemonick describes how one astronomer felt he was induced by a journalist to say that “the chances for life” were “100 percent” on a new planet, the very existence of which is now challenged.

Mirror Earth captures the excitement of planet hunting and the ups and downs of discovery. Yet today, the quest for an Earth twin remains just out of reach. To identify a truly Earth-like world will require a more sophisticated space telescope than Kepler, to look for water and signs of life. Lemonick speculates on how that might happen. It may take decades to see how the story really unfolds. Stay tuned.

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Correspondence to Sara Seager.

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Seager, S. Planetary science: The search for Earth's twin. Nature 490, 479 (2012) doi:10.1038/490479a

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