Review: May 2009

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How many solar systems are there in the universe, and how typical is the one we live in? Two decades ago it was impossible to say, since astronomers knew of no other solar systems but our own. Since 1995, however, when the detection of the first planet orbiting a star like our Sun was announced by Swiss astronomers, the number of known extrasolar planets has grown exponentially. As I write this review, the official number (at exoplanet.eu) stands at more than 340, with new ones being added almost weekly. The majority are detected, not by direct photography, but by measuring the subtle wobble their gravitation induces in the motion of their parent stars.

We can now say, with some confidence, that planets are common, though many of the newly discovered worlds are strangely different from our own. Many are what astronomers call “hot Jupiters,” giant balls of gas several hundred times more massive than the Earth, but orbiting only a few million miles from their stars—so close that they take only a few days to go around. Others move in elongated looping orbits. Until the discovery of these new worlds, astronomers had assumed that most planetary systems would be like our own, where Jupiter-size giant planets are located hundreds of times farther out from their star, taking tens of Earth years to orbit, and where all the planets travel on nearly circular paths.

Alan Boss, an astronomer at the Carnegie Institution of Washington, has been at the forefront of research in the origin of solar systems. As the number of known systems has grown, he and his colleagues have been able to get a clearer picture of their origins and a fuller understanding of how our own solar system fits into the picture. In this short and lucid review of his field, he traces the developments of the last fifteen years in chronological, diarylike entries, so that we can share with him the excitement of discovery.

In 1999, we read, astronomers first observed one of the newly discovered planets passing in front of its star, causing the star’s light to dim. The observation enabled the astronomers, by timing the eclipse, to determine the size and density of the object. A few years later, astronomers were routinely using such eclipses (called transits) as a complement to searching for planet-induced wobbles. By 2007, the roster of planets included “super-Earths”—rocky bodies ranging from several times to more than twenty times the mass of Earth—some of them in close, hot orbits, others far enough from their stars to be frigid. Astronomers have even been able to use space telescopes to measure the constituents of some of the new planets’ atmospheres, detecting both water and methane, two of the “biomarkers” exobiologists look for in searching for inhabited worlds.

The tone of Boss’s book, accordingly, is excited and hopeful, but there’s also a note of wry irony in his descriptions of the political trials astronomers have gone through trying to promote their research. And despite the successes of the past decade, Boss senses that it may be increasingly difficult for astronomers to attract the sums needed to continue the search for habitable planets. Readers of this book, I am certain, will hope his fears are unsubstantiated.