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Review: The Life of Super-Earths

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The Life of Super-Earths: How the Hunt for Alien Worlds and Artificial Cells Will Revolutionize Life on Our Planet
By Dimitar Sasselov
Basic Books, 2012
hardcover, 240 pp., illus.
ISBN 978-0-465-02193-2

The rapid growth of extrasolar planet studies since the mid-1990s has added new words to the scientific lexicon, not the least of which is “exoplanet”, a contraction of “extrasolar planet.” Another new term that has emerged is “super-Earth”, which is used for a class of exoplanets from one to ten times the mass of the Earth: larger than the Earth, but not so large as to be “ice giants” like Neptune and Uranus. The discovery of super-Earths around other stars has been widely seen as a milestone towards the ultimate goal of discovering Earth-sized planets in orbits like the Earth—and thus being likely candidates to support life. However, in The Life of Super-Earths, one scientist argues that super-Earths are important in their own right, perhaps more so than their smaller counterparts.

Sasselov makes the case that super-Earths have better odds for harboring life than Earth-sized planets. Their larger size make its easier for them to retain their atmospheres, and they are more likely to have active plate tectonics.

Dimitar Sasselov, a professor of astronomy at Harvard who has been involved in exoplanet studies since the 1990s, spends the first part of his book discussing the discovery of exoplanets in general, and super-Earths in particular. (Sasselov takes credit for coining the term “super-Earth,” first using it in a NASA proposal circa 2000. He said he was following the descriptive, if uncreative, tradition in astronomy to connote size by prepending “super” to existing terms; think of “supergiant” and “supernova.”) This part of the book covers some standard ground about the science of exoplanet searches and the history of their discoveries, including the role Sasselov and his colleagues played in identifying exoplanets as they transited the disk of the stars they orbiters. He also examines what these super-Earths may be like, based on models of planetary formation: some may be larger versions of rocky planets like the Earth, while others could be ocean worlds, with think atmospheres of water vapor of exotic forms of ice in their interiors.

In the second part of the book, Sasselov shifts gears from super-Earths to examine the origin and development of life. It’s here that Sasselov makes his more interesting arguments. Planets, he says, are “the best, if not the only” places for life to form, with access to energy and chemicals as well as protection from the hostile space environment. Some planets, of course, are more habitable than others: compare the Earth to, say, Mercury or Neptune. Sasselov, though, makes the case that super-Earths have better odds for harboring life than Earth-sized planets. Their larger size make its easier for them to retain their atmospheres, and they are more likely to have active plate tectonics, supporting a CO2 cycle that regulates the temperature of the planet’s atmosphere.

The last part of the book’s subtitle, “…and Artificial Cells Will Revolutionize Life on Our Planet”, is discussed only briefly in the final chapter of the book. Sasselov suggests that another emerging field, synthetic biology, will help scientists better understand what different forms of life are possible on other worlds: must alien life on a super-Earth have the same biochemistry as our own, or are there alternatives? He doesn’t go into much detail, though, at least compared to his exoplanet science and astrobiology discussions in earlier chapters; he claims that synthetic biology, along with the “completion of the Copernican revolution” (i.e., discovery of Earth-like exoplanets) and the globalization of society, are three key “milestones” for the future of life on this planet—the closest he gets to discussing how they will “revolutionize life” here.

Sasselov is an optimist about the prospects for life in the universe. In the book he estimates that there are about 100 million Earth-sized planets and super-Earths n our galaxy alone that have “habitable potential”: orbiting the right stars in the right orbits. He admits that the emphasis there is on “potential” given our limited knowledge of astrobiology: “We do not really know if life emerges with ease.” Even if doesn't, he believes our growing knowledge of super-Earths and other exoplanets will help us find “friendly harbors” beyond Earth that one day will allow us to spread life beyond our world.