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Landscapes of lava, vast waterscapes, and “eyeball worlds” are among the exoplanets that await discovery

The Planet Factory: Exoplanets and the Search for a Second Earth

Elizabeth Tasker
Bloomsbury Sigma
336 pp.
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Thirty years ago, the idea of planets orbiting other suns was relegated to the realm of science fiction. So much has changed since then that the astrophysicist Neil deGrasse Tyson recently asserted that not only do such planets likely “outnumber the sum of all sounds and words ever uttered by every human who has ever lived” but that “[t]o declare that Earth must be the only planet with life in the universe would be inexcusably bigheaded of us.”

The sea change began in the 1990s, when the only planets we knew were the nine we learned in kindergarten (now there are eight, but that’s a different story). In 1995, astronomers Michel Mayor and Didier Queloz announced they had detected the first “exoplanet” orbiting a main-sequence star, 51 light years away.Two months later, a team of astronomers led by Geoff Marcy confirmed Mayor’s detection and found two more. Within a decade, hundreds of planets orbiting other stars had been detected. NASA’s Kepler Telescope, launched into space in 2009, found a few thousand more. The Planet Factory, by Elizabeth Tasker, is the story of these 3000+ planets, “the travel log of how they came to form from dust particles to worlds so diverse that even Hollywood has failed to be weirder.”

Planet formation seems to follow an adage that frequently rings true in particle physics: Whatever can happen will happen. Some denizens of the planetary zoo appear to be made of diamond, others of 500° ice. Some are covered entirely by lava, others by a supercritical state of water in which it is “impossible to tell where the oceans meet the sky.” Some rain rocks, others cyanide. There are “eyeball worlds” tidally locked to their stars, sweltering on the perpetual dayside and icy in the dark, with a possible ribbon of habitability at the interface, “a twilight zone … with the star permanently on the horizon in a deep red sunset.” There are rogue planets wandering the darkness with no star to orbit, which might harbor life beneath fat hydrogen atmospheres; water worlds akin to the Jovian moons Europa, Ganymede, and Callisto, which may hide deep-sea creatures;
and planets pulled by two suns, like Luke Skywalker’s Tatooine. The staggering variety captures the imagination and makes Tasker’s “travel log” a riveting journey.


Three small exoplanets in the compact KOI-961 system are depicted here in an artist’s rendering.

Throughout the book, Tasker calls on an impressive array of physics—the expected applications of Newtonian mechanics but also atomic and nuclear physics, thermodynamics, stellar evolution, electromagnetism, and general relativity. She rounds out the stories with forays into chemistry and geology. The diverse and unexpected detections of exoplanets have turned the story of the formation of our own solar system on its end, proving that planet formation is more complicated than once believed and prompting a renewed interest in the planets in our own backyard. Tasker details the wild story of the solar system’s formation of planets, moons, and smaller objects. By describing the complexity and variety of ecosystems— the runaway greenhouse effect on Venus, fiery Mercury, barren Mars, and the gas giants’ watery moons—she reminds us that the story of every exoworld is as complex as our own.

Tasker is an astrophysicist, and her scientific explanations are accessible, clear, and complete. Although well described, the book’s science may be too dense for some readers; in many places, it could hold its own in a college course on planetary science. The italicized terms and figure captions are more appropriate to a textbook than a popular science book. (The simple, intuitive figures illustrate the narrative so well that the captions hardly seem necessary in the first place.)

In an interview in 2011, when asked why he started hunting for planets at a time when it was considered scientifically illegitimate, Marcy articulated “a question that’s dear to me on a human level, a personal level…. Are we alone?” As a British astrophysicist who worked in the United States and Canada before joining faculties in Japan, Tasker brings a refreshingly international perspective to the story of this pursuit, reminding us that the question of whether or not we are alone is one pondered by Earthlings everywhere.

About the author

The reviewer is a physics professor and freelance writer in Los Angeles, CA, USA.