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Review: Envisioning Exoplanets


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Envisioning Exoplanets: Searching for Life in the Galaxy
by Michael Carroll
Smithsonian Books, 2020
hardcover, 224 pp., illus.
ISBN 978-1-58834-691-9
US$34.95

More than a quarter of a century after the modern era of exoplanet discovery began, scientists can still only guess what those worlds look like. The tremendous distances and differences in brightness mean that most exoplanets are discovered by indirect means, such as the periodic Doppler shifts in spectral lines of stars caused by the gravitational tug of orbiting planets, or the miniscule drops in brightness of those stars as planets pass in front of them. Those and other techniques have allowed astronomers to measure the sizes and orbits of these planets, and spectroscopy has helped identify the composition of some. But they can only hypothesize what those planets look like.

That has not stopped speculation, though, about the appearance of exoplanets. It’s not uncommon for a press release by NASA or a university about an exoplanet discovery to include an artist’s rendition, a sort of informed guess of what those worlds might look like that’s easier for public understanding than a wiggling spectral line or a lightcurve. The worry some scientists have, though, is that the public confuses those illustrations with actual images of those exoplanets, something not possible with current telescopes or even those on the drawing boards.

On the spectrum between science and fiction, these fall much close to the former than the latter, since the illustrations are based on what we currently know about those planets, and offer one vision of what they may look like.

In Envisioning Exoplanets, author and artist Michael Carroll looks to combine science and art to describe our current knowledge of exoplanets. The text of the book provides a good overview of the history of exoplanet science, how exoplanets are discovered, and a discussion of some of the most promising candidates for “Earth-like” exoplanets. Much of that is familiar territory for those who have been following the subject, although there are some interesting and less well-known elements to that, such as the concept of the “Earth Similarity Index” proposed by scientists to estimate how much an exoplanet might be like the Earth based on its size and the stellar flux it receives from its parent star.

The real draw of the book, though, is the art. The book features dozens of illustrations of what some exoplanets may look like, from Proxima b to the various Earth-sized worlds orbiting TRAPPIST-1. The book also includes illustrations of more exotic exoplanets, from “hot Jupiters” orbiting close to their stars to “ploonets,” or moons stripped from larger exoplanets that have become planets.

The illustrations are beautiful, but they’re also, at best, speculation about what those worlds look like. That’s not a bad thing, though. On the spectrum between science and fiction, these fall much close to the former than the latter, since the illustrations are based on what we currently know about those planets, and offer one vision of what they may look like. They do, though, stir the imagination, showing both how strange the universe is but also, potentially, how familiar some exoplanets may be. And, they let one wonder what else is out there, yet to be discovered.


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