Image: A sandworm rears up out of the desert of Arrakis on the March, 1965 cover of Analog. I can never resist the chance to display the artwork of the remarkable John Schoenherr. What memories…


Stranger Than Fiction

by PAUL GILSTER on MARCH 8, 2013

Just what does it take to make a habitable world? Keith Cooper is editor of Astronomy Now, the British monthly whose first editor was the fabled Patrick Moore. An accomplished writer on astronautics and astronomy as well as a Centauri Dreams regular, Keith has recently become editor of Principium, the newsletter of the Institute for Interstellar Studies, whose third issue has just appeared. In this essay, Keith looks at our changing views of habitable zones in light of recent work, and takes us to two famous science fictional worlds where extreme climates challenge life but do not preclude it. How such worlds emerge and how life might cope on them are questions as timely as the latest exoplanet findings.

by Keith Cooper

Literally overnight, two habitable planets – tau Ceti f and HD 85512b – were rendered barren and lifeless. What was the cause of this cataclysm? A nearby supernova? Asteroid impacts? On the contrary, it was something far more mundane.

A dozen light years away, scientists at Penn State University were re-analysing the extent to which habitable zones penetrate the space around stars; in other words, at what distance liquid water could potentially exist on a planetary surface assuming an Earth-like atmosphere. The basics for habitable zone theory had been worked out in part by, among others, Penn State’s James Kasting in decades previous. Building on his work, Ravi Kumar Kopparapu and Ramses Ramirez discovered that habitable zones are found further from their stars than had been envisaged (see Habitable Zones: A Moving Target for more).

The result was bad news for our two exoplanets. Suddenly, as the habitable zone shifted imperceptibly around them, they found themselves on the wrong side of the inner habitable zone boundary, too close to their respective stars. Consequently the Planetary Habitability Laboratory at the University of Puerto Rico, Arecibo, declared them uninhabitable. Too bad for any life-forms living there.

Despite only knowing the scarcest of details about these worlds – mass, radius, density, the amount of heating from their stars – these two worlds have been cast into the obsolescence in a manner that seems shockingly final. We know so little about these planets, how can we possibly say whether they are habitable or not, especially when the only standard we are holding them to is habitability for human beings?

Read more: Stranger Than Fiction — Centauri Dreams.

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