Days of Powder and Planets

I’m writing from the ExoClimes 2012 Conference in Aspen, Colorado, where astronomers, planetary scientists, and Earth scientists have gathered in early morning and evening sessions to share results and ideas about the climates of extrasolar planets. Our afternoons are free for skiing.

The combination of mental stimulation and physical exhilaration facilitates insight and collaboration among the disparate specialists. Although it’s been a bad winter here for skiing, we emerged from the first day of talks to find the valley filled with freshly fallen snow. What a planet! It’s large enough for plate tectonics to raise these uppity mountains, and warm enough for water to be at the “triple point” — where it can be liquid for biochemistry, vapor for climate regulation, and solid for skiing. But how rare is this winning combination that supports our biosphere of aspens, archeobacteria, and astrobiologists? 

Since the first ExoClimes conference two years ago, Kepler and other telescopes have shown us that planets are common, their sizes range from smaller than Earth to larger than Jupiter, and that some orbit in their host star’s habitable zone. It’s only a matter of time before Kepler finds its holy grail — an Earth-sized planet in the habitable zone. How many of these might be habitable, and what it really takes for a planet to qualify, are hot topics here. Bright young researchers are applying clever new techniques to glean a surprising amount from the scant signals of distant, modulated starlight.

But what can we really say about the detailed environments of these worlds? How many rocky planets have plate tectonics, which is so important on Earth? Adrian Lenardic (Rice University) presented models suggesting that it’s insufficient to know a planet’s mass, density, and temperature — two planets with these same parameters may have different global tectonics, depending on their histories. It’s difficult to imagine knowing an exoplanet’s geologic history when we still don’t know Venus’s history! But these behaviors would produce different atmospheres, which we might in time be able to predict and observe.

Several researchers describe the role of less obvious gases and parameters in determining planetary climate, suggesting that our standard notion of the habitable zone — having enough greenhouse gases to support liquid water at a certain distance from a star — needs revision. Ray Pierrehumbert (University of Chicago) shows how large amounts of hydrogen might warm climates enough to expand the zone’s outer edge, but also how methane-producing organisms on such worlds might eat up the hydrogen and carbon dioxide molecules, collapsing the greenhouse and dooming their own world. As Colin Goldblatt (University of Victoria, Canada) stated, “Habitability and inhabitance are inseparable.” 

Perhaps our search for “another Earth” is driven by naïve assumptions. But our growing awareness of exoplanets is providing incentive for new models with a much wider range of parameters, such as including less obvious greenhouse gases (like nitrogen) that may be more important than we have realized. And so it goes: the observers give us theorists new puzzles and we provide them with new clues to search for as we strive to understand what kinds of planets are out there.

This article originally appeared in print in the April 2012 issue of Sky & Telescope. Subscribe to Sky & Telescope.