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The Study of Climate on Alien Worlds

Characterizing atmospheres beyond our Solar System is now within our reach

Kevin Heng

Earthlike Exoplanets

We are only starting to understand the basic properties of hot Jupiters, including why some appear more inflated than others and why some appear to redistribute heat more efficiently from their day-side to their night-side hemispheres. In the cases of HD 189733b and HD 209458b, simulated spectra and phase curves—the latter of which constrains the efficiency of heat redistribution—that are computed using GCMs are able to match their observed counterparts fairly well. Until the next generation of space telescopes becomes operational, these examples remain the cornerstones of our understanding of exoplanetary atmospheres.

HD 189733b is noninflated, meaning that its radius and mass are well matched by standard evolutionary theories of exoplanets (which predict the size of an exoplanet as it cools down from the primordial heat of formation). It also appears to be shrouded in haze of unidentified chemistry, because its spectrum in the optical—obtained via the Hubble Space Telescope by Frédéric Pont and David Sing of Exeter University, together with their collaborators—reveals a smooth, featureless slope consistent with Rayleigh scattering (the same process that causes the color of the sky as observed from Earth by preferentially affecting bluer sunlight).

By contrast, HD 209458b is free of haze, but it is markedly larger than expected from evolutionary calculations. Theoretical ideas for radius inflation include the suggestion that partially ionized, hot Jovian atmospheres behave like giant electrical circuits, which when advected past an ambient magnetic field invoke Lenz’s law on a global scale: Nature abhors a change in magnetic flux. Electric currents and opposing forces are induced to counteract the horizontal winds; the consequent conversion of mechanical energy into heat, called Ohmic dissipation, is believed to be responsible for keeping some hot Jupiters inflated. However, it remains to be proven if hot Jupiters even possess magnetic fields like those of Earth and some of the Solar System planets. This field of research remains active.

2012-07HengF6.jpgClick to Enlarge ImageThe study of hot Jupiters remains relevant because we already have the data to inform our hypotheses and modeling efforts, thereby affording us the opportunity to sharpen our theoretical tools—as much of the salient physics is identical—before applying them to Neptunelike or even Earthlike exoplanets, for which the data are currently scarce or nonexistent. For many researchers, the ultimate prize is more familiar: to detect the spectrum of an Earthlike exoplanet orbiting a Sunlike star, and thereby answer age-old questions about the existence of extraterrestrial life. More succinctly, one wishes to establish if the solitary example of an Earth twin in orbit around a solar twin is the only possible cradle for life in the Universe. At the moment, such a quest remains elusive and appears out of the reach of even the next generation of space telescopes.

Instead, astronomers such as David Charbonneau of Harvard University and Jill Tarter of the SETI Institute have argued that a promising route toward detecting potentially habitable super Earths—Earthlike exoplanets with masses and radii somewhat larger than those of Earth—is to hunt for them around M stars (also known as red dwarfs). These diminutive cousins of our Sun, with only a tenth to half of its mass, comprise about three-quarters of the stellar population in our galactic neighborhood. There are several advantages to scrutinizing M stars: They are cooler in temperature than Sunlike stars, implying that their exoplanets may reside 10 to 100 times closer and yet still be able to harbor liquid water on their surfaces. Being more proximate to their M stars renders these exoplanets more amenable to detection via current, established astronomical techniques—namely, transit and radial velocity measurements. However, the price to pay is that they are expected to be spin synchronized and possess permanent day- and night-side hemispheres, much like their hot Jovian brethren. Such an expectation has led to theoretical concerns that their atmospheres may collapse due to the main constituent molecules condensing out on the frigid night sides. The astronomical approach to this conundrum is to charge forward with making new and better observations—after all, the answer is ultimately revealed by the data. For example, for the super Earth GJ 1214b, transmission spectra have already been obtained, but interpretations about its atmospheric composition remain controversial.

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