The Study of Climate on Alien Worlds
Characterizing atmospheres beyond our Solar System is now within our reach
The first studies of exoplanetary atmospheres were performed on a class of objects known as hot Jupiters. A combination of the transit technique with a measurement of the radial velocity (which is the gravitational wobble of a star as its exoplanet orbits around their common center of mass) yields the radius and mass of a hot Jupiter, respectively, and reveals that they are similar in these aspects to our own Jupiter. The startling difference is that hot Jupiters are found about a hundred times closer to their parent stars than Jupiter, which raises their surface temperatures to between 1,000 and 3,000 degrees Kelvin. With spatial separations of a hundredth to a tenth of an astronomical unit (the average distance from the Earth to the Sun) from their stars, the discovery of hot Jupiters caught the astronomical community by surprise, because their existence was neither predicted from astrophysical theory nor subsequently explained by it.
Their large sizes render hot Jupiters easier to observe and thus the most obvious laboratories for extrasolar atmospheric studies. Furthermore, the belief that their atmospheres are dominated by molecular hydrogen—which is consistent with the densities of the exoplanets, inferred from the astronomical observations to be about 1 gram per cubic centimeter—offers some hope that the atmospheres are primary, reflecting the composition of the primordial nebulae from which they formed, rather than secondary and reprocessed by geological mechanisms (such as on Earth).
Given enough time, an exoplanet’s position and rotation tend to relax toward a state of minimum energy—a spin synchronized state, such that one hemisphere of the exoplanet always faces its parent star with the other hemisphere shrouded in perpetual darkness. The characteristic time scale associated with this process is typically 1,000 times less than the age of the star. (As a more familiar example, the Moon is in a spin synchronized state with respect to the Earth, notwithstanding its tiny rotational corrections called librations.) In other words, one hot Jovian day is equal to one hot Jovian year. The unfamiliar configuration of permanent day- and night-side hemispheres on hot Jupiters opens up an unexplored regime of atmospheric circulation with no precedent in the Solar System and motivates theoreticians to test their tools in unfamiliar territory.
Understanding these hot Jovian atmospheres requires clarifying the complex interplay between irradiation, atmospheric dynamics, chemistry and possibly magnetic fields. On the most irradiated hot Jupiters, the exoplanet viewed from the poles resembles a sphere painted half white and half black—the phase curve is a sinusoidal function that peaks at secondary eclipse and becomes dimmest at transit. Any shift of this peak from its reference point at secondary eclipse may be interpreted as being due to the presence of horizontal winds in the atmosphere, which act to transport heat from the day- to the night-side hemisphere. This angular shift was first measured for an exoplanet, the hot Jupiter HD 189733b, by Heather Knutson of the California Institute of Technology and her collaborators, who reported a peak shift of about 30 degrees east—in the direction of rotation. This angular shift was also measured for the hot Jupiters Ups And b (by Ian Crossfield of the University of California at Los Angeles and his collaborators) and WASP-12b (by Cowan and his collaborators).
Other astronomers continue to push the envelope. Ignas Snellen of Leiden University and his colleagues, using the ground-based European Very Large Telescope (VLT), used a technique called absorption spectroscopy to measure the speed of the horizontal winds on the hot Jupiter HD 209458b. The technique compares the relative size of the exoplanet across a range of wavelengths. At a wavelength where an atmospheric atom or molecule is the most absorbent, the exoplanet appears larger. By monitoring the shift in wavelength of an absorption line of carbon monoxide, the group determined that HD 209458b’s winds clock in at about 2 kilometers per second, roughly 100 times faster than those on Earth. More attempts to measure atmospheric wind speeds are in the works, and these measurements remain at the cutting—if not the bleeding—edge of what astronomers can achieve.
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