The Study of Climate on Alien Worlds
Characterizing atmospheres beyond our Solar System is now within our reach
The importance of these discoveries to astronomy cannot be overstated—they signal the dawn of exoplanetary meteorology, or at least legitimize its study in the eyes of astronomers and astrophysicists.
Astronomers now possess a tool kit not only to measure the masses and sizes of exoplanets but also to characterize their atmospheric dynamics and chemistry. Besides galvanizing the astronomical community, this newfound field is starting to exert a profound sociological impact on related fields of study: atmospheric and climate science, geophysics and planetary science. It marks the first great confluence of these fields with astrophysics, a gathering of scientists with different scientific and modeling philosophies, which is especially evident at interdisciplinary conferences where we struggle to understand one another’s jargon. Atmospheric and climate scientists, as well as geophysicists, are firmly grounded in a data-rich regime, living within the system they study. Awash in an abundance of data from the terrestrial atmosphere and the geological record, no single model is capable of accounting for all of the observed phenomena. Instead, a hierarchy of models with different degrees of sophistication is utilized, with each model isolating key pieces of physics. The strategy is to first divide and conquer, then to unify and rule.
The knowledge gleaned from studying Earth and the Solar System planets serves as an invaluable guide, but there is a cautionary tale to be told. As a rule of thumb, there are two characteristic length scales describing an atmosphere: the Rhines length is the typical width of zonal (east-west) jets, whereas the Rossby length is the typical size of vortices or eddies. For Solar System objects, both length scales are much smaller than planetary radii. On close-in exoplanets, the Rhines and Rossby lengths are comparable to exoplanetary radii, implying that the atmospheric features are global in extent, an expectation that is borne out in three-dimensional simulations. The atmospheres of close-in exoplanets are thus in a circulation regime that is unprecedented in the Solar System. Atmospheric circulation simulations therefore have to be global instead of local, and other physical implications—such as the mixing of atmospheric constituents and its effect on the spectral appearance of the exoplanet—remain to be fully understood.
The study of exoplanets is essentially confined to the scrutiny of point sources in the night sky. Although we may obtain detailed spectral and temporal information on these point sources, the procurement of detailed spatial information remains a grand challenge for posterity. Planetary scientists benefit from the ability to obtain photographs of the Martian surface and Jovian weather patterns, a privilege unavailable to astrophysicists. It is important to recognize that astrophysicists are therefore trapped in a data-poor regime, with its myriad restrictions on how to construct models and interpret data. When faced with multiple explanations that are consistent with a given data set, astrophysicists often apply the principle of Occam’s Razor: In the absence of more and better data, the simplest explanation is taken as the best one. To put it more tongue-in-cheek, one aims to be roughly accurate rather than precisely wrong. The need to recalibrate our scientific expectations and philosophies lies at the heart of this confluence of expertise.
From studying the atmospheres of Earth and the Solar System planets, researchers have realized that atmospheres are complex entities subjected to positive and negative feedback loops, exhibiting chemical, dynamical and radiative signatures over a broad range of time scales. Isaac Held of the Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey, has argued that to truly understand these complex systems, one has to construct a hierarchy of theoretical models. These simulations range from one-dimensional, pen-and-paper models that isolate a key piece of physics to full-blown, three-dimensional general circulation models (GCMs)—used for climate and weather forecasting—that incorporate a complicated soup of ingredients to capture the intricate interactions between the atmosphere, land and oceans on Earth. For instance, GCMs concurrently solve a set of equations (called the Navier-Stokes equation) that treat the atmosphere as a fluid, along with a thermodynamic equation and diverse influencing factors such as orography (mountain formation) and biology. Many of these intricacies are unwarranted in theoretical investigations of exoplanetary atmospheres, and thus one of the key challenges is to realize how and when to simplify an Earth-centric model.
Whether knowingly or unknowingly, a hierarchy of one- to three-dimensional theoretical models has emerged in the astrophysical literature. Because the treatment of hot Jupiters and brown dwarfs—substellar objects not massive enough to sustain full-blown nuclear fusion at their cores—share several similarities, many of the pioneering models (by researchers such as Adam Burrows of Princeton University and Ivan Hubeny of the University of Arizona) were carried over from the latter to the former class of objects. Furthermore, the early models focused on the spectral appearance of hot Jupiters, with the most sophisticated variants borrowing from an established technique in atmospheric and climate science known as abundance and temperature retrieval. Given the spectrum of an exoplanet, this technique obtains the atmospheric chemistry and temperature-pressure profile consistent with the data. In the case of the hot Jupiter WASP-12b, Nikku Madhusudhan of Yale University and his collaborators inferred, using the retrieval technique, that the exoplanet possesses a carbon-to-oxygen ratio at least twice that of its star. If this result is confirmed—and the carbon-to-oxygen ratio is measured for other exoplanets—it offers a valuable link between the properties of an exoplanetary atmosphere and the formation history of the exoplanet.
Astrophysicists have been quick to realize that atmospheric chemistry and dynamics intertwine in a nontrivial manner to produce the observed characteristics of a hot Jupiter. Adam Showman, a planetary scientist at the University of Arizona, became one of the first researchers to harness the power of GCMs in studying hot Jovian atmospheres. Several other researchers from the astrophysical community (including myself) followed soon after. My collaborators and I generalized a benchmark test, which solves for the basic climatology of a (exo)planet using two methods of solution, to hot Jupiters. The transport of heat from the day-side to the night-side hemisphere of a spin synchronized exoplanet is—by definition—at least a two-dimensional problem. For extrasolar gas giants, the characteristic time scale on which the atmosphere reacts to such radiative disturbances spans many orders of magnitude, thus necessitating its theoretical consideration in three dimensions, an endeavor that is only tractable using GCMs. Several groups have now successfully adapted GCMs to model exoplanetary atmospheres and are obtaining consistent results. Some outstanding technical issues remain, but it is clear that three-dimensional models are necessary if one wishes to predict not just the spectral appearance of exoplanets, but simultaneously their phase curves and temporal behavior. As the astronomical state-of-the-art advances, the exoplanets being discovered will be more Earthlike, both in size and temperature, with the implication that GCMs will become even more relevant.