One fifth of the giant planets discovered around other stars are orbiting at distances less than 0.1 Astronomical Unit from their stars. These planets' small orbits increase the likelihood that they will transit their stars as viewed from Earth. One such planet, HD209458b, has already been observed to transit its star every 3.5 days. From the dimming of the stellar light as the planet passes in front of its star, we now know that the planet's radius is 1.35 times that of Jupiter. Dozens more transit detections are expected within the next several years, allowing additional planets' radii to calculated, and direct measurements of albedo, day-night temperature differences, and atmospheric composition are also likely. Furthermore, the Kepler mission, scheduled for launch in 2006, should allow the radii and albedos of at least 100 transiting hot Jupiters to be inferred. An understanding of these measurements will require knowledge of possible atmospheric circulation regimes. The day-night temperature difference depends on the speed of advection (i.e., winds) across the planet. The albedo depends on where clouds form, and the abundance of condensible species such as silicates or alkali compounds depends on the geometry of the circulation. Furthermore, understanding the radius will require knowledge of the circulation. The radius represents the planet's history of cooling and contraction, which is affected by the circulation through the circulation's influence on temperature profile, cloud abundance, and internal energy transport. Evolution models of HD209458b that use realistic atmospheric temperatures predict a radius that is too small, which suggests that some heat source is missing from the calculations. The best hypothesis is that kinetic energy produced by the atmospheric heat engine is transported into the interior, where it counteracts the loss of energy that causes planetary contraction.
In this project Dr. Adam Showman, University of Arizona, will investigate the atmospheric circulation and evolution of short-period planets (the so-called "hot Jupiters"). Dr. Showman and collaborators will conduct three-dimensional, nonlinear numerical simulations of the atmospheric circulation of HD209458b (and other hot Jupiters) to determine (i) the nature of the circulation, including wind speeds, day-night temperature differences, and vertical structure, with implications for the infrared light curve of these planets, (ii) locations of cloud formation, with implications for albedo and its spatial variation across the planet, and (iii) the magnitude and depth of kinetic energy production and dissipation, which is crucial in evaluating whether the atmospheric heat engine can "inflate" the radius of HD209458b enough to satisfy its measured value. The simulations will be tightly linked to existing and upcoming observations and are among the first to investigate the effects of dynamics on the observable properties of these planets.
Broader impacts of the project are: (i) Graphic visualization of extrasolar planets will be performed, consistent with the simulations, to show what these planets may look like. These will be used in public and scientific presentations and interviews to increase public interest in extrasolar planets. (ii) The results of the research will be widely disseminated to the scientific community and general public through scientific talks, public presentations, and interviews. (iii) A graduate student will perform the work described here for a Ph.D. thesis. (iv) Dr. Showman will use the research to illustrate the excitement of discovery in the graduate and undergraduate courses that he teaches. ***
