A New Frontier: Observations of Exoplanet Atmospheres with the James Webb Space Telescope

Sara Seager, seager@mit.edu

The future of exoplanet atmospheric studies resides in the James Webb Space Telescope. With its large aperture and infrared spectroscopic capability, Webb will open opportunities to study planets as small as terrestrial size, taking us far beyond current studies of exoplanet atmospheres, which are limited mostly to hot Jupiters. Webb will access a wide variety of exoplanets, falling into two categories. First are the self-luminous young planets (down to about Saturn’s mass) with wide orbital separations, i.e., with apparent separations greater than a few arcseconds, so the images of the star and planet are distinct (e.g., Beichman et al. 2010). Second are the transiting planets of all sizes, where adequate angular separation is not an issue, but the required orbital alignment will be fortuitous and infrequent, limiting the numbers of transiting planets accessible for study.

Here we focus on the transiting mini Neptunes and super Earths, which are enigmatic planets with no solar system counterparts.  The so-called mini Neptunes—planets of about 2 to 3 Earth radii (or about 10 to 30 Earth masses), which by their sizes and/or densities must have hydrogen or hydrogen-helium gas envelopes—are extremely common, as found by Kepler (Fig. 1; Fressin et al. 2013), by radial velocity observations (Howard et al. 2010), and by microlensing surveys (Cassan et al. 2012). Mini Neptunes are likely to be ten times more common than Jupiter-size or Jupiter-mass planets. Nevertheless, we do not have a good understanding of the origin or composition of the mini Neptunes. Super Earths—planets that are predominantly rocky with thin atmospheres—are so interesting because those with cool enough surface temperatures may be suitable for life.

Webb will be an exoplanet-characterization machine, following up known transiting exoplanets, one planet at a time.