Zodiacal Exoplanets in Time: Spitzer's View of Two Young Exoplanets

The plethora of planets discovered by Kepler and earlier surveys have revolutionized our understanding of old (>1 Gyr) planetary systems around other stars. However, Kepler results tell us little about how planets evolve over their lifetimes. Stellar ages are difficult to determine, and most exoplanet detection methods are biased against the detection of young planets that undergo the most rapid evolution. By observing nearby young open clusters and star forming regions, the re-purposed Kepler mission (K2) has enabled the discovery and characterization of some of the first infant planetary systems. From K2 data, our Zodiacal Exoplanets in Time (ZEIT) survey has already identified planets spanning ages from 10 to 650 Myr. These planets have provided the first glimpse into how planets change over their lifetimes, demonstrating that young planets are statistically larger than their older counterparts, and that at least some close-in planets migrate through interactions with their protoplanetary disk or form in situ. We have targeted two young planets (K2-33b and K2-25b) for detailed follow-up with Spitzer. These two planets span the age range of the sample (10 and 650 Myr), and are both within reach of transmission spectroscopy (transit depth as a function of wavelength) with existing instruments. Here we propose for simultaneous analysis of this Spitzer data with the discovery K2 light curves. Our major goals are: 1) to see if their transmission spectra are consistent with hazy/flat atmospheres seen for similar-sized planets at older ages, 2) measure the dynamical properties of these planets at wavelengths where the impacts of stellar variability are reduced, 3) probe why young planets are statistically larger than their older counterparts, and 4) plan for more comprehensive study of young planetary atmospheres with JWST and/or HST.