Exoplanet atmospheric exploration and categorization through transmission spectroscopy

Abstract

Transiting exoplanets provide an amazing sample through which transmission spectroscopy observations, combined with atmospheric retrieval, can characterize the atmospheres of those planets. Out of that sample, super Earth exoplanets are particularly interesting because it is expected that their atmospheres will have a large diversity - from terrestrial-like to mini-Neptune-like. Discovering and understanding this large diversity will lead to fundamental progress forward in categorizing exoplanets and planet formation theory. Due to this intrigue of investigating super- Earth-sized exoplanets, the following thesis is separated into two projects involving the characterization of super Earth atmospheres. TESS will provide an unprecedented sample of nearby transiting super Earth exoplanets. Due to the large number of anticipated detections, developing prescreening techniques is especially important in order to determine which objects are the most desirable for in-depth investigations. The first study focuses on how we can use transmission spectroscopy to separate low-cloud H2-dominated super Earth atmospheres from other types of super Earth atmospheres. To do such, I define a metric called the "relative amplitude" of the spectral features in transmission. I find that spectral features in low-cloud H₂-dominated super Earth atmospheres will have a relative amplitude approximately 2.5 times larger than both high-cloud H2-dominated super Earth atmospheres and low-mean-molecular-mass super Earth atmospheres. I use this metric to predict the number of planets in which we could make the low-cloud H₂-dominated characterization for three different categories of exoplanets [currently detected exoplanets, anticipated TESS-planets, and anticipated transiting planets on the sky] and three different telescope options [0.3 m aperture microsatellites, a 0.76 m aperture telescope (same photon collecting area as FINESSE), and a 6.5 m aperture telescope (same photon collecting area as JWST)]. For the anticipated TESS-planets, a 6.5 m telescope would be able to characterize approximately 14 planets in 100 hours of observation time, 50 planets in 550 hours of observation time, 100 planets in 1600 hours of observation time, and 200 planets in 5300 hours of observation time. In comparison, the smaller telescopes (0.76 m and 0.3 m) require much longer amounts of time to build up the SNR required to make the low-cloud H2-dominated characterization. Specifically, the 0.76 m telescope (or the 6 microsatellite configuration) would characterize between 15 - 26 planets within the first three years of observation. A four microsatellite configuration would characterize 5 - 11 planets within its first three years of observation. In summary, if the field is unwilling to spend more than 500 hours of 6.5 m telescope time on prescreening exoplanets, then we must launch a characterization mission in the near future. In the second study, I focus specifically on the super Earth GJ 1214b, the most studied super Earth to date. GJ 1214b is particularly interesting due to the difficulty in characterizing its atmosphere because of observed flat transmission spectra. The currently accepted characterization for the flatness in the observed spectra is that GJ 1214b's atmosphere is dominated by high-altitude clouds. In this study, I investigated, using a Monte Carlo Markov Chain atmospheric retrieval algorithm, whether planetary parameters could compensate for each other to create the apparent flatness in the observed transmission spectrum of GJ 1214b. My analysis confirms the conclusions of previous studies that high-altitude cloud deck characterizations are consistent with the GJ 1214b transmission spectrum. However, by probing how different planetary parameters can compensate for each other, I have uncovered a small portion of phase space (approximately five percent of the MCMC solution set phase space) with low-altitude clouds (cloud tops at pressures greater than 100 Pa) and realistic atmospheric temperatures (temperatures greater than 400 Kelvin). Further investigation of this space is required to determine if such atmospheric compositions could exist in chemical equilibrium. Ultimately, future work may definitively prove that the only realistic characterization for GJ 1214b is high-altitude clouds. To this end, important future work for GJ 1214b includes determining its effective temperature, constraining the pressure level probed in transmission, and estimating its albedo. All three of these investigations will provide for better constraints on the possible characterizations for GJ 1214b. In conjunction, future higher signal-to-noise ratio, broader wavelength coverage, and higher spectral resolution observations between 1 - 5 [tm are needed at an SNR greater than ten times the current observations in order to better constrain the atmospheric composition of GJ 1214b. The James Webb Space Telescope is the optimal facility for these measurements.