A Population Study of Gaseous Exoplanets

Abstract

We present here the analysis of 30 gaseous extrasolar planets, with temperatures between 600 and 2400 K and radii between 0.35 and 1.9 R Jup . The quality of the HST/WFC3 spatially scanned data combined with our specialized analysis tools allow us to study the largest and most self-consistent sample of exoplanetary transmission spectra to date and examine the collective behavior of warm and hot gaseous planets rather than isolated case-studies. We define a new metric, the Atmospheric Detectability Index (ADI) to evaluate the statistical significance of an atmospheric detection and find statistically significant atmospheres around 16 planets out of the 30 analysed. For most of the Jupiters in our sample, we find the detectability of their atmospheres to be dependent on the planetary radius but not on the planetary mass. This indicates that planetary gravity plays a secondary role in the state of gaseous planetary atmospheres. We detect the presence of water vapour in all of the statistically detectable atmospheres, and we cannot rule out its presence in the atmospheres of the others. In addition, TiO and/or VO signatures are detected with 4 σ confidence in WASP-76 b, and they are most likely present in WASP-121 b. We find no correlation between expected signal-to-noise and atmospheric detectability for most targets. This has important implications for future large-scale surveys.

Statistical Analysis of Hubble/WFC3 Transit Spectroscopy of Extrasolar Planets

“…The 30 spectra presented in Tsiaras et al (2017) were derived using a uniform and consistent data analysis method (Tsiaras et al 2016). However, it is still advantageous to compare them with independent spectra derived by other groups (Table 1, right columns).…”

“…We use observed spectra from Tsiaras et al (2017) and derive the magnitude of the water absorption from the data as directly as possible. Essentially, we find the minimum and maximum values of the data and convert the difference between them to scale heights of absorption.…”

“…For reasons that we explain below, our fundamental observational quantity is the number of atmospheric scale heights that are opaque in the water band during transit (Stevenson 2016). Section 2 describes how we determine that quantity based on the spectra from Tsiaras et al (2017), and Section 3 describes the correlation of the inferred absorption with other planetary properties. Section 4 summarizes our conclusions and discusses implications for future measurements.…”

“…Recently, Tsiaras et al (2017) announced a catalog of hotJupiter absorption spectra observed in multiple programs using HST/WFC3 in spatial scanning mode. In this Letter, we use these spectra with four additional spectra (Huitson et al 2013;Mandell et al 2013;Knutson et al 2014b;Kreidberg et al 2014b) in a statistical analysis of transit water absorption in relation to planetary bulk parameters for a sample of 34 hot-Jupiter (to hot-Neptune) exoplanets.…”

An Observational Diagnostic for Distinguishing between Clouds and Haze in Hot Exoplanet Atmospheres

“…The only two targets expected to produce higher signal-to-noise spectra are WASP-76b and WASP-33b. An initial study of the former planet does not suggest evidence for aerosols in its transmission spectrum (Tsiaras et al 2017). The latter has not yet been characterized in transmission and is also not an ideal target because its host star is a δ-Scuti variable (Herrero et al 2011).…”