Constraining Exoplanet Metallicities and Aerosols with the Contribution to ARIEL Spectroscopy of Exoplanets (CASE)

Abstract

2 Launching in 2028, ESA's 0.64 m 2 Atmospheric Remote-sensing Exoplanet Largesurvey (ARIEL) survey of ∼1000 transiting exoplanets will build on the legacies of NASA's Kepler and TESS and complement JWST by placing its high precision exoplanet observations into a large, statistically-significant planetary population context. With continuous 0.5-7.8 µm coverage from both FGS (0.50-0.6, 0.6-0.81, and 0.81-1.1 µm photometry; 1.1-1.95 µm spectroscopy) and AIRS (1.95-7.80 µm spectroscopy), ARIEL will determine atmospheric compositions and probe planetary formation histories during its 3.5-year mission. NASAs proposed Contribution to ARIEL Spectroscopy of Exoplanets (CASE) would be a subsystem of ARIELs FGS instrument consisting of two visible-to-infrared detectors, associated readout electronics, and thermal control hardware. FGS, to be built by the Polish Academy of Sciences Space Research Centre, will provide both fine guiding and visible to near-infrared photometry and spectroscopy, providing powerful diagnostics of atmospheric aerosol contribution and planetary albedo, which play a crucial role in establishing planetary energy balance. The CASE team presents here an independent study of the capabilities of ARIEL to measure exoplanetary metallicities, which probe the conditions of planet formation, and FGS to measure scattering spectral slopes, which indicate if an exoplanet has atmospheric aerosols (clouds and hazes), and geometric albedos, which help establish planetary climate. Our simulations assume that ARIEL's performance will be 1.3× the photon noise limit. This value is motivated by current transiting exoplanet observations: Spitzer/IRAC and Hubble/WFC3 have empirically achieved 1.15× the photon noise limit. One could expect similar performance from ARIEL, JWST, and other proposed future missions such as HabEx, LUVOIR, and Origins. Our design reference mission simulations show that ARIEL could measure the mass-metallicity relationship of its 1000-planet single-visit sample to > 7.5σ and that FGS could distinguish between clear, cloudy, and hazy skies and constrain an exoplanet's atmospheric aerosol composition to 5σ for hundreds of targets, providing statistically-transformative science for exoplanet atmospheres.2 Note that the wavelengths used here are adopted from the ARIEL Assessment Study Report, ESA/SCI(2017)2 (http://sci.esa.int/cosmicvision/59109-ariel-assessment-study-report-yellowbook): VISPhot photometry at 0.50-0.55, 0.6-1.0, and 1.0-1.2 µm; NIRSpec R=10 spectroscopy from 1.251.95 µm; AIRS spectroscopy 1.95-7.80 µm with R=30-100

ArielRad: the Ariel radiometric model

“…If this were not the case, it would be more appropriate to assume that the noise floor is not reduced by averaging multiple observations. However, there is currently no evidence supporting this as Hubble/WFC3 and Spitzer/IRAC observations have not yet reached a noise floor [34] .…”

“…where R p and R are the planet and star radii respectively and z(λ) is the atmospheric height. Because the interest is on the detection of the exoplant atmosphere, the "signal" is just the rightmost quantity [34,45] in the expression above, i.e.…”

Alfnoor: assessing the information content of Ariel's low resolution spectra with planetary population studies.

“…If a planetary spectrum has all 4 bands marked as flat, it is classified as a flat spectrum. This strategy is similar to that presented in Zellem et al (2019), however, while in that work the authors were only focused on the Ariel FGS channels, here we are considering the full Ariel spectral coverage.…”

Hot Jupiters: Origins, Structure, Atmospheres

“…Determining if the trend in phase curve amplitude with equilibrium temperature is statistically significant requires further observations (Parmentier & Crossfield, 2018) and an improved understanding of detector systematics (Bell et al., 2020; May & Stevenson, 2020). Analysis of the growing sample of TESS phase curves (e.g., Daylan et al., 2019; Jansen & Kipping, 2020; Shporer et al., 2019; von Essen et al., 2020; Wong et al., 2019), unpublished Spitzer phase curves (Deming & Knutson, 2020), and future phase curve observations with ARIEL (Tinetti et al., 2018; Zellem et al., 2019) and JWST (Bean et al., 2018) will provide a more complete understanding of how observable properties of hot Jupiter atmospheres depend on their planetary parameters.…”