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Prospects for Characterizing the Haziest Sub-Neptunes with High Resolution Spectroscopy

Presentation #210.05 in the session “Exoplanets and Systems: Giant Planet Atmospheres 1”.

Published onOct 26, 2020
Prospects for Characterizing the Haziest Sub-Neptunes with High Resolution Spectroscopy

Observations to characterize planets larger than Earth but smaller than Neptune have been largely inconclusive at low spectral resolution due to hazes or clouds that obscure molecular features in their spectra. However, here we show that high-resolution spectroscopy (R~25,000 to 100,000) will enable us to probe the regions in these atmospheres above the clouds where the cores of the strongest spectral lines are formed. We present models of transmission spectra for a suite of sub-Neptune planets from 1–5 microns at a range of resolutions relevant to current and future ground-based spectrographs. We are particularly interested in the cloudiest planets where low-resolution transmission spectroscopy has yielded no constraints. We model GJ1214b-like planets with thick photochemical hazes, as well as hotter and cooler variants of GJ 1214b. Furthermore, we compare the utility of the cross-correlation function that is typically used with a new likelihood function derived for this technique. We show distinct advantages for the likelihood function, including the ability to detect haze opacity. We calculate the signal-to-noise of these spectra required to robustly detect a host of molecules, such as CO, CO2, H2O, and CH4, and photochemical products like HCN, as a function of wavelength range and spectral resolution to aid in planning future observations. We discuss how these requirements compare to what is achievable with current and future instruments, showing that high spectroscopy of these small planets is soon within reach.

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