Simulated JWST/NIRISS Transit Spectroscopy of Anticipated TESS Planets Compared to Select Discoveries from Space-Based and Ground-Based Surveys

TL;DR

This study uses simulations to evaluate the potential of TESS-discovered exoplanets for atmospheric characterization with JWST/NIRISS, comparing expected signal-to-noise ratios to known exoplanets and estimating observational requirements.

Contribution

It introduces a simulation-based approach to predict JWST/NIRISS S/N for anticipated TESS planets and compares these to known exoplanets, informing follow-up strategies.

Findings

Several hundred TESS planets will yield higher S/N than known exoplanets in the 1.5-2.5 R⊕ range.

Few TESS discoveries in the terrestrial regime will surpass known exoplanets like TRAPPIST-1 in S/N.

A 60-100 hour JWST program can effectively map the atmospheric transition region.

Abstract

The Transiting Exoplanet Survey Satellite (TESS) will embark in 2018 on a 2-year wide-field survey mission, discovering over a thousand terrestrial, super-Earth and sub-Neptune-sized exoplanets potentially suitable for follow-up observations using the James Webb Space Telescope (JWST). This work aims to understand the suitability of anticipated TESS planet discoveries for atmospheric characterization by JWST's Near InfraRed Imager and Slitless Spectrograph (NIRISS) by employing a simulation tool to estimate the signal-to-noise (S/N) achievable in transmission spectroscopy. We applied this tool to Monte Carlo predictions of the TESS expected planet yield and then compared the S/N for anticipated TESS discoveries to our estimates of S/N for 18 known exoplanets. We analyzed the sensitivity of our results to planetary composition, cloud cover, and presence of an observational noise floor. We found that several hundred anticipated TESS discoveries with radii from 1.5 to 2.5 times the Earth's radius will produce S/N higher than currently known exoplanets in this radius regime, such as K2-3b or K2-3c. In the terrestrial planet regime, we found that only a few anticipated TESS discoveries will result in higher S/N than currently known exoplanets, such as the TRAPPIST-1 planets, GJ1132b, and LHS1140b. However, we emphasize that this outcome is based upon Kepler-derived occurrence rates, and that co-planar compact multi-planet systems (e.g., TRAPPIST-1) may be under-represented in the predicted TESS planet yield. Finally, we apply our calculations to estimate the required magnitude of a JWST follow-up program devoted to mapping the transition region between hydrogen-dominated and high molecular weight atmospheres. We find that a modest observing program of between 60 to 100 hours of charged JWST time can define the nature of that transition (e.g., step function versus a power law).