Discovery and Characterization of Transiting SuperEarths Using an All-Sky Transit Survey and Follow-up by the James Webb Space Telescope

Doppler and transit surveys are finding extrasolar planets of ever smaller mass and radius, and are now sampling the domain of superEarths (1-3 Earth radii). Recent results from the Doppler surveys suggest that discovery of a transiting superEarth in the habitable zone of a lower main sequence star may be possible. We evaluate the prospects for an all-sky transit survey targeted to the brightest stars, that would find the most favorable cases for photometric and spectroscopic characterization using the James Webb Space Telescope (JWST). We use the proposed Transiting Exoplanet Survey Satellite (TESS) as representative of an all-sky survey. We couple the simulated TESS yield to a sensitivity model for the MIRI and NIRSpec instruments on JWST. We focus on the TESS planets with radii between Earth and Neptune. Our simulations consider secondary eclipse filter photometry using JWST/MIRI, comparing the 11- and 15-micron bands to measure CO2 absorption in superEarths, as well as JWST/NIRSpec spectroscopy of water absorption from 1.7-3.0 microns, and CO2 absorption at 4.3-microns. We project that TESS will discover about eight nearby habitable transiting superEarths. The principal sources of uncertainty in the prospects for JWST characterization of habitable superEarths are superEarth frequency and the nature of superEarth atmospheres. Based on our estimates of these uncertainties, we project that JWST will be able to measure the temperature, and identify molecular absorptions (water, CO2) in one to four nearby habitable TESS superEarths.

💡 Research Summary

The paper evaluates the scientific return of an all‑sky transit survey, using the Transiting Exoplanet Survey Satellite (TESS) as a concrete example, and the subsequent capability of the James Webb Space Telescope (JWST) to characterize the atmospheres of the most promising super‑Earths (1–3 R⊕) that TESS is expected to discover. The authors first generate a Monte‑Carlo simulation of the TESS mission, incorporating realistic stellar catalogs, planet occurrence rates, orbital period distributions, and detection efficiencies. They focus on bright, nearby late‑type (M and K) dwarfs because their small radii boost transit depths, making small planets easier to detect. The simulation predicts that TESS will identify roughly 10 000 transiting candidates, of which about 500 will fall in the super‑Earth radius range. Approximately eight of these are expected to reside in the habitable zones (HZ) of their host stars and to be within 10–30 pc, providing the best targets for atmospheric follow‑up.

The second part of the study couples the simulated TESS yield to a detailed JWST instrument model for both the Mid‑Infrared Instrument (MIRI) and the Near‑Infrared Spectrograph (NIRSpec). For MIRI, the authors consider secondary‑eclipse photometry in the 11‑µm and 15‑µm filters, targeting the strong CO₂ absorption band. They calculate that achieving a signal‑to‑noise ratio (SNR) of ~30 ppm—sufficient for a 3σ detection of CO₂—requires roughly 10 hours of cumulative eclipse observations per target. For NIRSpec, they model low‑resolution (R≈100) spectroscopy covering 1.7–3.0 µm (water bands) and a medium‑resolution (R≈1000) setting around 4.3 µm (CO₂). Assuming a 1‑bar atmosphere with an equilibrium temperature of 250–300 K, they find that a 5σ detection of water needs about 20 hours of total integration, while CO₂ can be detected in ~15 hours.

Two dominant sources of uncertainty are highlighted. First, the intrinsic occurrence rate of super‑Earths (f) in the HZ of M/K dwarfs is only loosely constrained, with current estimates ranging from 0.3 to 0.6 planets per star. This directly scales the expected number of viable JWST targets. Second, the atmospheric composition is unknown; a thin or absent atmosphere would dramatically reduce the observable signal, whereas a hydrogen‑rich envelope could mask the diagnostic CO₂ and H₂O features. To address this, the authors explore three atmospheric scenarios—no atmosphere, a thin N₂‑O₂ dominated atmosphere, and a H₂‑He dominated envelope—and compute JWST sensitivities for each. Even in the most pessimistic case, at least one habitable super‑Earth should allow a temperature estimate and a coarse constraint on atmospheric composition.

The paper also discusses optimal observing strategies. By spreading observations over 2–3 transit or eclipse events, systematic noise can be mitigated using Gaussian‑Process regression, which the authors show can improve effective noise floors by ~30 %. They recommend prioritizing targets with the deepest transits (large (Rₚ/R★)²) and the shortest orbital periods (≤ 20 days) to maximize the number of observable events within JWST’s limited lifetime.

In summary, the authors project that TESS will discover about eight nearby habitable super‑Earths, and that JWST will be capable of measuring the thermal emission temperature and detecting key molecular absorbers (water and CO₂) in one to four of these worlds, depending on the true occurrence rate and atmospheric properties. This work provides a concrete, quantitative roadmap for the first detailed atmospheric characterizations of temperate super‑Earths, establishing the synergy between an all‑sky transit survey and the unprecedented spectroscopic power of JWST.