Still Chasing Shadows: Transiting Exoplanets in the Era of JWST

As of March 2024, 75% of the exoplanets listed in NASA’s exoplanet archive were identified with the transit method and 80% of those were identified by the Kepler photometer. Demographic data from Kepler challenges theorists to model the physical processes that drive the observed diversity of exoplanets. Testing synthetic populations from theory against the Kepler data continues to yield important insights about atmospheric escape processes. Now contemplating its third extended mission, NASA’s TESS Mission continues to identify nearby transiting exoplanets. 75% of the known transiting sub-Neptunes within 50 pc were identified by TESS. Mass characterization of these nearby planets yields growing evidence of “water-worlds” in relatively short-period orbits, though mapping density to bulk composition is fraught with model degeneracies. The James Webb Space Telescope began science operations in 2022, giving us a new lens on planet diversity via transmission spectroscopy. Over 70 transiting exoplanets were observed in Cycle 1 alone, and the community is patiently awaiting answers to questions about the nature of mini-Neptunes and super-Earths and the existence (or not) of atmospheres of small planets orbiting M dwarfs. We’ve seen a panoply of molecule detections in the panchromatic spectrum of a hot Jupiter, evidence of photochemistry and climate, and the first detection of methane in a temperate mini-Neptune. We have also seen spectra consistent with flat lines and/or spot contamination - a poignant reminder that the answers to our most pressing questions will not be easily won. STScI embraced the challenge by dedicating hundreds of hours of discretionary time to transiting exoplanets in Cycle 3. So while microlensing of planets beyond 1AU and directing imaging of potentially habitable planets loom large on the horizon, we continue chasing the shadows of transiting planets. By mission’s end, JWST promises to deliver detailed characterization of planetary atmospheres in numbers sufficient for population-level studies. That work is just beginning.