The Chemical Compositions of Sub-Neptune Atmospheres

Sub-Neptunes (exoplanets smaller than Neptune yet substantially larger than Earth) are some of the most abundant planets, but the makeup of their interiors and atmospheres compositions are not yet well known. The bulk interior makeup (as determined from mass and radius) is inherently degenerate, but the atmospheric composition can be measured via transmission spectroscopy. I will report on our HST/WFC3 near-infrared transmission survey of four sub-Neptunes orbiting nearby bright stars. The sample includes clear molecular detections in several planets, and the survey further elucidates population-level trends in the atmospheres of these planets. These results will also have strong implications for planning future exoplanet studies with JWST.