Our ability to detect and characterize small planets in diverse environments is expanding rapidly with the development and continued improvement of the transit and radial velocity methods. Better models, instruments, and telescopes are producing greater planet yields and tighter planetary radius and mass constraints, which in turn provide new targets for atmospheric characterization and produce new insights on planet composition, formation, and evolution. In this thesis, I present work on the characterization and mass determination of small planets with the radial velocity method, the detection of new planets via the transit method, and the study of a planet’s atmosphere through transmission spectroscopy and its implications for planet formation and planet population features.

First, I report on my mass estimation and characterization of the long-period exoplanet Kepler-538b. This sub-Neptune with a period of P = 81.7 days is the only planet known to be orbiting its Sun-like star (0.892 M⊙). By simultaneously modeling Kepler photometry and radial velocities (RVs), I find a semi-amplitude of 1.68 ± 0.39 m s−1 and a planet mass of 10.6 ± 2.5 M⊕, which made Kepler-538b the smallest planet beyond P = 50 days with an RV mass measurement at the time of publication.

Next, I discuss K2-136c, a sub-Neptune with a period of P = 17.3 days and the largest of three transiting planets orbiting a late-K dwarf (0.742 M⊙) in the young Hyades open cluster (650 ± 70 Myr). Collecting and analyzing RV data from the HARPS-N and ESPRESSO spectrographs jointly with photometry from the K2 and TESS space telescopes, I find K2136c induces a semi-amplitude of 5.46 ± 0.45 m s−1, corresponding to a mass of 18.0 ± 1.7 M⊕. K2-136c is now the smallest planet to have a measured mass in an open cluster and one of the youngest planets ever with a mass measurement.

I then present the TATER planet detection pipeline and apply it to high-cadence photometry of 914 known planet systems observed during TESS Cycle 3. I report the validation of 4 new short-period planets. This study provides independent modeling and vetting of hundreds of planet candidates while also expanding the known planet population and providing possible new targets for follow-up radial velocity and transmission spectroscopy observations

Finally, I report on the atmospheric characterization of WASP-166b, a short-period superNeptune (P = 5.44 d, Mp = 32.1±1.6 M⊕, Rp = 7.1±0.3 R⊕). WASP-166b is located in the Hot Neptune Desert, a sparse region of exoplanet parameter space at high instellation flux and intermediate planet radii. Using transmission spectroscopy of WASP-166b (two transit observations with the James Webb Space Telescope), we make novel detections of H2O and CO2, report non-detections of SO2, NH3, and a cloud deck, place constraints on planetary metallicity and the C/O ratio, and find evidence for planetary migration, suggesting that planetary migration is a crucial component of the origin story of the Hot Neptune Desert