So thrilled for our group at UCLA to celebrate our first conferral of a Ph.D. to Dr. Mason MacDougall. Mason joined my group in 2019 and his thesis project involved characterizing both the typical and extreme values of small planet eccentricities to gain insight into their dynamical formation histories. Eccentricity refers to the elliptically of a planet's orbit. One of the great surprises from observations of giant exoplanets was the wide range of eccentricities that range from e = 0.0 (circular) to e > 0.9 (comet-like). (As a point of reference, the solar system planets are nearly circular with an average e of ~0.06). Today, far less is known about the eccentricities of Earth-to-Neptune-size planets because they produce smaller Doppler wobbles.
Mason searched for small planets with extreme eccentricities using brightness measurements (photometry) from NASA's TESS satellite. Basically, planets with anomalously short (or long) transits have a greater chance of being on eccentric orbits. Mason then followed up high-eccentricity candidates with precise Doppler measurements from the Keck Telescope as part of the TESS-Keck Survey (TKS) and confirmed the high-eccentricities of HIP97166 b and TOI-1272 b. Curiously, both planets have non-transiting companions planets that could play a role in establishing or maintaining their high eccentricities.
Mason's other goal was to measure the typical values eccentricities in a sample of ~100 small transiting planets from the TKS with just photometry. It's challenging to get precise measurements of a single planet's eccentricity from photometry alone, but it's possible to characterize the distribution of eccentricity in a population of planets, provided the light curves are modeled correctly. Mason and postdoc Greg Gilbert worked to develop an efficient and accurate framework to do this (see this paper too).
Mason applied this technique to the TKS sample and found that systems with only one transiting planet are more eccentric than those with more than one. This is likely due to the fact that multi-transiting systems are the outcome of more orderly planet formation histories with weaker gravitational interactions between planets. This tendency had been observed previously in analyses of Kepler planets (e.g. this paper), but the magnitude of this offset is different in the TKS sample, probably due to the different populations of planets probed by Kepler and TESS (see this paper, and a forthcoming paper).
It's been a real treat to mentor Mason these past four years, and I wish him the best as he starts a new job at Google as a data scientist in the fall.