CV (updated September 2024).

My publication list on ADS.

My research focuses on the demographics and atmospheres of exoplanets smaller than Neptune. There is strong evidence that these small planets are much more common in our galaxy than larger planets. I am particularly interested in super-Earths and sub-Neptunes, planets with radii between those of the Earth and Neptune. Theoretically, these planets can have a variety of compositions ranging from terrestrial to miniature gas giants. Further, no such planets are known to exist in our own Solar System, so the only constraints on their interior structure and composition come from observations of super-Earths and sub-Neptunes around other stars.

To improve our understanding of the demographics, composition and formation of small exoplanets, we need to build a sample with measurements of as many planetary and stellar properties as possible. The best way to do this is by studying small planets transiting bright stars.

Detection and Characterization of Long-Period Exoplanets

 

The Transiting Exoplanet Survey Satellite (TESS)

Now in its seventh year, the TESS mission has enabled the discovery of over 500 new transiting exoplanets and more than 7000 planet candidates. Most of these systems transit bright, nearby stars, allowing both their planetary radius and their mass to be measured, and facilitating atmospheric characterization. To carry out a sky survey in a limited amount of time, even with its 94 x 26 degree field of view TESS observes most of the sky for a window of time (or sector) that lasts just 27 days. This means that the planets it most easily finds have relatively short orbital periods and are very close to their host star.

To find transiting planets with period greater than ~30 days, it is necessary to identify and confirm planets that show only one transit within one or more sectors of TESS data. We then use ground-based telescopes to determine the period and mass of the planets with radial velocity measurements. Studying exoplanets in these "intermediate orbits" represents a stepping stone in the journey of understanding exoplanets increasingly similar to those in our Solar System.

                                                                                                                                                                                   Figure credit: Hugh Osborn

I am also a member of the TESS Follow-Up Observing Programs (TFOP) steering committee, the chair of the TFOP space-based photometry subgroup, and lead of the TESS Single Transit Planet Candidate Working Group.

Atmospheres of Small Exoplanets

With the advent of JWST, astronomers can now peer into the atmospheres of exoplanets with unprecedented sensitivity. I am currently interested in using JWST to better understand small hot exoplanets. As part of a team led by scientists at NASA JPL, we recently published the first clear detection of an atmosphere on a rocky exoplanet. This planet - a super-Earth called 55 Cancri e - orbits its star in just under 18 hours and had a temperature of ~2000 K. JWST observations of its secondary eclipse (shown upper left) allowed us to detect absorption from CO/CO2 (shown lower left), which indicates that in spite of the extreme irradiation it receives, 55 Cancri e has an atmosphere.

   Figures from Hu et al. (2024)

While 55 Cancri e's atmosphere likely represents only a small fraction of the planet's radius, there is a small population of hot exoplanets that do have a substantial atmosphere despite their high temperature. Unlike hot Jupiters, the much lower mass of these "hot Neptunes" was not expected to be sufficient to hold onto such a larger atmosphere against the stellar irradiation. In other words, there should be a hot Neptune desert. But within it, we have discovered a few stragglers. Why are they there? Specifically, how did they manage to keep their atmosphere for billions of years?
Next up, we will use JWST to investigate the atmospheric properties of such a hot Neptune - LTT 9779b. Stay tuned for what we find!