Research

My research involves observing exoplanets to understand the chemistry, radiative processes, and circulation patterns they exhibit across a range of atmospheric regimes. To do this, I use primary transit, secondary eclipse, and phase curve observations. The ultimate goal driving my work is to one day measure and robustly interpret spectra for temperate rocky planets ('exo-Earths'), searching for evidence of life beyond our solar system. These observations will likely become possible in the 2030s, if one of NASA's next-generation space observatories (LUVOIR, Origins, or HabEx) is funded, or perhaps even with upcoming giant ground-based telescopes such as ELT, GMT, and TMT.

Right now, I'm using the Hubble and Spitzer Space Telescopes to characterize hot Jupiters and sub-Neptunes. In particular, I've led a broad observing program to study the ultrahot Jupiter WASP-121b, painting one of the most detailed portraits we have for an exoplanet. Below is a figure from Mikal-Evans et al. (2020), showing the emission spectrum we've measured WASP-121b's dayside hemisphere, which at a temperature of 2700 Kelvin is as hot as an M6 dwarf star. It shows a beautiful H2O emission feature, along with a slope caused by emission from H- ions.

The 2020s will be an especially exciting time for studying sub-Neptunes, the mysterious population of planets with radii between ~2-4x the Earth's radius (Neptune's radius is about 4x Earth radii). Here's a transmission spectrum we published for one such planet, HD3167c, which is consistent with models assuming chemical equilibrium and a high enrichment of heavy elements:

The above spectrum is from Mikal-Evans et al. (2021) and represents the state-of-the-art achievable with the Hubble telescope. When the James Webb Space Telescope becomes operational in late 2021, our spectroscopic wavelength coverage will be greatly expanded to 0.6-11 micron. This will provide an opportunity to detect numerous molecular species, including H2O, CH4, NH3, HCN, CO, and CO2, greatly increasing our ability to constrain complex atmospheric chemistries. By contrast, with Hubble we've essentially only been sensitive to a single H2O band at near-infrared wavelengths.

Over the coming years, we can expect plenty of outstanding targets for atmospheric characterization to be detected by the MIT-led TESS survey. The following figure I put together highlights the dramatic contribution of TESS. The axis on the left shows the population of transiting planets with published mass measurements (as of late-2020), plotted as planet temperature versus planet radius, and color-coded by the host star spectral type. The axis on the right shows the same planet population as small grey circles, with the predicted TESS discoveries around bright stars from Barclay et al. (2018) overplotted as colored circles.

In particular, TESS will vastly increase the sample of known sub-Neptunes around stars brighter than 10th magnitude. The latter is important, as bright systems are needed for detailed follow-up observations, such as precision radial velocity (to measure masses) and atmospheric studies. Furthermore, many of these sub-Neptunes will inhabit the 'warm' (~400-800K) and 'temperate' (~200-400K) temperature regimes, which remain largely uncharted by atmospheric studies. As the colors indicate, a lot of these cooler sub-Neptunes will be discovered around M dwarfs and will be ripe targets for James Webb. What's more, the predicted TESS planets shown above only include those expected to be discovered around the subset of high priority targets (i.e. the 2-minute cadence sample) during the 2-year prime mission. The full-frame images should contain many thousands of additional planets, although recovery of these will likely be limited by the availability of follow-up resources required to confirm them.