Space-Based Telescopes

I have an excellent track record of making significant discoveries using space-based telescopes. In my most recent high-impact, first-author publication, I dedicated 61 Hubble Space Telescope (HST) orbits to measure the spectroscopic phase curve of exoplanet WASP-43b (the first measurement of its kind for any instrument) between 1.1 and 1.7 μm. The level of precision obtained from observing three full orbits of WASP-43b is unprecedented in exoplanet characterization. My colleagues and I were able to construct a 2D map of the planet's atmospheric thermal structure and precisely constrain the planet's atmospheric water abundance. A time-lapse video of WASP-43b over one planet rotation is available here. With these data, WASP-43b is now one of the most intensely scrutinized exoplanets to date.

I am currently involved in two projects that make use of the Spitzer Space Telescope. The first is an extension of the above HST program to obtain broadband photometric phase curves of WASP-43b at 3.6 and 4.5 μm. The planet's day-night heat redistribution (as measured by HST) is in contention with previously-published Spitzer measurements of exoplanets with similar equilibrium temperatures. The new WASP-43b observations will connect what we have learned from past Spitzer phase curves to current and future HST data, thus permitting a more complete understanding of atmospheric circulation in these benchmark exoplanets. The second is an ongoing project to determine the complete architectures of several Kepler candidate planets undergoing transit-timing variations (TTVs). The ill-timed failure of Kepler has left us with an incomplete picture for several long-period multi-planet systems due to insufficient time baseline. We are using Spitzer to confirm and characterize these dynamically interacting systems of cool planets.

Ground-Based Telescopes

Since 2013, I have been awarded 40+ nights of telescope time (as PI or Co-I) at large-telescope facilities such as W. M. Keck, Gemini, and Magellan to obtain transmission and emission spectra from a diverse group of exoplanets. I work in close collaboration with Professor Jacob Bean, who pioneered the ground-based technique for transit spectroscopy in both the optical and near infrared. As part of my extraction and modeling algorithm, I developed a new technique called Divide-White that uses the band-integrated (white) light curves to remove wavelength-independent systematics in both the target and comparison stars. This method has produced ground-based transmission spectra with the precision and wavelength resolution comparable to that which can be achieved with HST. The process of comparative exoplanetology is ongoing as I search for trends and examine correlations between objects with similar temperatures and/or gravities.

Transit Geometry

During the primary transit, the planet passes in front of its host star as seen from Earth. We measure the transmission spectrum of the planet as light from the host star is absorbed by chemical species in the planet's atmosphere. These data are sensitive to the relative chemical abundances and the presence of cloud or haze particles. During the secondary eclipse, the planet passes behind its host star as seen from Earth. As its light is blocked, we measure the dayside emission spectrum of the planet. Emission spectroscopy is sensitive to the absolute chemical abundances and the thermal profile. We observe the phase variation to map the planet's emission as a function of longitude. This probes the dynamics of energy transport in the planet's atmosphere.

Light Curves

The above figure depicts the measured system flux relative to the stellar flux throughout the planet's orbit around its host star. Values above unity represent thermal emission from the planet. The measured flux increases as the planet's dayside rotates into view, peaks just prior to secondary eclipse, and decreases as the planet's night-side returns to view. The measured flux drops well below unity as the planet blocks light emitted by the star during primary transit. Measurements at other wavelengths produce similar light curves that can vary in amplitude and peak offset. These wavelength-dependent variations constitute the planet's spectrum.

Emission & Transmission Spectra

The top panel depicts the planet's dayside emission spectrum (with best-fit model) as measured by the wavelength-dependent secondary-eclipse depths. The broad spectral feature from 1.35 to 1.6 μm is caused by the absorption of water vapor within the planet's atmosphere. The bottom panel depicts the planet's transmission spectrum as measured by the wavelength-dependent planet-to-star radius ratio during primary transit. In this case, the absorption of water causes an increase in apparent planet size at the corresponding wavelengths.