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 Small Exoplanets

The Transiting Exoplanet Survey Satellite (TESS)

The TESS mission will discover thousands of small planets that transit bright, nearby stars, allowing both their radii and their masses to be measured, and facilitating atmospheric characterization. As a result, we will soon have the text-align: left means to generate an ensemble of hundreds of well-characterized exoplanet systems.

My specific research interests in TESS are:

  • Increasing TESS' yield of long-period planets by following up planet candidates with only one transit in the TESS data.
  • Measuring the masses of small TESS planets with a wide range of periods (not just those in the habitable zone) and host stars (for example, spanning spectral types G to M) in order to trace the formation of small planets in different environments.

I am also a member of the TESS Follow-Up Observing Programs (TFOP) steering committee, and the chair of the TFOP space-based photometry subgroup.

The Microvariability and Oscillations in STars (MOST) space telescope

For my thesis I used the MOST space telescope to search for and find bright transiting systems by monitoring stars with radial velocity-detected planets. If an exoplanet transits a nearby star, we can often measure both its mass and radius, and therefore its density. For super-Earths, the density provides constraints on their composition. MOST is a suitcase-sized satellite with an aperture diameter of just 6 inches, but can reach a photometric precision of less than 100 parts per million. It has discovered the first two super-Earths known to transit stars brighter than V = 9.

Image credit: Canadian Space Agency/

In 2013, I used MOST to discover that HD 97658b, a super-Earth orbiting a K dwarf, crosses the disk of its host star and produces transits. HD 97658b is the second super-Earth found to transit a star brighter than V = 9. It is a prime candidate for atmospheric characterization.

MOST also discovered the transiting nature of 55 Cnc e, a super-Earth whose host star is so bright that it can be seen with the naked eye. It is twice the size of the Earth and has an orbital period of just under 18 hours, which makes 55 Cnc e one extremely hot planet.

Atmospheres of Small Exoplanets

At present, we know of only about a dozen exoplanets similar in size or smaller than Neptune whose atmospheres can be observed with existing telescopes. For most of these, only space-based observatories like the Spitzer and Hubble Space Telescope have the sensitivity required for such detailed observations.

We have HST and Spitzer observations in hand for about half of these sub-Neptunes and super-Earths. The remainder of the sample will be observed over the next year. The program consists of several proposals in which I participate as either principal investigator or co-investigator, but I am most involved with the analysis of the 55 Cnc e and HD 97658b data sets.

In the next few years, TESS will find many more small planets transiting nearby stars and the James Webb Space Telescope (JWST) will study these planets in more detail than ever before. I am excited at the prospect of new avenues of research, such as understanding the relationship between the presence of clouds and planetary/stellar properties, and uncovering links between the composition of small planet atmospheres and their formation.

The Neptune-size GJ 3470b orbits a small cool star, producing deep transits that are easily observable even with small ground-based telescopes. I used the 1.0m and 2.0m telescopes that make up the LCO network to observe transits of GJ 3470b at two different optical wavelengths. I found that this planet's atmosphere shows a strong scattering signature which indicates the presence of hazes. In cases where our view of a planet's atmosphere is obscured by clouds or hazes, measuring scattering in this way can tell us about the properties and composition of these aerosols.