The same pheonomenon that causes the siren of a passing ambulence to change pitch also causes a moving star to change color. It is called the Doppler effect and while it is a much smaller effect for light than for sound, modern instrumentation allows us to track the subtle motions of stars down to human walking speeds! The wobble, or “reflex motion” of stars due to orbiting planets produces a recognizeable signature, and we use this method to find planets and to confirm planets found by other methods such as transits. If the properties of the host star are known, the Doppler method gives us the mass of the planet, and the eccentricity and radius of the planetary orbit.

Research Areas


The tiny, periodic diminutions of star light due to an exoplanet moving across the face of a star (eclipse, or transit) can be detected allowing the existence of the exoplanet to be inferred. The transit method of planet detection is performed by monitoring the light of stars with very high precision in search of these characteristic signatures. Transits allow us to measure planetary radii and combined with Doppler measurements give planetary mass. These properties are combined to reveal the bulk density providing insight into planetary compositions and interior structures. High-sensitivity follow-up observations with spectroscopy and photometry can be used to measure planetary temperatures, weather patterns, spin-orbit alignments, and atmospheric properties.

High-contrast Imaging

Cutting-edge astronomical instrumentation now allows astonomers to detect objects very near a star that are a hundred thousand times fainter. This technology of high-contrast imaging has led us in the last 5 years to the direct detection of exoplanets. This planet finding technique is most sensitive to massive exoplanets with orbits larger than that of Jupiter in our own solar system, and is complementary to the results of the radial velocity and transit methods. These direct images of exoplanets give us information about their atmospheric chemistry, compositions and thermodyanamic properties.


According to Einstein's general theory of relativity, massive bodies like stars and planets can bend space and time. If a foreground star (lens) is aligned on the sky with a backround star (source), the light from the source star can be lensed and magnitifed as viewed from Earth. If a planet orbiting the lens star happens to lie within the Einstein ring radius of the lens geometry, then the planet can induce an additional magnification--a "blip" on the microlensing event light curve. By carefully modeling the observed light curve, the properties of the planetary system can be measured, and the ensemble collection of observed lens events provide powerful statistical probes of the population of planets that are currently out of the reach of other detection techniques. 

Stellar Astrophysics

To understand exoplanets requires a good understanding their host stars; the derived mass and orbit of the planet depends on the mass of the star, the derived radius of the planet depends on the radius of the star, and the planet equilibrium temperature is a function of the temperature and luminosity of the star. We therefore strive to observationally determine stellar properites with accurate observations. The extremely precise and repetitive observations of the Kepler mission can detect small variations in stellar brightness due to sound waves within the star. In a similar way that seismology is used to determine the structure of the Earth's interior, these observations can be used to derive stellar mass, age, and composition. Also, the large, growing library of Keck HIRES spectra gathered for low-mass stars over the past 15 years is being used to identify subtle features in the starlight that can be calibrated to determine stellar mass and metallicity.