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Professor of Physics and Astronomy, University of California, Los Angeles

Extrasolar Planets are my principal subject of interest today. My students and I pursue both theoretical and observational programs on a variety of topics.

On the observational side, along with my former student Ian Crossfield , and long-time collaborator Travis Barman, we have pursued a program over the last several years to observe transmission spectra of extrasolar giant planets. These observations offer the promise of constraining the chemical compositions of planetary atmospheres, and the techniques we develop will hopefully prove useful one day in performing similar measurements on lower mass, earth-class planets. More detail on this work can be found here . In the last few years, my role in this particular enterprise has diminished as Ian has successfully assumed leadership of this and related projects.

Recent theoretical work has focussed on the origin and formation of systems of multiple low-mass planets around nearby stars. Observations now suggest that these may be the dominant population of planets in our galaxy, especially if such systems are found around low-mass M dwarfs, which are the dominant stellar demographic in the Galaxy. Along with Norm Murray, at CITA, I have been studying the in situ assembly of rocky planets on sub-AU scales around stars of different mass. Our models are able to provide an encouraging match to the observed distribution, and a much better fit that the more conventionally assumed models based on the concept of migration. More detail on this work can be found here .

I also have an ongoing interest in how our own Solar system fits into the framework of the planets discovered around other stars. I have worked on the formation of the terrestrial planets and am currently working on the formation of moons in the Solar system and around other stars.

I also maintain a long-standing interest in the existence and survival of planets around evolved stars. This dates back to my days as a graduate student, when I worked on models for the formation of the pulsar planets, and continues today with studies of planetary systems around white dwarfs. My former graduate student, Shane Frewen, studied the evolution of planetary systems around both white dwarfs and subgiant stars. More detail on this work can be found here .

White Dwarfs are the end product of stellar evolution for most stars (except for those massive enough to explode as supernovae). As such, the study of old white dwarfs can tell us a lot about the history of star formation in our Galaxy. By the time a star has reached the white dwarf stage, it has exhausted all the nuclear burning resources with which it was born. With no remaining reservoir of energy supply white dwarfs slowly cool and fade over time. I have worked a lot on theoretical models for the evolution and appearance of white dwarfs of different kinds. With my observational collaborators, I have used these models to estimate the ages of various stellar components of our Galaxy, including both the stars in the immediate vicinity of the sun and some of the nearest globular clusters. One of our recent results is a setting a lower limit on the age of our Galaxy of approximately 11 billion years.

Neutron Stars and Black Holes are the first objects I worked on, during my PHD. My most recent interest in this subject concerns the black hole at the center of our Galaxy and the immediate environment. With my students Steve Berukoff and Elliot Koch, I have been studying the dynamics of thousand-solar mass black holes orbiting the main black hole. We believe these `intermediate mass' black holes may have played an important role in the transport of young stars to the Galactic center and they will have an important influence on the resulting dynamics.


  • –present
    Professor of Physics and Astronomy, University of California, Los Angeles


  • 1996 
    California Institute of Technology, PhD/Astronomy