Interferometric Astrometry
Astrometry refers to measuring the positions of stars with high precision. If the star has a planet around it, the planet will cause the star to "wobble" or move around its mean position. This wobble can be detected and used to infer the presence of a planet. The value of astrometry comes from the fact that it allows one to unambiguously determine the mass of a planet - something other techniques do not always provide.
Optical interferometers are particularly suited to astrometry because of the great "leverage" or sensitivity provided by the widely separated apertures. With baselines of 100 meters or more an interferometer can measure stellar positions with precisions that are 10-1000 more precise than what can be achieved with a single telescope.
For several years I have been working with collaborators at MIT (in particular Matthew Muterspaugh), Caltech and JPL to develop an astrometric planet search. Our approach is particularly suitable for binary stellar systems; this choice was motivated in part by the simple fact that most Solar-type stars occur in multiples, and hence a complete census of planets will require that searches include binaries. However, it is also the case that the presence of a binary companion poses challenges to current theories of planet formation (core accretion vs. gravitation instability), and hence the discovery of large numbers of planets in binary systems may help constrain such theories.
The PHASES Project
Our development efforts have resulted in a dedicated search using a long-baseline optical interferometer (PTI – “Palomar Testbed Interferometer”) located on Palomar Mountain, CA (“PHASES” - Palomar High-precision Astrometric Search for Exoplanetary Systems; I am a Co-I). We have been obtaining 10-20 micro-arcsecond precision astrometry of approximately 50 targets over the last two years and expect to be sensitive to planets with masses in the 0.1-1 Jupiter-mass range in periods around a year (right).
Phase Referencing
One of the biggest challenges facing a successful program of narrow-angle astrometry is the availablility of sufficiently bright and close reference stars. Phase referencing is a way to improve the sensitivity of an interferometer (and hence allow the use of fainter reference stars) by using a bright star to sense and correct atmospheric turbulence in real time. As such it is very similar to using a guide star in adaptive optics. PTI was designed to make use of this technique, and I have been responsible for its implementation and testing.
Phase referencing provides a factor of ~100 improvment in astrometric precision in our system.
