The small star to the right is GL229B, which was found 100 light years away orbiting the larger star to the left, GL229A. It is a brown dwarf, slightly more massive than Jupiter.

The last several years have seen the detection, via precise radial velocities, of massive planets around nearby stars (from the Extrasolar Planet Search site of UC Berkeley).  In addition, a brown dwarf was discovered orbiting the star GL229.  These discoveries have challenged our understanding of the structure and evolution of planets and planetary systems. Planets with a orbital radii less than 0.3 AU (Mercury is at 0.4 AU) are often referred to as "Hot Jupiters". 51 Peg B, the first of these discovered, is the prototypical Hot Jupiter. These planets are so close to the parent star that their surface temperature is quite high, 900K or hotter, and they can be detected from their direct emission.

A Jupiter-sized planet at a temperature of 900K is about 10,000 times dimmer than a solar-type star. Our own Jupiter, on the other hand,  is about a million times fainter than the Sun in the thermal infrared, and a billion times fainter in the visible and near infrared. Direct detection of a "cold Jupiter" can only be done from space, but direct detection of a hot Jupiter can be done with the Keck Interferometer. The Keck Interferometer will have the capability of detecting the radiated light from Jupiter-sized planets at a separation of 0.15 AU from parent stars, and at a distance of 10 pc, through the use of multi-color phase-difference interferometry.

The detection approach planned for the Keck Interferometer is a complementary approach to the high-precision radial-velocity technique which was first used to discover these objects. It also allows, for example, unambiguous mass determinations and validations of atmospheric models. The measurement is challenging because of the relative faintness of the planet compared to the star. However, with large telescopes, the signal-to-noise ratio is good, and the measurement technique must deal primarily with systematic errors. The approach (in figure below) exploits the wavelength-dependent phase shift of the fringe position of the star-planet system: at longer wavelengths, the centroid of the star-planet system moves toward the cooler planet. Simultaneous measurements of the fringe phase at multiple wavelengths with a single beam combiner make many errors common mode. The multiple wavelengths are also used to calibrate residual temperature and water vapor turbulence feedthrough.

A chart showing the principle of the differential-phase technique. The center of light of the star-planet system shifts toward the planet at longer wavelengths. (Click image to enlarge)