The Attempt To Observe Outer-planets In Non-single-stellar Environments
(My acronyms are becoming absurd)
TATOOINE is being developed as a complementary program to PHASES and other planets-in-binary-star-systems searches. Whereas previous searches have sought planets orbiting just one stellar component of more widely spaced binaries, TATOOINE is looking for exoplanets which simultaneously orbit both stars simultaneously, in systems where the stars are closer together. The prototype of this configuration exists only in the world of science fiction---the Skywalkers’ home world of Tatooine exists in such a configuration in Star Wars---none of these planets have yet been detected. Thus, we are initiating this search in Fall 2006 with a program specifically designed to find these. If we find evidece that these systems are common in nature, it will force a revolution in the way we understand how planetary systems are formed.
To find these systems, we use a variation of the precision radial velocity method. A technical description of this variation can be found in this paper; the modifications are necessary to apply the method to binary stars whose separations are too small to be separated by the telescopes we use. We are very thankful to have been allowed access to the spectrograph and iodine cell developed by the California and Carnegie Extrasolar Planet Search team at Lick Observatory. This system has found most of the planets known around isolated stars.
A circumbinary planet causes two measurable effects in the radial velocities of the host stars. First, the mass-weighted average velocity of the binary changes periodically due to velocity Doppler shift; the equal-and-opposite reaction of the binary to the planet moving around it can be measured—this is the same effect as observed in single-star planet searches, and is most sensitive to short-period planets. Second, the equal-and-opposite reaction of the binary to the planet brings the binary slightly closer and slightly further from the Earth, and the time required for the light signal to arrive at earth varies. This appears as a variation in the binary’s orbital period, and the effect is most sensitive to long period planets.
Sensitivity of radial velocity measurements to circumbinary planets, companions with masses greater than the plotted lines can be detected. The two vertical lines at the left represent the approximate critically stable orbits around 5-day (to the left) and 10-day period binaries--shorter period orbits are unstable. Note the perturbations due to the finite speed of light (LTE) allow one to detect lower mass planets as the period increases. For all companion periods, planets as small as Jupiter can be detected. Stars whose rotation rates are tidally locked to orbital periods less than about 5 days show sufficient rotational line broadening to prevent 20 m/s radial velocity precisions. The calculations assume the binary consists of two stars each massing 1 Solar Mass.