While it can be shown planets can have stable orbits if they are close to one star in a widely separated binary ("S-type" planets) or if they have a much larger orbit around both stars in a much more closely separated binary ("P-type", or circumbinary, planets), it is not known whether planets could form in such a gravitationally complex environment. Searches for planets in binary systems explore the degree to which stellar multiplicity inhibits or promotes planet formation. If present, the planets must have formed rapidly with additional turbulence aiding the process, favoring the gravitational collapse model of giant planet formation. Alternatively, if giant planets are much more rare in binaries than around single stars, it would imply giant planet formation is a fragile process, perhaps requiring much longer timescales than allowed by the chaotic binary environment, indicating the core accretion model is dominant in nature.
PHASES and TATOOINE are two dedicated surveys for planets in binaries. The recently completed PHASES project used optical interferometry to perform precision differential astrometry on binary stars to find perturbations to the binary orbit caused by S-type planets. I will discuss the detection limits for planets in the binaries for which no companions were found (the zero-signal control sample), the triple star systems observed as control systems with known signals, and the candidate substellar companions to a few binaries. The ongoing TATOOINE survey uses precision Doppler velocimetry to identify P-type (circumbinary) planets. I will present the current detection limits from that survey.
Finally, the triple star systems used as PHASES control targets have interesting dynamics in their own right. PHASES observations enabled full characterization of the triple systems, the first few of which indicate their geometries are very different than that of the solar system---they do not lie flat in a single plane as do the planets. I will discuss the future directions of triple star research, including the new VISION interferometric beam combiner being built for the NPOI.
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