55 CANCRI: STELLAR ASTROPHYSICAL PARAMETERS, A PLANET IN THE HABITABLE ZONE, AND IMPLICATIONS FOR THE RADIUS OF A TRANSITING SUPER-EARTH

The bright star 55 Cancri is known to host five planets, including a transiting super-Earth. The study presented here yields directly determined values for 55 Cnc's stellar astrophysical parameters based on improved interferometry: R = 0.943 +/- 0.010 R-circle dot, T-EFF = 5196 +/- 24 K. We use isochrone fitting to determine 55 Cnc's age to be 10.2 +/- 2.5 Gyr, implying a stellar mass of 0.905 +/- 0.015 M-circle dot. Our analysis of the location and extent of the system's habitable zone (HZ; 0.67-1.32 AU) shows that planet f, with period similar to 260 days and M sin i = 0.155M(Jupiter), spends the majority of the duration of its elliptical orbit in the circumstellar HZ. Though planet f is too massive to harbor liquid water on any planetary surface, we elaborate on the potential of alternative low-mass objects in planet f's vicinity: a large moon and a low-mass planet on a dynamically stable orbit within the HZ. Finally, our direct value for 55 Cancri's stellar radius allows for a model-independent calculation of the physical diameter of the transiting super-Earth 55 Cnc e (similar to 2.05 +/- 0.15 R-circle plus),which, depending on the planetary mass assumed, implies a bulk density of 0.76 rho(circle plus) or 1.07 rho(circle plus).