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: $R=0.943\pm 0.010R_{\odot }$, $T_{\mathrm{E}\mathrm{F}\mathrm{F}}=5196\pm 24$ K. We use isochrone fitting to determine 55 Cnc’s age to be 10.2 $\pm$ 2.5 Gyr, implying a stellar mass of $0.905\pm 0.015M_{\odot }$. Our analysis of the location and extent of the system’s habitable zone (0.67–1.32 AU) shows that planet f, with period $\sim$ 260 days and $M\sin i=0.155M_{Jupiter}$, spends the majority of the duration of its elliptical orbit in the circumstellar habitable zone. 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 habitable zone. 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 ($\sim 2.05 \pm 0.15 R_{\earth}$), which, depending on the planetary mass assumed, implies a bulk density of 0.76 ${\rho }_{\text{earth}}$ or 1.07 ${\rho }_{\text{earth}}$.