Detection of a Neptune-mass planet in the ρ1 Cancri system using the Hobby-Eberly telescope

We report the detection of the lowest mass extrasolar planet yet found around a Sun-like star-a planet with an M sin i of only 14.21+/-2.91 M-circle plus in an extremely short period orbit (P=2.808 days) around rho(1) Cancri, a planetary system that already has three known planets. Velocities taken from late 2003-2004 at McDonald Observatory with the Hobby-Eberly Telescope revealed this inner planet at 0.04 AU. We estimate an inclination of the outer planet rho(1) Cancri d, based on Hubble Space Telescope Fine Guidance Sensor measurements that suggest an inner planet of only 17.7+/-5.57 M-circle plus, if coplanarity is assumed for the system.