Detection of a NEPTUNE-mass planet in the \(\rho^{1}\) Cancri system using the Hobby-Eberly Telescope

We report the detection of the lowest mass extra-solar planet yet found around a Sun-like star - a planet with an \msini of only 14.21 \(\pm\) 2.91 Earth masses in an extremely short period orbit (P=2.808 days) around \(\rho^{1}\) Cancri, a planetary system which already has three known planets. Velocities taken from late 2003-2004 at McDonald Observatory with the Hobby-Eberly Telescope (HET) revealed this inner planet at 0.04 AU. We estimate an inclination of the outer planet \(\rho^{1}\) Cancri d, based upon {\it Hubble Space Telescope} Fine Guidance Sensor (FGS) measurements. This inclination suggests an inner planet of only 17.7 \(\pm\) 5.57 Earth masses, if coplanarity is assumed for the system.