A NEPTUNE-MASS PLANET ORBITING THE NEARBY G DWARF HD 16417

Precision Doppler measurements from an intensive 48 night Rocky Planet Search observing campaign on the Anglo-Australian Telescope (AAT) have revealed the presence of a low-mass exoplanet orbiting the G1 dwarf HD 16417. Subsequent Doppler observations with the AAT, as well as independent observations obtained by the Keck Planet Search, have confirmed this initial detection and refine the orbital parameters to period 17.24 +/- 0.01 d, eccentricity 0.20 +/- 0.09, orbital semimajor axis 0.14 +/- 0.01 AU, and minimum planet mass 22.1 +/- 2.0 M(Earth). HD 16417 raises the number of published exoplanets with minimum masses of less than 25 MEarth to 18. Interestingly, the distribution of detected sub-25M(Earth) planets over the spectral types G, K, and M is almost uniform. The detection of HD 16417b by an intensive observing campaign clearly demonstrates the need for extended and contiguous observing campaigns when aiming to detect low-amplitude Doppler planets in short-period orbits. Perhaps most critically it demonstrates that the search for low-mass Doppler planets will eventually require these traditional bright-time projects to extend throughout dark lunations.