An exoplanet in orbit around tau^1 Gruis

Abstract

We report the detection of a new candidate exoplanet around the metal-rich star tau^1 Gruis. With M sin $i$ = 1.23+/-0.18 M_JUP, a period of 1326+/-300 d and an orbit with an eccentricity of 0.14+/-0.14 it adds to the growing population of long period exoplanets with near-circular orbits. This population now comprises more than 20% of known exoplanets. When the companion to tau^1 Gruis is plotted together with all exoplanets found by the Anglo-Australian Planet Search and other radial velocity searches we find evidence for a peak in the number of short-period exoplanets, followed by a minimum of planets between around 7 and 50 days and then an apparent rise in the number of planets per unit radius that seems to set in by a hundred days, indicating more planets farther from the host star. This is very different from the gaussian-like period distribution found for stellar companions. This lends support to the idea that once a clearing in the inner protoplanetary disk develops, it halts the inward migration of planets. In particular, the smooth distribution of exoplanets arising from planetary migration through a disk is altered by an accumulation of exoplanets at the point where the disk has been cleared out.