The HARPS search for southern extra-solar planets. XXVII. Up to seven planets orbiting HD 10180: probing the architecture of low-mass planetary systems

摘要

Context. Low-mass extrasolar planets are presently being discovered at anincreased pace by radial velocity and transit surveys, opening a new window onplanetary systems. Aims. We are conducting a high-precision radial velocitysurvey with the HARPS spectrograph which aims at characterizing the populationof ice giants and super-Earths around nearby solar-type stars. This will leadto a better understanding of their formation and evolution, and yield a globalpicture of planetary systems from gas giants down to telluric planets. Methods.Progress has been possible in this field thanks in particular to the sub-m/sradial velocity precision achieved by HARPS. We present here new high-qualitymeasurements from this instrument. Results. We report the discovery of aplanetary system comprising at least five Neptune-like planets with minimummasses ranging from 12 to 25 M_Earth, orbiting the solar-type star HD 10180 atseparations between 0.06 and 1.4 AU. A sixth radial velocity signal is presentat a longer period, probably due to a 65-M_Earth object. Moreover, another bodywith a minimum mass as low as 1.4 M_Earth may be present at 0.02 AU from thestar. This is the most populated exoplanetary system known to date. The planetsare in a dense but still well-separated configuration, with significant secularinteractions. Some of the orbital period ratios are fairly close to integer orhalf-integer values, but the system does not exhibit any mean-motionresonances. General relativity effects and tidal dissipation play an importantrole to stabilize the innermost planet and the system as a whole. Numericalintegrations show long-term dynamical stability provided true masses are withina factor ~3 from minimum masses. We further note that several low-massplanetary systems exhibit a rather "packed" orbital architecture with little orno space left for additional planets. (Abridged)