2015A&A...584A..72M

We know now from radial velocity surveys and transit space missions that planets only a few times more massive than our Earth are frequent around solar-type stars. Fundamental questions about their formation history, physical properties, internal structure, and atmosphere composition are, however, still to be solved. We present here the detection of a system of four low-mass planets around the bright (V=5.5) and close-by (6.5pc) star HD219134. This is the first result of the Rocky Planet Search programme with HARPS-N on the Telescopio Nazionale Galileo in La Palma. The inner planet orbits the star in 3.0935±0.0003days, on a quasi-circular orbit with a semi-major axis of 0.0382±0.0003AU. Spitzer observations allowed us to detect the transit of the planet in front of the star making HD219134b the nearest known transiting planet to date. From the amplitude of the radial velocity variation (2.25±0.22m/s) and observed depth of the transit (359±38ppm), the planet mass and radius are estimated to be 4.36±0.44M_⊕ and 1.606±0.086R_⊕, leading to a mean density of 5.76±1.09g/cm³, suggesting a rocky composition. One additional planet with minimum-mass of 2.78±0.65M_⊕ moves on a close-in, quasi-circular orbit with a period of 6.767±0.004-days. The third planet in the system has a period of 46.66±0.08-days and a minimum-mass of 8.94±1.13M_⊕, at 0.233±0.002AU from the star. Its eccentricity is 0.46±0.11. The period of this planet is close to the rotational period of the star estimated from variations of activity indicators (42.3±0.1days). The planetary origin of the signal is, however, the preferred solution as no indication of variation at the corresponding frequency is observed for activity-sensitive parameters. Finally, a fourth additional longer-period planet of mass of 71M_⊕ orbits the star in 1842-days, on an eccentric orbit (e=0.34±0.17) at a distance of 2.56AU.