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 (6.5 pc) star HD 219134. This is the first result of the
Rocky Planet Search program with HARPS-N on the Telescopio Nazionale Galileo in La Palma. The inner planet orbits the star in 3.0937 ± 0.0004 days, on a quasi-circular orbit with a semi-major axis of 0.0382 ± 0.0003 AU.
Spitzer observations allowed us to detect the transit of the planet in front of the star making HD 219134 b the nearest known transiting planet to date. From the amplitude of the radial-velocity variation (2.33 ± 0.24 ms
−1) and observed depth of the transit (359 ± 38 ppm), the planet mass and radius are estimated to be 4.46 ± 0.47 M
⊕ and 1.606 ± 0.086 R
⊕ , leading to a mean density of 5.89 ± 1.17 g cm
−3, suggesting a rocky composition. One additional planet with minimum mass of 2.67 ± 0.59 M
⊕ moves on a close-in, quasi-circular orbit with a period of 6.765 ± 0.005 days. The third planet in the system has a period of 46.78 ± 0.16 days and a minimum mass of 8.7 ± 1.1 M
⊕ , at 0.234 ± 0.002 AU from the star. Its eccentricity is 0.32 ± 0.14. The period of this planet is close to the rotational period of the star estimated from variations of activity indicators (42.3 ± 0.1 days). 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 62 ± 6 M
⊕ orbits the star in 1190 days, on an eccentric orbit (
e = 0.27 ± 0.11) at a distance of 2.14 ± 0.27 AU.
