The HARPS search for southern extra-solar planets. XLIII. A compact system of four super-Earth planets orbiting HD 215152

TL;DR

This paper reports the discovery of four super-Earth planets orbiting HD 215152 using 13 years of HARPS radial velocity data, revealing a rare, compact multi-planet system similar to Kepler discoveries.

Contribution

First detection of a compact four-planet super-Earth system via radial velocity, demonstrating the technique's capability for such systems.

Findings

Four super-Earths with periods of 5.76, 7.28, 10.86, and 25.2 days.

System stability suggests low eccentricities below 0.03.

System resembles Kepler's compact multi-planet systems.

Abstract

We report the discovery of four super-Earth planets around HD 215152, with orbital periods of 5.76, 7.28, 10.86, and 25.2 d, and minimum masses of 1.8, 1.7, 2.8, and 2.9 M_Earth respectively. This discovery is based on 373 high-quality radial velocity measurements taken by HARPS over 13 years. Given the low masses of the planets, the signal-to-noise ratio is not sufficient to constrain the planet eccentricities. However, a preliminary dynamical analysis suggests that eccentricities should be typically lower than about 0.03 for the system to remain stable. With two pairs of planets with a period ratio lower than 1.5, with short orbital periods, low masses, and low eccentricities, HD 215152 is similar to the very compact multi-planet systems found by Kepler, which is very rare in radial-velocity surveys. This discovery proves that these systems can be reached with the radial-velocity technique, but characterizing them requires a huge amount of observations.