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

J.-B Delisle, D. Segransan, X. Dumusque, R. F. Diaz, F. Bouchy, C. Lovis, F. Pepe, S. Udry, R. Alonso, W. Benz, A. Coffinet, A. Collier Cameron, M. Deleuil, P. Figueira, M. Gillon, G. Lo Curto, M. Mayor, C. Mordasini, F. Motalebi, C. Moutou, D. Pollacco, E. Pompei, D. Queloz, N. C. Santos, A. Wyttenbach

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 respectively. This discovery is based on 373 high-quality radial velocity measurements taken by HARPS over 13 yr. 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.

Keywords
planets and satellites: general, planets and satellites: detection, planets and satellites: dynamical evolution and stability

Astronomy and Astrophysics
Volume 614, Article Number A133, Number of pages 9
2018 June

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Instituto de Astrofísica e Ciências do Espaço Universidade do Porto Faculdade de Ciências da Universidade de Lisboa Fundação para a Ciência e a Tecnologia
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