1. Observatoire de Genève, 51 ch. des Maillettes, 1290 Sauverny, Switzerland
2. Physikalisches Institut der Universität Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
3. Observatoire de Haute-Provence, 04870 St Michel l'Observatoire, France
4. Institut d'Astrophysique de Paris, 98bis Bd Arago, 75014 Paris, France
5. Departamento de Física da Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal
6. Astronomie et Systèmes Dynamiques, IMCCE-CNRS UMR 8028, 77 Av. Denfert-Rochereau, 75014 Paris, France
7. Centro de Astronomia e Astrofísica da Universidade de Lisboa, Observatório Astronómico de Lisboa, Tapada da Ajuda, 1349-018 Lisboa, Portugal
8. Centro de Geofísica de Évora, Colégio Luis Verney, Rua Romão Ramalho, 59, 7002-554 Évora, Portugal
9. Service d'Aéronomie du CNRS, BP 3, 91371 Verrières-le-Buisson, France
10. Laboratoire d'Astrophysique de Marseille, Traverse du Siphon, 13013 Marseille, France

Correspondence to: Christophe Lovis¹Michel Mayor¹ Correspondence and requests for materials should be addressed to C.L. (Email: christophe.lovis@obs.unige.ch) or M.M. (Email: michel.mayor@obs.unige.ch).

Abstract

Over the past two years, the search for low-mass extrasolar planets has led to the detection of seven so-called 'hot Neptunes' or 'super-Earths' around Sun-like stars. These planets have masses 5–20 times larger than the Earth and are mainly found on close-in orbits with periods of 2–15 days. Here we report a system of three Neptune-mass planets with periods of 8.67, 31.6 and 197 days, orbiting the nearby star HD 69830. This star was already known to show an infrared excess possibly caused by an asteroid belt within 1 au (the Sun–Earth distance). Simulations show that the system is in a dynamically stable configuration. Theoretical calculations favour a mainly rocky composition for both inner planets, while the outer planet probably has a significant gaseous envelope surrounding its rocky/icy core; the outer planet orbits within the habitable zone of this star.

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