Letter to the Editor

Formation and structure of the three Neptune-mass planets system around HD 69830

¹ Physikalisches Insitut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland e-mail: yann.alibert@space.unibe.ch
² CRAL, École Normale Supérieure, 46 allée d'Italie, 69007 Lyon, France
³ Observatoire de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland
⁴ Institut d'Astrophysique de Paris, 98bis Bd Arago, 75014 Paris, France

Received: 26 May 2006
Accepted: 6 July 2006

Abstract

Since the discovery of the first giant planet outside the solar system in 1995 (Mayor & Queloz 1995, Nature, 378, 355), more than 180 extrasolar planets have been discovered. With improving detection capabilities, a new class of planets with masses 5–20 times larger than the Earth, at close distance from their parent star is rapidly emerging. Recently, the first system of three Neptune-mass planets has been discovered around the solar type star HD 69830 (Lovis et al. 2006, Nature, 441, 305). Here, we present and discuss a possible formation scenario for this planetary system based on a consistent coupling between the extended core accretion model and evolutionary models (Alibert et al. 2005a, A&A, 434, 343; Baraffe et al. 2004, A&A, 419, L13; Baraffe et al. 2006, A&A, 450, 1221). We show that the innermost planet formed from an embryo having started inside the iceline is composed essentially of a rocky core surrounded by a tiny gaseous envelope. The two outermost planets started their formation beyond the iceline and, as a consequence, accrete a substantial amount of water ice during their formation. We calculate the present day thermodynamical conditions inside these two latter planets and show that they are made of a rocky core surrounded by a shell of fluid water and a gaseous envelope.