Three planets around HD 27894

A close-in pair with a 2:1 period ratio and an eccentric Jovian planet at 5.4 AU^⋆

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117, Heidelberg, Germany
e-mail: trifonov@mpia.de
² Institut für Astrophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
³ Main Astronomical Observatory, National Academy of Sciences of the Ukraine, Ukraine
⁴ Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
⁵ Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong
⁶ Department of Physics, The University of Hong Kong, Pokfulman Road, Hong Kong
⁷ European Southern Observatory (ESO), Alonso de Cordova 3107, Vitacura, Santiago de Chile, Chile
⁸ UCO/Lick Observatory, University of California, Santa Cruz, CA, 95064, USA

Received: 25 April 2017
Accepted: 30 May 2017

Abstract

Aims. Our new program with HARPS aims to detect mean motion resonant planetary systems around stars which were previously reported to have a single bona fide planet, often based only on sparse radial velocity data.

Methods. Archival and new HARPS radial velocities for the K2V star HD 27894 were combined and fitted with a three-planet self-consistent dynamical model. The best-fit orbit was tested for long-term stability.

Results. We find clear evidence that HD 27894 is hosting at least three massive planets. In addition to the already known Jovian planet with a period P_b≈ 18 days we discover a Saturn-mass planet with P_c≈ 36 days, likely in a 2:1 mean motion resonance with the first planet, and a cold massive planet (≈5.3 M_Jup) with a period P_d ≈ 5170 days on a moderately eccentric orbit (e_d = 0.39).

Conclusions. HD 27894 is hosting a massive, eccentric giant planet orbiting around a tightly packed inner pair of massive planets likely involved in an asymmetric 2:1 mean motion resonance. HD 27894 may be an important milestone for probing planetary formation and evolution scenarios.