Three planets around HD 27894. A close-in pair with a 2:1 period ratio and an eccentric Jovian planet at 5.4 AU

Three planets around HD 27894. A close-in pair with a 2:1 period ratio and an eccentric Jovian planet at 5.4 AU 

Authors:

Trifonov et al

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 Pb ≈ 18 days we discover a Saturn-mass planet with Pc ≈ 36 days, likely in a 2:1 mean motion resonance with the first planet, and a cold massive planet (≈ 5.3 MJup) with a period Pd ≈ 5170 days on a moderately eccentric orbit (ed = 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.