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

TL;DR

This paper reports the discovery of a three-planet system around HD 27894, including a close-in pair in a 2:1 resonance and a distant eccentric Jovian planet, providing insights into planetary formation.

Contribution

It presents the first detection of a three-planet system around HD 27894, revealing a resonant pair and a distant massive planet, based on combined HARPS radial velocity data.

Findings

Discovery of a three-planet system around HD 27894.

Identification of a 2:1 mean motion resonance.

Detection of a distant eccentric Jovian planet.

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 $P_{\rm b}$ $\approx$ 18 days we discover a Saturn-mass planet with $P_{\rm c}$ $\approx$ 36 days, likely in a 2:1 mean motion resonance with the first planet, and a cold massive planet ($\approx$ 5.3 $M_{\mathrm{Jup}}$) with a period $P_{\rm d}$ $\approx$ 5170 days on a moderately eccentric orbit ($e_{\rm d}$ = 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.