Characterization of the outer substellar companion around HD 72659 using HARPS-N and Gaia

An Italian research team has precisely characterized the mass and orbital parameters of HD 72659 c, an exoplanet located in the so-called “brown dwarf desert”, by combining data from HARPS-N (High Accuracy Radial velocity Planet Searcher – Northern Hemisphere) installed at the Telescopio Nazionale Galileo (TNG), astrometric measurements from Gaia (ESA), direct imaging from SPHERE on the Very Large Telescope in Chile and data from literature.

The host star has a mass, metallicity, and age similar to the Sun and already hosts another planet discovered in 2003. The GAPS (Global Architecture of Planetary Systems) team has been monitoring the system from 2003 to 2023, collecting a total of 91 spectra, with HARPS-N spectra covering the period from 2012 onwards; observations with SPHERE attempted to directly image the newly announced companion from 2022, without success, but the combination of radial velocities and astrometry allowed the team to determine HD 72659 c’s mass and orbit with unprecedented precision, improving on previous results.

In particular, the team found that the exoplanet has an orbital period of 97 years (with only 3 years uncertainty) and a mass of 19.4 Jupiter masses. Previous studies had found a similar mass but estimated a period roughly half of the newly determined value.

The team also investigated potential dynamical interactions between the two planets, finding that the Kozai-Lidov mechanism may occasionally cause planet b’s orbit to become retrograde.

This work highlights the crucial role of HARPS-N at TNG in studying distant planetary systems, combining high-precision spectroscopy with astrometry and cutting-edge direct imaging.

Link to the paper.

Upper limits in mass for the long-period substellar companion of HD72659 derived from SPHERE observation and PMa sensitivity curves calculated following Kervella et al. (2022). The positions of planet b (from RVs only) and planet c (as derived from our RV+astrometry analysis) are also shown. More information in the paper.

Best-fit model for HD 72659 obtained using two Keplerian terms. More information in the paper.