Two long-period giant planets around metal-rich stars characterized using HARPS-N and Gaia data

An Italian research team has discovered a new long-period giant planet orbiting HD 75898 and refined the parameters of a recently announced planet around HD 11506, using data from the HARPS-N spectrograph in combination with astrometric measurements from the Gaia mission.

The previously known planets in these two systems were discovered between 2007 and 2009. Since 2012, the Global Architecture of Planetary Systems (GAPS) team has been monitoring both stars, collecting 122 HARPS-N spectra for HD 75898 and 72 for HD 11506. These data, combined with archival observations from the HIRES spectrograph and Gaia astrometry, allowed the team to investigate the presence of additional long-period companions.

The newly identified planet, HD 75898 c, is located 7.4 astronomical units (au) from its host star and has a mass of 8.5 Jupiter masses. Both this planet and its sibling exhibit low orbital eccentricities, suggesting that the system is dynamically stable. Additionally, the researchers detected an activity cycle in the host star with a period of about two years, which contributes to the overall radial velocity (RV) signal.

For HD 11506, the study reveals that the planet HD 11506 d has a mass of approximately 13 Jupiter masses and an orbital period of 72.6 years, with an uncertainty of less than 10 months—a remarkably precise result for such a long-period object. These updated values are nearly twice as large as those reported by Feng et al. (2022), likely due to the extended HARPS-N dataset.

Interestingly, dynamical simulations indicate that the HD 11506 system is in a state of "stable chaos": the innermost planet, HD 11506 c, undergoes chaotic variations in orbital radius and eccentricity, but these changes are confined within a stable region, ensuring the system's long-term stability over billions of years.

The team also emphasized that future data releases from Gaia (particularly DR4), along with continued HARPS-N monitoring, could enable the precise determination of the true masses of the innermost planets in both systems, which are currently only known by their minimum masses.

Link to the paper

HD75898_PMA_RUWE_sensitivity: Gaia DR3 sensitivity curve with the mass and orbital radius derived for HD 75898 c from RVs only (black) and in combination with astrometry (red). Credits: A. Ruggieri et al. A&A 2024

HD11506_PMA_RUWE_sensitivity: Gaia DR3 sensitivity curve with the mass and orbital radius derived for HD 11506 d from RVs only (black) and in combination with astrometry (red). Credits: A. Ruggieri et al. A&A 2024