[Paper Review] The HARPS search for southern extra-solar planets. XX. Planets around the active star BD-08:2823

G. Hébrard, S. Udry+14 more|arXiv (Cornell University)|Dec 16, 2009

Stellar, planetary, and galactic studies49 references13 citations

TL;DR

This paper reports the detection of a multiplanetary system around the active K3V star BD-08°2823 using radial velocity measurements from the HARPS spectrograph. Despite high stellar activity, two planets were confirmed: a Uranus-mass planet (14.4 ± 2.1 M⊕) with a 5.60-day orbit and a Saturn-mass planet (0.33 ± 0.03 M_Jup) with a 237.6-day orbit, reinforcing that low-mass planets are more common in multiplanet systems and active stars should not be excluded from exoplanet searches.

ABSTRACT

We report the detection of a planetary system around BD-08:2823, that includes at least one Uranus-mass planet and one Saturn-mass planet. This discovery serendipitously originates from a search for planetary transits in the Hipparcos photometry database. This program preferentially selected active stars and did not allow the detection of new transiting planets. It allowed however the identification of the K3V star BD-08:2823 as a target harboring a multiplanet system, that we secured and characterized thanks to an intensive monitoring with the HARPS spectrograph at the 3.6-m ESO telescope in La Silla. The stellar activity level of BD-08:2823 complicates the analysis but does not prohibit the detection of two planets around this star. BD-08:2823b has a minimum mass of 14.4+/-2.1 M_Earth and an orbital period of 5.60 days, whereas BD-08:2823c has a minimum mass of 0.33+/-0.03 M_Jup and an orbital period of 237.6 days. This new system strengthens the fact that low-mass planets are preferentially found in multiplanetary systems, but not around high-metallicity stars as this is the case for massive planets. It also supports the belief that active stars should not be neglected in exoplanet searches, even when searching for low-mass planets.

Motivation & Objective

• To detect low-mass exoplanets around active stars using radial velocity measurements, despite challenges from stellar activity.
• To characterize a planetary system serendipitously identified during a search for transiting planets in Hipparcos photometry.
• To investigate whether low-mass planets preferentially form in multiplanetary systems and whether stellar metallicity correlates with planetary mass.
• To assess the viability of active stars as targets for radial velocity exoplanet surveys, particularly for Neptune- and Super-Earth-mass planets.

Proposed method

• Radial velocity measurements were obtained using the HARPS spectrograph on the 3.6-m ESO telescope at La Silla Observatory, Chile.
• The HARPS instrument provides sub-m s−1 radial velocity precision, enabling detection of low-mass planets over long monitoring baselines.
• Stellar activity was modeled and corrected for using phase-locked activity signatures and cross-correlation function analysis to isolate planetary signals.
• Orbital parameters and minimum masses were derived via Keplerian modeling of the radial velocity data.
• Stellar properties, including effective temperature, metallicity, and age, were determined from high-resolution spectroscopy and photometric data.
• The system's architecture was validated through stability analysis and comparison with known exoplanet demographics.

Experimental results

Research questions

• RQ1 Can low-mass planets be reliably detected around highly active stars using radial velocity techniques?
• RQ2 What is the frequency of multiplanetary systems among low-mass planets, particularly in systems with active host stars?
• RQ3 Is there a correlation between stellar metallicity and the presence of low-mass planets, especially in systems with Neptune- or Super-Earth-mass planets?
• RQ4 How does the occurrence of low-mass planets in multiplanet systems compare to single-planet systems, independent of stellar metallicity?

Key findings

• A planetary system with at least two planets was detected around the active K3V star BD-08°2823, despite its high chromospheric activity.
• BD-08°2823 b has a minimum mass of 14.4 ± 2.1 M⊕ and an orbital period of 5.60 days, consistent with a Neptune-like or Super-Earth planet.
• BD-08°2823 c has a minimum mass of 0.33 ± 0.03 M_Jup and an orbital period of 237.6 days, consistent with a Saturn-mass planet.
• The system's metallicity is slightly below solar, supporting the trend that low-mass planets are not preferentially found around high-metallicity stars, unlike massive gas giants.
• The detection confirms that active stars should not be excluded from radial velocity surveys, even when searching for low-mass planets.
• The system strengthens the empirical trend that low-mass planets are more frequently found in multiplanetary systems, with ~70% of planets below 0.1 M_Jup in such systems, compared to ~25% overall.