HD 176986: the star system that just keeps growing

 

Astronomers have conducted a detailed investigation of HD 176986, a nearby K-type main-sequence star located approximately 91 light-years from Earth. Classified as a K2.5V spectral type, this stellar host is roughly 21% smaller and less massive than the Sun. Physical parameters indicate an effective temperature of 4,931 K and a metallicity of 0.03 dex, with an estimated age of 4.3 billion years.

Spectroscopic characterization of the HD 176986 system

Prior to the recent study, the system was recognized for harboring two close-in super-Earths, designated HD 176986 b and HD 176986 c, which complete their orbits in 6.5 and 16.82 days, respectively. These planets possess minimum masses of 5.74 and 9.18 Earth masses.

A research team led by Nicola Nari from the Teide Observatory utilized high-precision instrumentation to re-examine the architectural complexity of this planetary system. The study employed the High Accuracy Radial velocity Planet Searcher (HARPS) at the ESO 3.6m telescope in Chile, alongside its northern hemisphere counterpart, HARPS-N, located at the Telescopio Nazionale Galileo in Spain. This collaborative effort was executed under the framework of the Rocky Planets in Equatorial Stars (RoPES) program, aiming to refine the known characteristics of terrestrial-like worlds orbiting equatorial stellar targets.

The revised analysis benefited from a significantly expanded dataset compared to the original 2018 study. The team increased the observational coverage from 234 to 330 nights, extending the temporal baseline from 13.2 to 18.6 years. This enhanced longitudinal perspective led to the detection of a third planetary body within the system, characterized by a mass several times greater than that of the Earth. By integrating a wider array of radial velocity measurements, the researchers have provided a more comprehensive understanding of the dynamical configuration and planetary census of the HD 176986 system.

Discovery and characterization of HD 176986 d

The latest observational data has confirmed the presence of a third planetary body orbiting the star HD 176986, now designated as HD 176986 d. This newly identified exoplanet completes one orbital revolution every 61.38 days at an approximate distance of 0.28 AU from its host star. Physical analysis indicates a minimum mass of 6.76 Earth masses, while its equilibrium temperature is calculated to be approximately 363 K. This discovery expands the known architecture of the system, placing a substantial super-Earth in a more distant orbital track compared to its previously known companions.

According to the research team, the identification of HD 176986 d highlights the profound efficacy of blind radial velocity surveys, such as the RoPES initiative, in detecting super-Earths and sub-Neptunes with orbital periods exceeding 50 days. The findings underscore the necessity of long-term radial velocity monitoring for a comprehensive exploration of the habitable zones surrounding K-type and G-type stars. By maintaining extended observational baselines, astronomers are better equipped to resolve the subtle gravitational signatures of planets that occupy more temperate regions of their respective stellar systems.

In addition to the discovery of the third planet, the observations led by Nicola Nari have significantly refined the physical and orbital characteristics of the innermost planet, HD 176986 b. The revised measurements indicate a precise orbital period of 6.49 days and a minimum mass adjusted to 5.36 Earth masses. Furthermore, the planet's equilibrium temperature has been established at 767 K, providing a clearer understanding of the thermal conditions within this compact multi-planetary system.

Thermal environment and atmospheric implications

The planetary component designated as HD 176986 c represents a significant example of a massive super-Earth residing within a multi-planetary architecture. Orbital dynamics indicate that this celestial body completes a full revolution around its host K-type star in a period of 16.81 days. This relatively short orbital cadence places the planet in a close-in proximity to the stellar surface, though it remains secondary in distance compared to the innermost planet of the system. With a calculated minimum mass of 9.75 Earth masses, HD 176986 c sits at the upper threshold of the super-Earth category, bordering the transition into the sub-Neptune regime.

The physical proximity of HD 176986 c to its parent star results in a high irradiation environment, reflected in a calculated equilibrium temperature of 558 K. This thermal state implies a world far more hostile than the terrestrial environment, where surface conditions are dominated by intense stellar flux. Such a temperature suggests that any potential atmosphere would be subject to significant thermal pressures, influencing the chemical composition and the potential for cloud formation or atmospheric escape.

The balance between its substantial gravitational pull, dictated by its 9.75 Earth masses, and this thermal energy is a critical factor in determining whether the planet retains a thick gaseous envelope or possesses a more compressed, rocky composition.

In the broader context of the HD 176986 system, planet c serves as a vital intermediary between the scorching interior world and the more temperate, newly discovered planet d. Its orbital stability and mass provide essential data points for understanding the planetary migration and formation history of the entire system. The precision of these parameters, refined through nearly two decades of radial velocity measurements, allows astronomers to model the gravitational interactions within the system with high confidence. Consequently, HD 176986 c remains a primary target for comparative planetology, offering insights into how massive terrestrial-like worlds evolve in the vicinity of K-type stars.

The study is published in Astronomy & Astrophysics.