Radial Velocity Discovery of an Eccentric Jovian World Orbiting at 18 au (1908.09925v2)

Abstract: Based on two decades of radial velocity (RV) observations using Keck/HIRES and McDonald/Tull, and more recent observations using the Automated Planet Finder, we found that the nearby star HR 5183 (HD 120066) hosts a 3$M_J$ minimum mass planet with an orbital period of $74^{+43}_{-22}$ years. The orbit is highly eccentric (e$\simeq$0.84), shuttling the planet from within the orbit of Jupiter to beyond the orbit of Neptune. Our careful survey design enabled high cadence observations before, during, and after the planet's periastron passage, yielding precise orbital parameter constraints. We searched for stellar or planetary companions that could have excited the planet's eccentricity, but found no candidates, potentially implying that the perturber was ejected from the system. We did identify a bound stellar companion more than 15,000 au from the primary, but reasoned that it is currently too widely separated to have an appreciable effect on HR 5183 b. Because HR 5183 b's wide orbit takes it more than 30 au (1") from its star, we also explored the potential of complimentary studies with direct imaging or stellar astrometry. We found that a Gaia detection is very likely, and that imaging at 10 $\mu$m is a promising avenue. This discovery highlights the value of long-baseline RV surveys for discovering and characterizing long-period, eccentric Jovian planets. This population may offer important insights into the dynamical evolution of planetary systems containing multiple massive planets.

• The paper reports the discovery of HR 5183 b, a 3 Jupiter-mass exoplanet with a highly eccentric (e≈0.84) orbit.
• It employs MCMC analysis with Gaussian priors on two-decade RV data from Keck/HIRES, McDonald/Tull, and the Automated Planet Finder to robustly constrain orbital parameters.
• The study underscores that extended radial velocity observations are essential for understanding the formation and dynamic evolution of long-period Jovian exoplanets.

An Eccentric Jovian Exoplanet Discovered via Radial Velocity Method: HR 5183 b

The detailed paper by Blunt et al. reveals the discovery of an eccentric Jovian exoplanet, HR 5183 b, orbiting the star HR 5183 at a semi-major axis distance of approximately 18 AU. This exoplanet, characterized by its notably eccentric orbit with an eccentricity of about 0.84, was detected through extended radial velocity (RV) measurements spanning over two decades. The paper uses data from the Keck/HIRES and McDonald/Tull observatories, alongside more recent data from the Automated Planet Finder.

Key Findings and Numerical Results

The exoplanet HR 5183 b exhibits a highly eccentric and long-period orbit, necessitating the long observational baseline to secure precise measurement of its orbital parameters. The planet possesses a minimum mass of approximately 3 Jupiter masses (M_J) and has an orbital period of around 43 years, shuttling between positions inside Jupiter’s orbit to beyond Neptune. The absence of any detectable gravitational perturbations by other stellar or planetary bodies suggests potential historical interactions that may have resulted in any such perturber being ejected from the system.

This paper rigorously assesses potential stellar companions by analyzing infrared images from HST and additional coronagraphic data. While a stellar companion ~15,000 AU from HR 5183 was identified, its influence on HR 5183 b is deemed negligible due to the vast distance. Given HR 5183 b's wide orbit, direct imaging and astrometric techniques, such as those potentially facilitated by Gaia, are suggested as future work to constrain further details of the HR 5183 system.

Methodology and Model Selection

The discovery leverages the RV method, employing MCMC analysis within the radvel toolkit framework to evaluate the orbital parameters and account for potential $\dot{\gamma}$ terms indicative of any line-of-sight acceleration imparted by wide-orbit perturbers. The use of Gaussian priors informed by the extensive baseline period ensures robust Bayesian inferences on the otherwise broad parameter space, typically challenging for long-period exoplanets.

The broader implications suggest that long-baseline RV data are critical to characterizing eccentric, long-period Jovian planets which are comparatively rare due to current observational and technological constraints. The HR 5183 system stands as a case paper for theorizing planet formation and dynamic evolution, particularly in systems that may have originally hosted multiple Jovian bodies.

Implications and Speculation

The discovery informs our understanding of extreme orbital dynamics within exoplanetary systems, highlighting architectures potentially similar to that of our solar system when Jupiter engages with longer reach orbits. It also invokes a discourse on planet formation histories where scattering and dynamical instability could result in such eccentric configurations, probing theories on high-eccentricity migration and massive perturbers. Detection of HR 5183 b utilizing direct imaging or future astrometric surveys may deliver insights into unseen orbiting bodies or confirm hypotheses around system evolution dynamics.

The paper also underscores the significant potential for Gaia in elucidating further dynamical attributes, possibly confirming astrometric signatures of HR 5183 b's orbit. Continued research leveraging high-resolution imaging and improved instrumentation aims to explore further the properties of such expansive systems, potentially contrasting with planets detected via direct imaging, often biased towards younger and more massive analogs.

In conclusion, the discovery of HR 5183 b represents a vital contribution to long-period Jovian exoplanet detection, modulating our perspective on planetary system evolution and encouraging continued examination of multispectral and methodological approaches to reveal the hidden dynamics of distant celestial bodies.