What is the mass of alpha Cen B b?

TL;DR

This study uses numerical simulations to constrain the true mass of alpha Cen B b by analyzing its orbital stability and inclination, suggesting it is likely a terrestrial planet with a mass under 2.7 Earth masses.

Contribution

The paper provides the first detailed dynamical analysis to estimate the true mass of alpha Cen B b considering Kozai mechanism effects and orbital stability constraints.

Findings

Alpha Cen B b's true mass is likely less than 2.7 Earth masses.

The planet's orbit is stable within certain inclination and semi-major axis boundaries.

Kozai mechanism can cause migration but is limited by stability constraints.

Abstract

We investigate the possibility of constraining the sin i degeneracy of alpha Cen B b -- with orbital period P=3.24 d; a = 0.042 AU; msini = 1.1 M_earth -- to estimate the true mass of the newly reported terrestrial exoplanet in the nearest stellar system to our Sun. We present detailed numerical simulations of the dynamical stability of the exoplanet in the alpha Cen AB binary system for a range of initial inclinations, eccentricities, and semi-major axes. The system represents a benchmark case for the interplay of the Kozai mechanism, general relativistic and tidal forces. From our simulations, there is only a small boundary in initial inclinations and initial semi-major axes that result in the migration via the Kozai mechanism of alpha Cen B b to its present location. Inside this boundary, the planet orbit is stable for up to 1 Gyr against the Kozai mechanism, and outside this boundary the planet collides with alpha Cen B or is ejected. In our three simulations where the planet migrates in towards the star via the Kozai mechanism, the final inclination is 46-53 degrees relative to the AB orbital plane, lower than the initial inclination of 75 degrees in each case. We discuss inclination constraints from the formation of alpha Cen B b in situ at its present location, migration in a proto-planetary disk, or migration in resonance with additional planets. We conclude that alpha Cen B b probably has a mass of less than 2.7 M_earth, implying a likely terrestrial composition warranting future confirmation.