Revisiting the dynamical masses of the transiting planets in the young AU Mic system: Potential AU Mic b inflation at $\sim$20 Myr

TL;DR

This study precisely measures the radii and masses of AU Mic's transiting planets, revealing potential atmospheric compositions and evolutionary states, and clarifies the existence of additional planetary candidates.

Contribution

It provides the first joint transit and radial velocity analysis of AU Mic planets, refining their physical properties and assessing atmospheric retention and evolution.

Findings

AU Mic b has a radius of 4.79 R_⊕ and a mass of 9.0 M_⊕, with a low density suggesting a possible H2 envelope.

AU Mic c has a radius of 2.79 R_⊕ and a mass of 14.5 M_⊕, likely composed of rock and water with a small H2 atmosphere.

The study rules out the existence of the proposed AU Mic 'e' planet and sets an upper mass limit for a potential additional planet.

Abstract

Understanding planet formation is important in the context of the origin of planetary systems in general and of the Solar System in particular, as well as to predict the likelihood of finding Jupiter, Neptune, and Earth analogues around other stars. We aim to precisely determine the radii and dynamical masses of transiting planets orbiting the young M star AU Mic using public photometric and spectroscopic datasets. We characterise the stellar activity and physical properties (radius, mass, density) of the transiting planets in the young AU Mic system through joint transit and radial velocity fits with Gaussian processes. We determine a radius of $R^{b}$= 4.79 +/- 0.29 R$_\oplus$, a mass of $M^{b}$= 9.0 +/- 2.7 M$_\oplus$, and a bulk density of $\rho^{b}$ = 0.49 +/- 0.16 g cm$^{-3}$ for the innermost transiting planet AU Mic b. For the second known transiting planet, AU Mic c, we infer a radius of $R^{c}$= 2.79 +/- 0.18 R$_\oplus$, a mass of $M^{c}$= 14.5 +/- 3.4 M$_\oplus$, and a bulk density of $\rho^{c}$ = 3.90 +/- 1.17 g cm$^{-3}$. According to theoretical models, AU Mic b may harbour an H2 envelope larger than 5\% by mass, with a fraction of rock and a fraction of water. AU Mic c could be made of rock and/or water and may have an H2 atmosphere comprising at most 5\% of its mass. AU Mic b has retained most of its atmosphere but might lose it over tens of millions of years due to the strong stellar radiation, while AU Mic c likely suffers much less photo-evaporation because it lies at a larger separation from its host. Using all the datasets in hand, we determine a 3$\sigma$ upper mass limit of $M^{[d]}\sin{i}$ = 8.6 M$_{\oplus}$ for the AU Mic 'd' TTV-candidate. In addition, we do not confirm the recently proposed existence of the planet candidate AU Mic 'e' with an orbital period of 33.4 days.