One year of AU Mic with HARPS: I -- measuring the masses of the two transiting planets

TL;DR

This study used a year-long HARPS observation campaign and advanced Gaussian Process modeling to measure the masses of two transiting Neptune-sized planets around AU Mic, revealing unexpected density and envelope properties challenging existing planet formation models.

Contribution

It presents the first detailed mass measurements of AU Mic's planets using a multidimensional Gaussian Process to handle stellar activity, providing new insights into their composition and formation.

Findings

Planet b has a mass of 11.7 ± 5.0 M⊕, possibly less dense.

Planet c has a mass of 22.2 ± 6.7 M⊕, denser than previously thought.

Results challenge standard core-accretion models and suggest smaller H/He envelopes.

Abstract

The system of two transiting Neptune-sized planets around the bright, young M-dwarf AU Mic provides a unique opportunity to test models of planet formation, early evolution, and star-planet interaction. However, the intense magnetic activity of the host star makes measuring the masses of the planets via the radial velocity (RV) method very challenging. We report on a 1-year, intensive monitoring campaign of the system using 91 observations with the HARPS spectrograph, allowing for detailed modelling of the $\sim 600\, \rm{m\,s^{-1}}$ peak-to-peak activity-induced RV variations. We used a multidimensional Gaussian Process framework to model these and the planetary signals simultaneously. We detect the latter with semi-amplitudes of $\rm{K_b} = 5.8 \pm 2.5\, \rm{m\,s^{-1}}$ and $\rm{K_c} = 8.5 \pm 2.5\, \rm{m\, s^{-1}}$, respectively. The resulting mass estimates, $\rm{M_b} = 11.7 \pm 5.0\, \rm{M_{\rm \oplus}}$ and $\rm{M_c} = 22.2 \pm 6.7\, \rm{M_{\rm \oplus}}$, suggest that planet b might be less dense, and planet c considerably denser than previously thought. These results are in tension with the current standard models of core-accretion. They suggest that both planets accreted a H/He envelope that is smaller than expected, and the trend between the two planets' envelope fractions is the opposite of what is predicted by theory.