New constraints on the planetary system around the young active star AU Mic. Two transiting warm Neptunes near mean-motion resonance

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E. Martioli, G. Hébrard, A. C. M. Correia, J. Laskar, A. Lecavelier des Etangs

AU Mic is a young, active star around which a transiting planet has been recently detected. Here we report an analysis of its TESS light curve where we model the quasi-periodic rotational modulation by starspots simultaneously to the flaring activity and planetary transits. We measured a flare occurrence rate of 6.35 flares-per-day for flares with amplitudes in the range 0.06%<fmax<1.5% of the star flux. We employ a Bayesian MCMC analysis to model the five transits of AU Mic b observed by TESS, improving the constraints on the planetary parameters. The measured planet-to-star radius ratio of 0.05345+0.00014−0.00015 implies a physical radius of 4.38±0.18 M⊕ and a planet density of 1.1±0.3 g\,cm−3, confirming that AU Mic b is a Neptune-size moderately inflated planet. Whereas a single feature possibly due to a second planet was previously reported in the former TESS data, we report the detection of two additional transit-like events in the new TESS observations of July 2020. This represents substantial evidence for a second planet (AU Mic c) in the system. We analyzed its three available transits and obtained an orbital period of 18.858991±0.000010 d and a planetary radius of 3.51±0.16 R⊕, which defines AU Mic c as a warm Neptune-size planet with an expected mass in the range 1.7 M⊕<Mc<27.7 M⊕. The two planets in the AU Mic system are in near 9:4 mean-motion resonance. We show that this configuration is dynamically stable and should produce TTV. Our non-detection of significant TTV in AU Mic b suggests an upper limit to the mass of AU Mic c of <7 M⊕. Being a young multi-planet system with at least two transiting planets, AU Mic becomes a key system for the study of atmospheres of infant planets as well as of planet-planet and planet-disk dynamics at the early stages of planetary evolution.

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