Z. Lin, G. Szabo, K Zielinski, Z. Guo, Z. Garai, R. P. Butler, A. Brandeker, J. Teske, D. Gandolfi, H. Yu, N. Billot, S. Aigrain, M. Cretignier, L. Wang, X Mao, W Yuan, H Lu, J Li, Y. Alibert, A. Boldog, V. Bourrier, G. Bruno, J. D. Crane, F. Dai, O. Demangeon, A. Heitzmann, Z. Hu, P Karmakar, L. Kriskovics, M. Lendl, H. Liu, P. F. L. Maxted, H. Osborn, G. Scandariato, S. A. Shectman, S. G. Sousa, S. Ulmer-Moll, M. Wang, T. G. Wilson, S. X. Wang

Abstract

Young planets offer a unique window into the early stages of planetary evolution. AU Mic is one of the nearest (9.8 pc) pre-main-sequence stars (∼20 Myr), hosting two transiting Neptune-sized planets and a debris disk. Previous studies have shown that the rotations of the central star, the debris disk, and the inner planet b are all aligned, suggesting that the system has not undergone violent evolution. Here we report new Rossiter─McLaughlin measurements for both AU Mic b and c, which happened to transit back-to-back on 2024 August 24 and 25, using the Magellan Planet Finder Spectrograph, together with contemporaneous photometry from LCOGT and CHEOPS. We confirm the aligned orbit of AU Mic b (λ_b = 1^∘ ± 12^∘) and find two possible solutions for AU Mic c: we slightly favor an aligned solution (λ_c = −10^∘ ± 16^∘) but cannot rule out a polar solution ( λc=87∘−29∘+36∘ ). Broader considerations, including dynamical stability and transit possibility, also support the mutually aligned scenario. An unexpected stellar signal during ingress and the poor transit-timing variation predictions of AU Mic c prevent a precise constraint on its obliquity, and various attempts using chromatic spectral analyses fail to outperform simple data exclusion in mitigating stellar contamination. Our observation highlights the importance of understanding stellar activity across multiple timescales and channels when characterizing young, active systems. A robust solution for the AU Mic architecture will require either a better understanding of stellar activity or future observations fortuitously free from strong stellar contamination. *This Letter includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile, and CHEOPS data observed as part of Guaranteed Time Observations (GTO) program CH_PR140071 (PI: Szabo).

Keywords
Exoplanets / Exoplanet dynamics / Planetary alignment / Exoplanet evolution / Stellar activity / Photometry / Radial velocity / Transits / 498 / 490 / 1243 / 491 / 1580 / 1234 / 1332 / 1711

The Astrophysical Journal Letters
Volume 1005, Number L25, Page 17
2026 July

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