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Spin-Orbit Geometry of AU Mic b and c from Back-to-Back Transits Observed Contemporaneously with Magellan PFS, LCOGT, and CHEOPS

This study presents new Rossiter-McLaughlin measurements of the young AU Mic system obtained during back-to-back transits of planets b and c, confirming the spin-orbit alignment of planet b while finding that planet c is likely aligned but potentially polar due to challenges posed by stellar activity and poor transit timing variation predictions.

Original authors: Zitao Lin, Gyula M. Szabó, Krzysztof Sz. Zieliński, Zhen Guo, Zoltán Garai, R. Paul Butler, Alexis Brandeker, Johanna K. Teske, Davide Gandolfi, Haochuan Yu, Nicolas Billot, Suzanne Aigrain, Michael C

Published 2026-05-29

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CC BY 4.0

Original authors: Zitao Lin, Gyula M. Szabó, Krzysztof Sz. Zieliński, Zhen Guo, Zoltán Garai, R. Paul Butler, Alexis Brandeker, Johanna K. Teske, Davide Gandolfi, Haochuan Yu, Nicolas Billot, Suzanne Aigrain, Michael Cretignier, Liang Wang, Xuan Mao, Wei M. Yuan, Hongpeng Lu, Jiayin Li, Yann Alibert, Ádám Boldog, Vincent Bourrier, Giovanni Bruno, Jeffrey D. Crane, Fei Dai, Olivier D. S. Demangeon, Alexis Heitzmann, Zhecheng Hu, Pradip Karkamar, Levente Kriskovics, Monika Lendl, He Y. Liu, Pierre F. L. Maxted, Hugh P. Osborn, Gaetano Scandariato, Stephen A. Shectman, Sérgio G. Sousa, Solène Ulmer-Moll, Mu-Tian Wang, Thomas G. Wilson, Sharon X. Wang

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a young, energetic star named AU Mic, located just a stone's throw away in cosmic terms (about 32 light-years). It's only about 20 million years old—basically a toddler in the universe's timeline. This star has two Neptune-sized planets, AU Mic b and AU Mic c, that orbit it. Because they are so close to us and so young, astronomers are very eager to understand how they formed and how they move.

The big question this paper tries to answer is: Are these planets' orbits lined up neatly with the star's spin, or are they tilted at wild angles?

Think of the star as a spinning top. If the planets orbit in the same flat plane as the star's equator, they are "aligned" (like cars driving in lanes on a highway). If they are tilted, they might be "misaligned" (like cars driving on a ramp that crosses the highway at a 90-degree angle).

The "Back-to-Back" Opportunity

Usually, watching planets cross in front of a star (a transit) is a rare event. But in August 2024, something special happened: AU Mic b and AU Mic c were scheduled to cross in front of the star on consecutive nights. This is a "once-a-decade" event.

The team used three different tools to watch this happen:

Magellan PFS: A giant telescope in Chile that listens to the star's "voice" (radial velocity) to see how the star wobbles.
CHEOPS: A space satellite that takes very precise photos of the star's brightness.
LCOGT: A network of ground-based telescopes that also took photos.

The Rossiter-McLaughlin Effect: The "Spinning Record" Analogy

To figure out if the planets are aligned, the scientists looked for something called the Rossiter-McLaughlin (RM) effect.

Imagine the star is a spinning vinyl record. One side is spinning toward you (blue), and the other is spinning away (red).

When a planet blocks the "blue" side, the star looks slightly redder.
When it blocks the "red" side, the star looks slightly bluer.

By watching how the star's color shifts as the planet crosses, scientists can tell if the planet is crossing the equator (aligned) or crossing the poles (misaligned).

The Results: One Clear, One Confused

Planet b (The Easy One):
The data for AU Mic b was clear. It crossed the star exactly over its equator. The orbit is perfectly aligned with the star's spin. This confirms what we suspected: young planets often start out in neat, flat lines.

Planet c (The Mystery):
AU Mic c was much harder to figure out. The data gave the scientists two possible answers, like a fork in the road:

The Aligned Path: The planet is crossing the equator, just like its sibling (Planet b).
The Polar Path: The planet is crossing the star from pole to pole, at a steep 90-degree angle.

The scientists slightly favor the Aligned Path, but they cannot completely rule out the Polar Path.

The "Stellar Tantrum" Problem

Why was Planet c so confusing? Because the star is a toddler, and toddlers throw tantrums. AU Mic is extremely active. During the observation of Planet c, the star had a massive "flare" (a burst of energy) that looked suspiciously like the signal of a tilted planet.

The scientists tried to filter out this "noise" using different mathematical tricks, but the star's activity was so chaotic that it was hard to separate the planet's signal from the star's tantrum. It's like trying to hear a whisper in a room where someone is constantly slamming doors.

Why Does It Matter?

The paper argues that the Aligned Path is the most likely scenario for a few reasons:

Dynamical Stability: If the planets were tilted wildly, they might crash into each other or get kicked out of the system. The system looks too stable for that.
The "Family" Rule: Most young planetary systems we've seen are neat and aligned. It would be very strange for one planet to be neat and its neighbor to be wild, unless something very specific and rare happened to tilt just one of them.

The Bottom Line

The team successfully confirmed that the inner planet (b) is neatly aligned. For the outer planet (c), they think it's probably aligned too, but the star's "tantrums" (flares and activity) made the data too messy to be 100% sure.

To solve the mystery of Planet c completely, we need to catch it crossing the star again during a "quiet" night when the star isn't throwing tantrums, or we need to get much better at mathematically filtering out the star's noise. For now, the best guess is that the whole family is lined up in a neat row.

Technical Summary: Spin-Orbit Geometry of AU Mic b and c from Back-to-Back Transits

Problem Statement
The alignment between a planet's orbit and its host star's spin (stellar obliquity) serves as a critical tracer of planetary system evolution. While young systems (<100 Myr) generally exhibit aligned orbits, suggesting primordial formation within a disk, the AU Mic system presents a complex case. AU Mic is a nearby (~9.8 pc), ~20 Myr-old M-dwarf hosting two transiting Neptune-sized planets (b and c) and a debris disk. Previous measurements confirmed the alignment of planet b and the debris disk. However, recent independent studies yielded conflicting results for planet c: one suggested a polar orbit (H. Yu et al. 2025), while others hinted at a retrograde configuration. Furthermore, the system's high level of stellar activity and the presence of a potential non-transiting planet (d) complicate the determination of the system's true architecture and mutual inclinations.

Methodology
The authors conducted a unique observational campaign targeting the back-to-back transits of AU Mic b and c on August 24 and 25, 2024. The study utilized a multi-instrument approach to capture simultaneous photometry and radial velocities (RVs):

Spectroscopy: High-resolution RVs were obtained using the Magellan Planet Finder Spectrograph (PFS) on the 6.5m Magellan Clay telescope, covering both transits.
Photometry: Simultaneous light curves were captured by the space-based CHEOPS satellite and the ground-based Las Cumbres Observatory Global Telescope (LCOGT) network.
Activity Monitoring: A two-month ground-based photometric campaign monitored long-term stellar variability.

Data Analysis and Modeling

Stellar Activity Mitigation: The dataset was heavily contaminated by stellar activity, including long-term spot modulation and short-term transient events (flares). The authors modeled long-term trends using low-order polynomials. A significant challenge arose from a transient stellar event during the ingress of planet c's transit, which produced a "blueshift-then-redshift" RV signal mimicking a retrograde Rossiter-McLaughlin (RM) effect. Chromatic RV analysis (examining specific spectral chunks) identified that these anomalies were linked to activity-sensitive lines (Mg I, Fe I) but failed to cleanly remove the contamination. Consequently, the affected time ranges were masked in the final analysis.
Transit Fitting: Mid-transit times ( $T_0$ ) were constrained using the light curves. Due to the sensitivity of $T_0$ to flare masking strategies and Transit Timing Variations (TTVs) induced by planet d, the authors adopted manually inflated error bars for $T_0$ priors in the RM modeling.
RM Modeling: The authors employed the rmfit package to model the RM effect, testing both fiducial models (polynomial trends) and models incorporating Gaussian Processes (GP) to account for correlated noise. They tested multiple priors for transit parameters derived from previous studies (Wittrock et al. 2023; Boldog et al. 2025).

Key Results

AU Mic b: The study confirms the aligned orbit of planet b, yielding a projected stellar obliquity of $\lambda_b = 1^\circ \pm 12^\circ$ , consistent with previous independent measurements.
AU Mic c: Two distinct solution families emerged for planet c's obliquity ( $\lambda_c$ $λ_{c}$ ):
1. Aligned Solution: $\lambda_c \approx -10^\circ \pm 16^\circ$ . This solution is slightly favored, particularly when assuming circular orbits and utilizing priors that favor shorter transit durations.
2. Polar Solution: $\lambda_c \approx 87^\circ^{+36^\circ}_{-29^\circ}$ . This solution cannot be ruled out, particularly when using Gaussian Process models or priors favoring longer transit durations.
Model Robustness: Injection-recovery tests indicated that while GP models provided a statistically better fit (higher Bayesian evidence), they did not guarantee more robust extraction of the RM signal and could introduce biases. The authors therefore adopted the fiducial (non-GP) model as their preferred solution.
Stellar Contamination: The transient activity event during planet c's transit prevented a precise constraint on its obliquity. The authors noted that standard chromatic analysis techniques were insufficient to mitigate this specific contamination.

Significance and Conclusions
The paper highlights the difficulty of characterizing young, active systems where stellar activity can mimic or obscure planetary signals. While the authors slightly favor an aligned, coplanar architecture for AU Mic b and c—consistent with theoretical expectations for young multi-planet systems formed via disk migration and resonance capture—they explicitly state that a polar orbit for planet c remains a viable possibility given the current data limitations.

The study concludes that a definitive resolution of the AU Mic system's architecture requires either a significantly improved understanding and modeling of stellar activity (specifically transient events) or fortuitous future observations during periods of low stellar activity. The authors emphasize that back-to-back observations, while intended to control for activity levels, were insufficient to overcome the stochastic nature of flares in this specific system. The results underscore the need for robust techniques to model stellar jitter to accurately interpret the dynamical history of young planetary systems.