AU Mic’s young planets show aligned orbits, but stellar activity complicates measurements

Star system: AU Microscopii | Distance from the Sun: 31.7 light years

AU Mic is a young, nearby star with two Neptune-sized planets and a debris disk. Researchers observed back-to-back transits of both planets, AU Mic b and c, using multiple telescopes and measured their spin-orbit alignment with the Rossiter-McLaughlin effect. They confirmed that planet b’s orbit is well aligned with the star’s rotation, while planet c’s orbit is likely aligned but a polar orbit cannot be ruled out due to data limitations.

The study found that stellar activity, especially during the transit of planet c, complicated the measurements and made it difficult to precisely determine its orbital tilt. Despite these challenges, the overall architecture of the system appears to be mutually aligned, supporting the idea that AU Mic’s planets and disk have not experienced violent dynamical evolution. The results highlight the need for better understanding and mitigation of stellar activity when studying young planetary systems.

Key findings
- AU Mic b’s orbit is well aligned with the star’s rotation.
- AU Mic c’s orbit is likely aligned, but a polar orbit is still possible.
- Stellar activity complicates precise measurements of young planetary systems.
- The system’s architecture supports a calm dynamical history.

Scientific study

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