A Search for Transit Duration Variations in M dwarf Multi-Planet Systems

TL;DR

This study searches for transit duration variations in M-dwarf multi-planet systems to detect dynamical effects like nodal precession, finding no significant TDVs within current measurement sensitivities, thus constraining possible orbital configurations.

Contribution

It provides the first systematic search for TDVs in M-dwarf multi-planet systems and compares observed data with secular theory predictions to constrain orbital dynamics.

Findings

All systems are consistent with no detectable TDV at 3σ.

TRAPPIST-1d shows a marginal 2.2σ indication of TDV.

Null results align with low-impact-parameter predictions, constraining orbital configurations.

Abstract

The nominal habitable zone for exoplanets orbiting M dwarfs lies close to the host star, making dynamical considerations especially important. One consequence of this proximity is the expectation of spin synchronization, with implications for atmospheric circulation. Several mechanisms can maintain non-zero obliquities over long timescales in compact multi-planet systems, including capture into Cassini State 2 (CS2) and other forms of secular spin-orbit coupling; such pathways are plausible in the orbital architectures of close-in M-dwarf planets. In this study, we search for transit duration variations (TDVs) consistent with the nodal precession rates predicted by Laplace-Lagrange secular theory in compact M-dwarf multi-planet systems. Our sample includes 23 exoplanets orbiting 12 stars. We compare recent, high-precision transit durations obtained from JWST white-light curves with measurements published at the discovery epoch and afterward. The resulting transit duration variation ranges from seconds to minutes, and we fit a linear trend to duration versus time for each planet. All systems are consistent with flat (no TDV) at the 3{\sigma} level. The strongest candidate is TRAPPIST-1d, whose fitted slope differs from zero with 2.2{\sigma} confidence. We calculate the expected TDV signals predicted by secular precession and compare them to the observed limits. Our null detection is consistent with the low-impact-parameter regime, where theoretical TDVs are only a few seconds per decade and below our sensitivity. Higher-impact-parameter configurations predict substantially larger TDVs and are disfavored: under uniformly distributed geometries, at least half of the allowed configurations would be excluded.