Validating AU Microscopii d with Transit Timing Variations

TL;DR

This study validates the existence of AU Mic d, a young Earth-mass exoplanet, through detailed TTV analysis, revealing its likely orbital period and resonance with other planets in the system, and highlighting its significance for studying young terrestrial planets.

Contribution

First validation of AU Mic d as a planet using TTVs, providing its orbital period, mass estimate, and resonance context, advancing understanding of young planetary systems.

Findings

AU Mic d likely orbits at ~12.736 days near a 4:6:9 resonance.

AU Mic d has an estimated mass of about 1 Earth mass.

The planet's confirmation offers insights into young terrestrial planet formation.

Abstract

AU Mic is a young (22 Myr) nearby exoplanetary system that exhibits excess TTVs that cannot be accounted for by the two known transiting planets nor stellar activity. We present the statistical "validation" of the tentative planet AU Mic d (even though there are examples of "confirmed" planets with ambiguous orbital periods). We add 18 new transits and nine midpoint times in an updated TTV analysis to prior work. We perform the joint modeling of transit light curves using EXOFASTv2 and extract the transit midpoint times. Next, we construct an O-C diagram and use Exo-Striker to model the TTVs. We generate TTV log-likelihood periodograms to explore possible solutions for the period of planet d and then follow those up with detailed TTV and RV MCMC modeling and stability tests. We find several candidate periods for AU Mic d, all of which are near resonances with AU Mic b and c of varying order. Based on our model comparisons, the most-favored orbital period of AU Mic d is 12.73596+/-0.00793 days (T_{C,d}=2458340.55781+/-0.11641 BJD), which puts the three planets near a 4:6:9 mean-motion orbital resonance. The mass for d is 1.053+/-0.511 M_E, making this planet Earth-like in mass. If confirmed, AU Mic d would be the first known Earth-mass planet orbiting a young star and would provide a valuable opportunity in probing a young terrestrial planet's atmosphere. Additional TTV observation of the AU Mic system are needed to further constrain the planetary masses, search for possible transits of AU Mic d, and detect possible additional planets beyond AU Mic c.