Transit Timing Variations for AU Microscopii b and c

We explore the transit timing variations (TTVs) of the young (22 Myr) nearby AU Mic planetary system. For AU
Mic b, we introduce three Spitzer (4.5 μm) transits, five TESS transits, 11 LCO transits, one PEST transit, one
Brierfield transit, and two transit timing measurements from Rossiter–McLaughlin observations; for AU Mic c, we
introduce three TESS transits. We present two independent TTV analyses. First, we use EXOFASTv2 to jointly
model the Spitzer and ground-based transits and obtain the midpoint transit times. We then construct an O − C
diagram and model the TTVs with Exo-Striker. Second, we reproduce our results with an independent
photodynamical analysis. We recover a TTV mass for AU Mic c of -10.8+2.22.3 M⊕. We compare the TTV-derived
constraints to a recent radial velocity (RV) mass determination. We also observe excess TTVs that do not appear to
be consistent with the dynamical interactions of b and c alone or due to spots or flares. Thus, we present a
hypothetical nontransiting “middle-d” candidate exoplanet that is consistent with the observed TTVs and candidate
RV signal and would establish the AU Mic system as a compact resonant multiplanet chain in a 4:6:9 period
commensurability. These results demonstrate that the AU Mic planetary system is dynamically interacting,
producing detectable TTVs, and the implied orbital dynamics may inform the formation mechanisms for this
young system. We recommend future RV and TTV observations of AU Mic b and c to further constrain the masses
and confirm the existence of possible additional planet(s).