2025AJ....170...65G

LP 791-18 d is a temperate Earth-sized planet (R_P = 1.03 R_⊕, P = 2.76 days) orbiting a late M dwarf, with an interior super-Earth (LP 791-18 b, R_P = 1.2 R_⊕, P = 0.95 days) and an exterior sub-Neptune (LP 791-18 c, R_P = 2.5 R_⊕, P = 4.99 days). Dynamical interactions between LP 791-18 d and c produce transit timing variations (TTVs) that can be used to constrain the planet masses and eccentricities. These interactions can also force a non-zero eccentricity for LP 791-18 d, which raises its internal temperature through tidal heating and could drive volcanic outgassing. We present three new transit observations of LP 791-18 c with Palomar/WIRC, including the most precise TTV measurements (<6 s uncertainty) of this planet to date. We fit these times with a TTV model to obtain updated constraints on the mass, eccentricity, and tidal heat flux of LP 791-18 d. We reduce the mass uncertainty by more than a factor of two (M_d = 0.91 ±0.19 M_⊕). We perform an updated fit assuming tidally damped free eccentricities and find ${e}_{d}=0.001{1}_{-0.0008}^{+0.0010}$ and e_c = 0.0001 ± 0.0001, consistent with circular orbits. We find that the observed TTVs are not sensitive to e ≤ ∼0.01. Without a tidally damped eccentricity prior, ${e}_{d}=0.05{6}_{-0.014}^{+0.015}$, much higher than the eccentricity predicted by n-body simulations incorporating the effects of dynamical excitation and tidal damping. We predict the timing offset relative to the prediction for a circular orbit of upcoming JWST secondary eclipse observations for LP 791-18 d (${\rm{\Delta }}t=-0.{2}_{-2.7}^{+2.0}$ minutes and ${\rm{\Delta }}t=-11{7}_{-47}^{+41}$ minutes for the damped and undamped eccentricity cases, respectively), which could tightly constrain the eccentricity and tidal quality factor of this Earth-sized exoplanet.