The NEID Earth Twin Survey. I. Confirmation of a 31 Day Planet Orbiting HD 86728

With close to 3 yr of observations in hand, the NEID Earth Twin Survey (NETS) is starting to unearth new astrophysical signals for a curated sample of bright, radial velocity (RV)-quiet stars. We present the discovery of the first NETS exoplanet, HD 86728b, a mpsini=9.16−0.56+0.55M⊕ planet on a circular, P=31.1503−0.0066+0.0062 day orbit, thereby confirming a candidate signal identified by L. A. Hirsch et al. We confirm the planetary origin of the detected signal, which has a semi-amplitude of just K=1.91−0.12+0.11 m s−1, via careful analysis of the NEID RVs and spectral activity indicators, and we constrain the mass and orbit via fits to NEID and archival RV measurements. The host star is intrinsically quiet at the ∼1 m s−1 level, with the majority of this variability likely stemming from short-timescale granulation. HD 86728b is among the small fraction of exoplanets with similar masses and periods that have no known planetary siblings.