GJ 3929: High-precision Photometric and Doppler Characterization of an Exo-Venus and Its Hot, Mini-Neptune-mass Companion

We detail the follow-up and characterization of a transiting exo-Venus identified by TESS, GJ 3929b (TOI-2013b), and its nontransiting companion planet, GJ 3929c (TOI-2013c). GJ 3929b is an Earth-sized exoplanet in its star's Venus zone (P (b) = 2.616272 +/- 0.000005 days; S-b = 17.3(-0.7)(+0.8) S-circle plus) orbiting a nearby M dwarf. GJ 3929c is most likely a nontransiting sub-Neptune. Using the new, ultraprecise NEID spectrometer on the WIYN 3.5 m Telescope at Kitt Peak National Observatory, we are able to modify the mass constraints of planet b reported in previous works and consequently improve the significance of the mass measurement to almost 4 sigma confidence (M-b = 1.75 +/- 0.45 M-circle plus). We further adjust the orbital period of planet c from its alias at 14.30 +/- 0.03 days to the likely true period of 15.04 +/- 0.03 days, and we adjust its minimum mass to m sin i = 5.71 +/- 0.92 M-circle plus. Using the diffuser-assisted ARCTIC imager on the ARC 3.5 m telescope at Apache Point Observatory, in addition to publicly available TESS and LCOGT photometry, we are able to constrain the radius of planet b to R-p = 1.09 +/- 0.04 R-circle plus. GJ 3929b is a top candidate for transmission spectroscopy in its size regime (TSM = 14 +/- 4), and future atmospheric studies of GJ 3929b stand to shed light on the nature of small planets orbiting M dwarfs.