Wolf 327b: A New Member of the Pack of Ultra-Short-period Super-Earths Around M Dwarfs

Planets with orbital periods shorter than 1 day are rare and have formation histories that are not completely understood. Small (R-p < 2 R-circle plus) ultra-short-period (USP) planets are highly irradiated, probably have rocky compositions with high bulk densities, and are often found in multi-planet systems. Additionally, USP planets found around small stars are excellent candidates for characterization using present-day instrumentation. Of the current full sample of approximately 5500 confirmed exoplanets, only 130 are USP planets and around 40 have mass and radius measurements. Wolf 327 (TOI-5747) is an M dwarf (R-star = 0.406 +/- 0.015 R-circle dot,M-star = 0.405 +/- 0.019M(circle dot),T-eff = 3542 +/- 70K, and V = 13mag) located at a distanced=28.5pc. NASA's planet hunter satellite, TESS, detected transits in this star with a period of 0.573 day (13.7 h) and with a transit depth of 818 ppm. Ground-based follow-up photometry, high resolution imaging, and radial velocity (RV) measurements taken with the CARMENES spectrograph confirm the presence of this new USP planet. Wolf 327b is a super-Earth with a radius of R-p=1.24 +/- 0.06R(circle plus)and a mass of M-p = 2.53 +/- 0.46M(circle plus), yielding a bulk density of7.24 +/- 1.66g cm(-3) and thus suggesting a rocky composition. Owing to its close proximity to its host star (a=0.01au), Wolf 327b has an equilibrium temperature of 996 +/- 22K. This planet has a mass and radius similar to K2-229b, a planet with an inferred Mercury-like internal composition. Planet interior models suggest that Wolf 327b has a large iron core, a small rocky mantle, and a negligible (if any) H/He atmosphere.