Tidally Distorted, Iron-enhanced Exoplanets Closely Orbiting Their Stars

The transiting planet candidate KOI 1843.03 ( $0.6\,{R}_{\oplus }$ radius, 4.245 hr orbital period, $0.46\,{M}_{\odot }$ host star) has the shortest orbital period of any known planet. Here we show, using the first three-dimensional interior structure simulations of ultra-short-period tidally distorted rocky exoplanets, that KOI 1843.03 may be shaped like an American football, elongated along the planet-star axis with an aspect ratio of up to 1.79. Furthermore, for KOI 1843.03 to have avoided tidal disruption (wherein the planet is pulled apart by the tidal gravity of its host star) on such a close-in orbit, KOI 1843.03 must be as iron-rich as Mercury (about 66% by mass iron compared to Mercury's 70% by mass iron). Of the ultra-short-period (P_orb ≲ 1 day) planets with physically meaningful constraints on their densities characterized to date, just under half (four out of nine) are iron-enhanced. As more are discovered, we will better understand the diversity of rocky planet compositions and the variety of processes that lead to planetary iron enhancement.