Ultra-short period planet
[1] At this short distance, tidal interactions lead to relatively rapid orbital and spin evolution.WASP-47e and 55 Cnc e have a lower density and are compatible with pure rock, or a rocky-iron body surrounded by a layer of water (or other volatiles).[1] A difference between hot Jupiters and terrestrial USP planets is the proximity of planetary companions.Hot Jupiters are rarely found with other planets within a factor of 2–3 in orbital period or distance.[1] A study by the TESS-Keck Survey using 17 USP planets found that USP planets predominantly have an Earth-like compositions with iron core mass of about 32% and have masses below runaway accretion.