Completing a Year in 4.2 Hours

Adams et al. (2016) present the detection of a planet candidate whose orbital period around its host star is one of the shortest known. Identified as EPIC 203533312, this planet candidate goes around its host star in only 4.2 hours. The host star of EPIC 203533312 has 0.73 ± 0.03 times the radius of the Sun and an effective temperature of 4630 K. By measuring how much light EPIC 203533312 obscures when it transits its host star, the planet candidate is found to have 1.92 ± 0.12 times the radius of Earth. EPIC 203533312 is so close to its host star that its density has to be at least 8.9 g/cm³ in order for it to be massive enough for its own self-gravity to keep it from being tidally disrupted by its host star.

Reference:
Adams et al. (2016), "Ultra Short Period Planets in K2: SuPerPiG Results for Campaigns 0-5", arXiv:1603.06488 [astro-ph.EP]