There is a growing number of known exoplanets with very short orbital periods of less than half a day. All of these shortest-period exoplanets are expected to be small Earth-mass planets. Larger planets, especially gas giant planets, are unlikely to survive in such short-period orbits. Effects such as tidally-induced orbital decay and evaporation can rapidly destroy a short period gas giant planet. An Earth-mass rocky planet is less susceptible to these effects and can survive almost indefinitely in a very close-in orbit around its parents star. Even so, there is a minimum distance an Earth-mass rocky planet can be from its parent star before tidal forces from the star disintegrate the planet. This minimum distance is known as the Roche limit and the denser a planet, the closer it can orbit its parent star.
KOI 1843.03 is a candidate exoplanet detected by the Kepler space telescope. It is 0.6 times the Earth’s diameter and its orbital period of 4.2 hours is probably the shortest known. The requirement that a planet must orbit outside of its Roche limit provides a lower limit to the mean density of this planet. As a result, the mean density of KOI 1843.03 must be at least 7 g/cm^3 or more. In comparison, Mercury has a mean density of 5.43 g/cm^3 and Earth has a mean density of 5.52 g/cm^3. This implies KOI 1843.03 has a significantly denser bulk composition than Mercury or Earth. Modelling the planetary interior of KOI 1843.03 show that its bulk composition is mostly iron with silicates comprising no more than 30 percent of the planet’s mass.
Rappaport et al. (2013), “The Roche limit for close-orbiting planets: Minimum density, composition constraints, and application to the 4.2-hour planet KOI 1843.03”, arXiv:1307.4080 [astro-ph.EP]