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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.
Reference:
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]