KOI-1843.03 - High-density terrestrial planet on a 4.5 hr(!) orbit

The Roche limit for close-orbiting planets: Minimum density, composition constraints, and application to the 4.2-hour planet KOI 1843.03
http://arxiv.org/abs/1307.4080

Quote :

The requirement that a planet must orbit outside of its Roche limit gives a lower limit on the planet's mean density. The minimum density depends almost entirely on the orbital period and is immune to systematic errors in the stellar properties. We consider the implications of this density constraint for the newly-identified class of small planets with periods shorter than half a day. When the planet's radius is known accurately, this lower limit to the density can be used to restrict the possible combinations of iron and rock within the planet. Applied to KOI 1843.03, with a radius of 0.6 Earth radii and the shortest known orbital period of 4.245 hr, the planet's mean density must be greater than approximately 7 g/cm^3. By modeling the planetary interior subject to this constraint, we find the composition of the planet must be mostly iron, with at most a modest fraction of silicates (less than approximately 30% by mass).

page 4 :

To survive at a period of 4.25 hours, an iron-silicate planet
with the measured radius of KOI 1843.03 must be composed
of 70% Fe by mass.

IIRC 70% metallic core is roughly the same proportion as Mercury.

Quote :

We note in this regard that the planet Mercury, with a mass of 0.055 M_⊕ and mean density of 5.4 g cm[sup]−3[/sup[, could not survive in a 4.245 hr orbit.

Extra compression in the more massive planet saves the day?