THE ROCHE LIMIT FOR CLOSE-ORBITING PLANETS: MINIMUM DENSITY, COMPOSITION CONSTRAINTS, AND APPLICATION TO THE 4.2 hr PLANET KOI 1843.03

Abstract

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