The minimum-mass extrasolar nebula: in situ formation of close-in super-Earths

Close-in super-Earths, with radii R ≈ 2-5R_⊕ and orbital periods P < 100 d, orbit more than half, and perhaps nearly all, Sun-like stars in the Universe. We use this omnipresent population to construct the minimum-mass extrasolar nebula (MMEN), the circumstellar disc of solar-composition solids and gas from which such planets formed, if they formed near their current locations and did not migrate. In a series of back-of-the-envelope calculations, we demonstrate how in situ formation in the MMEN is fast, efficient, and can reproduce many of the observed properties of close-in super-Earths, including their gas-to-rock fractions. Testable predictions are discussed.