Photoevaporation of the Solar Nebula and the Formation of the Giant Planets

Abstract

We review the prevailing theories for the formation of the jovian planets and comment that they do not provide a natural explanation for the oft-noted subdivision into two separate classes: the gas-rich giants, Jupiter and Saturn, and the gas-poor giants, Uranus and Neptune. To account for the observed differences in envelope mass relative to core mass, the conventional discriminants would seem to require special timing for the phases: assemblage of the protoplanetary cores, runaway accretion of gas, and the dispersal of nebular gases. We propose a discriminant that relies on photoevaporation by Lyman continuum photons of the outer parts of the disk as the primary hydrogen (and helium) loss mechanism. We show that Saturn's orbit constitutes the natural transitional radius between gas retainage and loss in this sort of picture and that the evaporative wind in the Uranus and Neptune regions would have been large enough to get rid of the hydrogen and helium gas before these planets assembled a critical core mass for runaway gas accretion, if the primitive Sun had an enhanced extreme ultraviolet luminosity for a duration comparable to those estimated for classical T Tauri stars.