b'25Incoming pebbles slowly sink through the atmosphere. As pebbles react in warmer regions, the ice evaporates, saturating the atmosphere with steamy water vapor. Water and ice precipitate toward the core, forming an ocean, when the core reaches about 0.04 Earth masses, and temperatures and pressure allow water, ice, and vapor to coexist.In the atmospheric convective region, water vapor in the saturated gas condenses as the gas rises and cools. When the rocky core mass reaches about 0.08 Earth masses, the interior temperature becomes too high for an ocean to exist. The water becomes a supercritical fluid that is mixed with hydrogen and helium gasCarnegies John Chambers modeled the atmosphere of a planetary accreted from the protoplanetary disk.core that grows by accreting ice-rich pebbles.The complicated evolution of atmospheric structure means that the critical mass for gas accretion varies with the details of pebble accretion. Chambers found that the core mass for an atmosphere of 50% hydrogen and helium might be a better indicator of the onset of gas accretion than the core mass alone. This mass is typically 1-3 Earth masses for pebbles that are 50% ice.This image, from the Atacama Large Millimeter/submillimeter Array (ALMA) radio telescope, shows a gas-rich protoplanetary disc around a young star named HL Tauri in the constellation Taurus. The substructures of the disc are particularly detailed. Image courtesy ALMA, European Southern Observatory, National Astronomical Observatory, National Radio Astronomy Observatory'