b'Earth/Planetary ScienceUnderstanding the Formation andEvolution of Planets24The solution may involve dirty snowballs.Carnegies John Chambers modeled the atmosphere of a planetary core that grows by accreting ice-Icy Pebbles rich pebbles. He found that the atmosphere goes Make Steam Worlds through several evolutionary stages, and it interacts with the incoming pebbles to cause the critical The mechanism of gas-giant planetary formation,mass to vary in a complicated way that depends on such as with Jupiter, is a central question in planetarypebble size, mass flow, and the opacity of dust in the science and for understanding how planetaryprotoplanetary material.systems develop, because the gravitational pull from large bodies influences the orbits and growth ofWhen the planets core reaches about 0.002 Earth smaller objects. The traditional model for gas-giantmasses, it begins to acquire an atmosphere. The formation, core accretion, involves the rapid growthatmospheric mass increases rapidly with increasing of a solid core to a size, the critical mass, needed tocore mass. The outer atmosphere becomes thin, gravitationally capture hydrogen and helium gas fromwith nearly constant temperatures, and energy is a stars protoplanetary disk. How cores grow to criticaltransferred by electromagnetic radiation to space. mass before the gas is blown away by stellar activityClose to the core, temperatures increase and a remains an unsolved problem in planet-formationconvective zone develops, where hotter material rises modeling. The solution may involve dirty snowballs. and colder falls.1) 2)rock / ice core This schematic shows evolutionary stages of a protoplanet accreting ice-rich pebbles. First, a rocky core arises, but the atmosphere is lost to space. precipitation Second, the planet begins to retain an atmosphere surrounding the solid radiation ice/rock surface. Energy in the outer atmosphere radiates out. Energy in 3) 4) the inner atmosphere moves via convection, with ice grains precipitating toward the surface. Third, the planet forms an ocean on the rocky core. The outer atmosphere is still radiative, while the inner atmosphere moves heat by convection with ice and water precipitation. Fourth, the planet is similar to stage three, but precipitation occurs at middle altitudes. The deeper atmosphere is now too hot for water to condense.rock core ocean convection Image courtesy John Chambers'