Exoplanets

Terrestrial Planet Formation

297 Terrestrial Planet Formation John Chambers Carnegie Institution for Science The standard planetesimal model of terrestrial planet formation is based on astronomical and cosmochemical observations, and the results of laboratory experiments and numerical simulations. In this model, planets grow in a series of stages beginning with the micrometersized dust grains observed in protoplanetary disks. Dust grains readily stick together to form millimeter- to centimeter-sized aggregates, some of which are heated to form chondrules. Growth beyond meter size via pairwise sticking is problematic, especially in a turbulent disk. Turbulence also prevents the direct formation of planetesimals in a gravitationally unstable dust layer. Turbulent concentration can lead to the formation of gravitationally bound clumps that become 10 –1000-km planetesimals. Dynamical interactions between planetsimals give the largest objects the most favorable orbits for further growth, leading to runaway and oligarchic growth and the formation of Moon- to Mars-sized planetary embryos. Large embryos acquire substantial atmospheres, speeding up planetesimal capture. Embryos also interact tidally with the gas disk, leading to orbit modification and migration. Oligarchic growth ceases when planetesimals become depleted. Embryos develop crossing orbits, and occasionally collide, leaving a handful of terrestrial planets on widely spaced orbits. The Moon probably formed via one such collision. Most stages of planet formation probably took place in the asteroid belt, but dynamical perturbations from the giant planets removed the great majority of embryos and planetesimals from this region. 1. INTRODUCTION When we think of a planet, our first conception is a body like Earth with an atmosphere, continents, and oceans. However , the Sun’s planets are a diverse group of objects that come in several varieties, and exoplanets are more diverse still (Fig. 1). In this chapter I will focus on terrestrial planets — planets that are mostly composed of refractory materials such as silicates and metal. These objects are large enough to be roughly spherical due to their own gravity. They have a solid surface one could walk around on. They may have an atmosphere, but gases make up a negligible fraction of their total mass. Terrestrial planets are the only place we know for certain that life can exist. While living organisms might survive on icy satellites like Titan, or in the atmospheres of giant planets, terrestrial planets can provide a number of benefits for life. These include a solid substrate, access to abundant sunlight, and the possibility of liquid water at or near the surface. Clues to the origin of the solar system and its planets come from several sources including astronomical observations, data returned from spacecraft, and cosmochemical analysis of samples from planets and asteroids. Numerical simulations are increasingly used to try to make sense of these data and examine different theoretical models for planet formation. The Sun’s planets all orbit in the same direction and have roughly coplanar orbits, suggesting the solar system formed from a disk-shaped structure. Many young stars are surrounded by solar-system-sized disks of gas and dust, and these structures are commonly referred to as protoplanetary disks. A typical protoplanetary disk is mostly composed of hydrogen and helium gas. Submicrometer- to centimeter-sized dust grains composed of silicates and water ice have been observed in these disks using infrared- and radiotelescopes. The Sun’s protoplanetary disk, also called the solar nebula, must have had a mass of at least 1–2% that of the Sun in order to provide the heavy elements seen in the planets today. However, this “minimum mass nebula” is only a lower limit — the solar nebula could have been an order of magnitude more massive than this. The solar nebula probably had a composition similar to the Sun itself since most primitive meteorites have fairly similar elemental abundances to the Sun (normalized to silicon) except for highly volatile elements such as hydrogen and the noble gases. Samples from Earth, the Moon, Mars, and most asteroids have roughly uniform isotopic abundances, after physical processes leading to mass-dependent fractionation are taken into account. (Oxygen is a notable exception.) The solar nebula was probably made up of a mixture of material from different stellar sources, which suggests widescale mixing took place, perhaps during an early hot phase. The great depletion of ice-forming elements on the terrestrial planets compared to the outer planets and their satellites suggests the inner nebula was too hot for ices to condense while the inner planets were forming, while the outer nebula must have been cooler. There are several indications that planets form shortly after...