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The Sun’s chemical and isotopic composition records the composition of the solar nebula from which the planets formed. If a piece of the Sun is cooled to 1,000 K at 1 mbar total pressure, a mineral assemblage is produced that is consistent with the minerals found in the least equilibrated (most chemically heterogeneous), oldest, and compositionally Sunlike (chondritic), hence most “primitive,” meteorites. This is an equilibrium or fractional condensation experiment. The result can be simulated by calculations using equations of state for hundreds of gaseous molecules, condensed mineral solids, and silicate liquids, the products of a century of experimental measurements and recent theoretical studies. Such calculations have revolutionized our understanding of the chemistry of the cosmos. The mid-20th century realization that meteorites are fossil records of the early solar system made chemistry central to understanding the origin of the Earth, Moon, and other bodies. Thus “condensation,” more generally the distribution of elements and isotopes between vapor and condensed solids and/or liquids at or approaching chemical equilibrium, came to deeply inform discussion of how meteoritic and cometary compositions bear on the origins of atmospheres and oceans and the differences in composition among the planets. This expansion of thinking has had profound effects upon our thinking about the origin and evolution of Earth and the other worlds of our solar system. Condensation calculations have also been more broadly applied to protoplanetary disks around young stars, to the mineral “rain” of mineral grains expected to form in cool dwarf star atmospheres, to the expanding circumstellar envelopes of giant stars, to the vapor plumes expected to form in giant planetary impacts, and to the chemically and isotopically distinct “shells” computed and observed to exist in supernovae. The beauty of equilibrium condensation calculations is that the distribution of elements between gaseous molecules, solids, and liquids is fixed by temperature, total pressure, and the overall elemental composition of the system. As with all sophisticated calculations, there are inherent caveats, subtleties, and computational difficulties. In particular, local equilibrium chemistry has yet to be consistently integrated into gridded, dynamical astrophysical simulations so that effects like the blocking of light and heat by grains (opacity), absorption and re-emission of light by grains (radiative transfer), and buffering of heat by grain evaporation/condensation are fed back into the physics at each node or instance of a gridded calculation over time. A deeper integration of thermochemical computations of chemistry with physical models makes the prospect of a general protoplanetary disk model as hopeful in the 2020s as a general circulation model for global climate may have been in the early 1970s.

For religious institutions in Latin western Europe of the 16th century, Earth was the center of the universe, and the orderly and predictable motion of the heavenly planets about the Earth (which included the Sun) reflected divine will and an inducement to moral improvement. The discovery by Copernicus that the Earth was not at the center of the universe, but was itself a planet orbiting the Sun, was revolutionary. The invention of the telescope resulted in the discovery of more planets by Galileo and others, initially thought to be planets orbiting planets. All planets were expected to feature geological processes seen on Earth. It was even speculated that these other worlds also supported intelligent life. The search for and discovery of a predicted “missing planet” at the beginning of the 19th century opened the door to a rapidly growing number of new small planets, which appeared as points of light in the sky, and were also referred to as “asteroids,” meaning “star-like.” All planets at this time, which then included asteroids, were thought to have formed from a disk of nebular dust and gas surrounding the early Sun. While in the 19th century it was thought by some that asteroids may have arisen from the breakup of a larger planet, it was not until the 1950s when the smaller members of this population were shown to be collisional fragments that there was a paradigm shift in the scientific literature away from their being considered a type of planet. Near the end of the 20th century, planets around other stars were discovered. These planets now number in the many thousands, greatly expanding the diversity of planet characteristics and solar system architectures. Since then, a growing number of small planets have been discovered in the Solar System (often referred to as ice dwarfs), many of which are hypothesized to have or have had subsurface oceans. Dwarfs are also satellites of planets. Ice dwarfs are the most common type of planet in the Solar System and are hypothesized to be the most common type of planet around other stars. If life can arise in subsurface oceans of these worlds, it raises the question of whether life might be common in the universe. The use of the term “planet” today in the scientific literature continues to reflect its heritage from the time of Galileo, that is, planets are geophysical objects, regardless of their orbits. So, by definition, planets would be those objects large enough to be gravitationally round, in generally hydrostatic equilibrium, at which point differentiation commences and geophysical processes, similar to those observed on Earth, are observed to “turn on.”