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.”
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Article
Defining Planets
Mark V. Sykes, Elisabeth Adams, Kirby Runyon, S. Alan Stern, and Philip T. Metzger
Article
Technosignatures and Astrobiology
Jacob Haqq-Misra
Astrobiologists are engaged in the search for signs of extraterrestrial life in all forms, known as biosignatures, as well as specific signs of extraterrestrial technology, known as technosignatures. The search for technosignatures and biosignatures attempts to identify characteristic evidence of life on other planets that could be detected using astronomical methods. The first scientific searches for technosignatures began in the 1960s, which used radio telescopes to examine nearby star systems for evidence of narrowband transmissions used for communication. The search for extraterrestrial intelligence has continued to search for anomalous radio and optical signals that would indicate intentional or unintentional extraterrestrial communication. Advances in ground- and space-based spectroscopy are also beginning to enable searches for technosignatures in exoplanetary systems such as atmospheric pollution, city lights, large-scale surface structures, and orbiting satellites. Some technosignature searches also attempt to search for nonterrestrial artifacts within the solar system on planetary bodies or in stable orbits. All of these technosignature concepts use known technology on Earth as a starting point for thinking about technology that could be plausible and detectable in extraterrestrial systems.
Technology is a relatively recent phenomenon in the history of life on Earth, so the search for technosignatures also employs methods from futures studies to explore numerous trajectories for extensions of known technology. The range of possibilities considered by technosignature science can include any known or plausible technology that could be remotely detected and would not violate any known physical laws. Megastructures are examples of theoretical large-scale planetary engineering or astroengineering projects that could be detectable in exoplanetary systems through infrared excesses or gravitational effects. Many other technosignatures remain possible, even if they do not draw upon Earth projections, but most astrobiological study of technosignatures focuses on predictions that could be tested with current or near-future missions. The positive discovery of extraterrestrial technology could be of great significance to humanity, but technosignature searches that yield negative results still provide value by placing qualitative upper limits on the prevalence of certain types of extraterrestrial technology.
Article
Planetary Systems Around White Dwarfs
Dimitri Veras
White dwarf planetary science is a rapidly growing field of research featuring a diverse set of observations and theoretical explorations. Giant planets, minor planets, and debris discs have all been detected orbiting white dwarfs. The innards of broken-up minor planets are measured on an element-by-element basis, providing a unique probe of exoplanetary chemistry. Numerical simulations and analytical investigations trace the violent physical and dynamical history of these systems from astronomical unit (au)-scale distances to the immediate vicinity of the white dwarf, where minor planets are broken down into dust and gas and accrete onto the white dwarf photosphere. Current and upcoming ground-based and space-based instruments are likely to further accelerate the pace of discoveries.
Article
Migration of Low-Mass Planets
Frédéric S. Masset
Planet migration is the variation over time of a planet’s semimajor axis, leading to either a contraction or an expansion of the orbit. It results from the exchange of energy and angular momentum between the planet and the disk in which it is embedded during its formation and can cause the semimajor axis to change by as much as two orders of magnitude over the disk’s lifetime. The migration of forming protoplanets is an unavoidable process, and it is thought to be a key ingredient for understanding the variety of extrasolar planetary systems. Although migration occurs for protoplanets of all masses, its properties for low-mass planets (those having up to a few Earth masses) differ significantly from those for high-mass planets.
The torque that is exerted by the disk on the planet is composed of different contributions. While migration was first thought to be invariably inward, physical processes that are able to halt or even reverse migration were later uncovered, leading to the realization that the migration path of a forming planet has a very sensitive dependence on the underlying disk parameters.
There are other processes that go beyond the case of a single planet experiencing smooth migration under the disk’s tide. This is the case of planetary migration in low-viscosity disks, a fashionable research avenue because protoplanetary disks are thought to have very low viscosity, if any, over most of their planet-forming regions. Such a process is generally significantly chaotic and has to be tackled through high-resolution numerical simulations. The migration of several low-mass planets is also is a very fashionable topic, owing to the discovery by the Kepler mission of many multiple extrasolar planetary systems. The orbital properties of these systems suggest that at least some of them have experienced substantial migration. Although there have been many studies to account for the orbital properties of these systems, there is as yet no clear picture of the different processes that shaped them. Finally, some recently unveiled processes could be important for the migration of low-mass planets. One process is aero-resonant migration, in which a swarm of planetesimals subjected to aerodynamic drag push a planet inward when they reach a mean-motion resonance with the planet, while another process is based on so-called thermal torques, which arise when thermal diffusion in the disk is taken into account, or when the planet, heated by accretion, releases heat into the ambient gas.
Article
Planetary Aurorae
Steve Miller
Planetary aurorae are some of the most iconic and brilliant (in all senses of that word) indicators not only of the interconnections on Planet Earth, but that these interconnections pertain throughout the entire Solar System as well. They are testimony to the centrality of the Sun, not just in providing the essential sunlight that drives weather systems and makes habitability possible, but also in generating a high velocity wind of electrically charged particles—known as the Solar Wind—that buffets each of the planets in turn as it streams outward through interplanetary space. Aurorae are created when electrically charged particles—predominantly negatively charged electrons or positive ions such as protons, the nuclei of hydrogen—crash into the atoms and molecules of a planetary or lunar atmosphere. Such particles can excite the electrons in atoms and molecules from their ground state to higher levels. The atoms and molecules that have been excited by these high-energy collisions can then relax; the emitted radiation is at certain well-defined wavelengths, giving characteristic colors to the aurorae. Just how many particles, how much atmosphere, and what strength of magnetic field are required to create aurorae is an open question. But giant planets like Jupiter and Saturn have aurorae, as does Earth. Some moons also show these emissions. Overall, the aurorae of the Solar System are very varied, variable, and exciting.
Article
Condensation Calculations in Planetary Science and Cosmochemistry
Denton S. Ebel
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.
Article
Hot Planetary Coronas
Valery I. Shematovich and Dmitry V. Bisikalo
The uppermost layers of a planetary atmosphere, where the density of neutral particles is vanishingly low, are commonly called exosphere or planetary corona. Since the atmosphere is not completely bound to the planet by the planetary gravitational field, light atoms, such as hydrogen and helium, with sufficiently large thermal velocities can escape from the upper atmosphere into interplanetary space. This process is commonly called Jeans escape and depends on the temperature of the ambient atmospheric gas at an altitude where the atmospheric gas is virtually collisionless. The heavier carbon, nitrogen, and oxygen atoms can populate the coronas and escape from the atmospheres of terrestrial planets only through nonthermal processes such as photo- and electron-impact energizing, charge exchange, atmospheric sputtering, and ion pickup.
The observations reveal that the planetary coronae contain both a fraction of thermal neutral particles with a mean kinetic energy corresponding to the exospheric temperature and a fraction of hot neutral particles with mean kinetic energy much higher than that expected for the exospheric temperature. These suprathermal (hot) atoms and molecules are the direct manifestation of the nonthermal processes taking place in the atmospheres. These hot particles populate the hot coronas, take a major part in the atmospheric escape, produce nonthermal emissions, and react with the ambient atmospheric gas, triggering the hot atom chemistry.
Article
Solar Elemental Abundances
Katharina Lodders
Solar elemental abundances, or solar system elemental abundances, refer to the complement of chemical elements in the entire Solar System. The Sun contains more than 99% of the mass in the solar system and therefore the composition of the Sun is a good proxy for the composition of the overall solar system. The solar system composition can be taken as the overall composition of the molecular cloud within the interstellar medium from which the solar system formed 4.567 billion years ago. Active research areas in astronomy and cosmochemistry model collapse of a molecular cloud of solar composition into a star with a planetary system and the physical and chemical fractionation of the elements during planetary formation and differentiation. The solar system composition is the initial composition from which all solar system objects (the Sun, terrestrial planets, gas giant planets, planetary satellites and moons, asteroids, Kuiper-belt objects, and comets) were derived.
Other dwarf stars (with hydrostatic hydrogen-burning in their cores) like the Sun (type G2V dwarf star) within the solar neighborhood have compositions similar to the Sun and the solar system composition. In general, differential comparisons of stellar compositions provide insights about stellar evolution as functions of stellar mass and age and ongoing nucleosynthesis but also about galactic chemical evolution when elemental compositions of stellar populations across the Milky Way Galaxy is considered. Comparisons to solar composition can reveal element destruction (e.g., Li) in the Sun and in other dwarf stars. The comparisons also show element production of, for example, C, N, O, and the heavy elements made by the s-process in low to intermediate mass stars (3–7 solar masses) after these evolved from their dwarf-star stage into red giant stars (where hydrogen and helium burning can occur in shells around their cores). The solar system abundances are and have been a critical test composition for nucleosynthesis models and models of galactic chemical evolution, which aim ultimately to track the production of the elements heavier than hydrogen and helium in the generation of stars that came forth after the Big Bang 13.4 billion years ago.
Article
Steam Atmospheres and Magma Oceans on Planets
Keiko Hamano
A magma ocean is a global layer of partially or fully molten rocks. Significant melting of terrestrial planets likely occurs due to heat release during planetary accretion, such as decay heat of short-lived radionuclides, impact energy released by continuous planetesimal accretion, and energetic impacts among planetary-sized bodies (giant impacts). Over a magma ocean, all water, which is released upon impact or degassed from the interior, exists as superheated vapor, forming a water-dominated, steam atmosphere. A magma ocean extending to the surface is expected to interact with the overlying steam atmosphere through material and heat exchange.
Impact degassing of water starts when the size of a planetary body becomes larger than Earth’s moon or Mars. The degassed water could build up and form a steam atmosphere on protoplanets growing by planetesimal accretion. The atmosphere has a role in preventing accretion energy supplied by planetesimals from escaping, leading to the formation of a magma ocean. Once a magma ocean forms, part of the steam atmosphere would start to dissolve into the surface magma due to the high solubility of water into silicate melt. Theoretical studies indicated that as long as the magma ocean is present, a negative feedback loop can operate to regulate the amount of the steam atmosphere and to stabilize the surface temperature so that a radiative energy balance is achieved. Protoplanets can also accrete the surrounding
H
2
-rich disk gas. Water could be produced by oxidation of
H
2
by ferrous iron in the magma. The atmosphere and water on protoplanets could be a mixture of outgassed and disk-gas components.
Planets formed by giant impact would experience a global melting on a short timescale. A steam atmosphere could grow by later outgassing from the interior. Its thermal blanketing and greenhouse effects are of great importance in controlling the cooling rate of the magma ocean. Due to the presence of a runaway greenhouse threshold, the crystallization timescale and water budget of terrestrial planets can depend on the orbital distance from the host star. The terrestrial planets in our solar system essentially have no direct record of their earliest history, whereas observations of young terrestrial exoplanets may provide us some insight into what early terrestrial planets and their atmosphere are like. Evolution of protoplanets in the framework of pebble accretion remains unexplored.
Article
Tidal Interactions Between Planets and Host Stars
Gordon Ogilvie
Hundreds of planets are already known to have orbits only a few times wider than the stars that host them. The tidal interaction between a planet and its host star is one of the main agents shaping the observed distributions of properties of these systems. Tidal dissipation in the planet tends make the orbit circular, as well as synchronizing and aligning the planet’s spin with the orbit, and can significantly heat the planet, potentially affecting its size and structure. Dissipation in the star typically leads to inward orbital migration of the planet, accelerating the star’s rotation, and in some cases destroying the planet.
Some essential features of tidal evolution can be understood from the basic principles that angular momentum and energy are exchanged between spin and orbit by means of a gravitational field and that energy is dissipated. For example, most short-period exoplanetary systems have too little angular momentum to reach a tidal equilibrium state.
Theoretical studies aim to explain tidal dissipation quantitatively by solving the equations of fluid and solid mechanics in stars and planets undergoing periodic tidal forcing. The equilibrium tide is a nearly hydrostatic bulge that is carried around the body by a large-scale flow, which can be damped by convection or hydrodynamic instability, or by viscoelastic dissipation in solid regions of planets. The dynamical tide is an additional component that generally takes the form of internal waves restored by Coriolis and buoyancy forces in a rotating and stratified fluid body. It can lead to significant dissipation if the waves are amplified by resonance, are efficiently damped when they attain a very short wavelength, or break because they exceed a critical amplitude.
Thermal tides are excited in a planetary atmosphere by the variable heating by the star’s radiation. They can oppose gravitational tides and prevent tidal locking, with consequences for the climate and habitability of the planet.
Ongoing observations of transiting exoplanets provide information on the orbital periods and eccentricities as well as the obliquity (spin–orbit misalignment) of the star and the size of the planet. These data reveal several tidal processes at work and provide constraints on the efficiency of tidal dissipation in a variety of stars and planets.
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