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The astronomical unit is a length scale used for Solar System distances, approximately $1.5\times 10^{13}\mathrm{cm}$. Coarse estimates sometimes round it to $10^{13}\mathrm{cm}$. Numerical coefficients in planet formation estimates depend on which approximation is used.

Eiichiro Kokubo uses numerical dynamics to investigate planet formation. His work with Shigeru Ida identified a regime of oligarchic growth, where neighboring large protoplanets accrete smaller planetesimals while remaining dynamically separated.

The large bodies alter the very population from which they grow: their gravity increases small-body random velocities, changing subsequent collision and capture rates. Following this feedback distinguishes the oligarchic regime from a simple extrapolation in which the largest object keeps outgrowing all its neighbors indefinitely.^[ref]

Michel Mayor develops precise stellar radial velocity measurements and their use in searches for unseen companions. With Didier Queloz, he reported a planetary companion to 51 Pegasi in 1995.

The host star's regularly changing Doppler shift revealed its reflex motion around a shared center of mass. The approximately four-day orbital period showed that a giant planet could exist much closer to its host star than the giant planets of the Solar system. The discovery made this previously unexpected orbital arrangement an observational constraint on planet formation.^[ref]

A protoplanetary disk is a flattened, rotating reservoir of gas and solids around a young star. Its radial gravity is usually dominated by the host, making its rotation close to Keplerian rotation, ${\Omega }_K=(G{M}_{\star }/r^3{)}^{1/2}$. Gas pressure provides vertical support and a small radial correction to the orbital speed; it does not make the disk a nonrotating spherical atmosphere.

At height $|z|\ll r$, vertical hydrostatic equilibrium in the stellar gravitational field givesFor a locally vertically isothermal ideal gas, $P_g={\rho }_g{c}_s^2$ with $c_s^2=k_{B}T/(\mu m_H)$, where $\mu$ is mean molecular weight. Integration givesHere ${\Sigma }_g$ is the vertically integrated gas surface density of a disk, and $H$ is the isothermal disk scale height. The disk aspect ratio $H/r=c_s/v_K$ is small in a geometrically thin disk. For $T\propto r^{-q}$ it scales as $r^{(1-q)/2}$, so $q<1$ produces disk flaring. This illustrative local vertical model does not determine a unique radial surface-density law or temperature normalization from the information given.

The gas is predominantly molecular hydrogen and helium. A much smaller component consists of heavier elements, partitioned between vapor, solid refractory grains and condensed volatile ices. A gas-to-solid mass ratio of order $100$ is a common initial fiducial model, not a universal measurement applying at every disk radius or evolutionary stage. Solids can grow and settle while gas accretes or disperses, changing the local ratio substantially.^[ref]

Thermal structure organizes composition. In the hottest inner region, grains can sublimate and much of their material remains in the gas. Refractory silicate minerals and metallic grains survive at lower temperatures; farther out, water and then other sufficiently volatile species can condense when their local condensation conditions are met. The water snow line can raise the solid inventory available to planet formation. Its location depends on stellar irradiation, accretion heating, opacity and gas pressure, and evolves with time; it is not fixed merely by naming a star or adopting a solar-system distance.

Stellar radiation heats the exposed surface of a passive disk. Beneath it, shielding can leave a cooler, molecule-rich interior with a higher mass density. In the inner actively accreting disk, dissipation can instead make the optically thick midplane the hottest region. These are distinct heating regimes; a vertically isothermal profile is an analytic approximation rather than a claim that every real layer has the same temperature. Dust opacity controls how radiation is absorbed and re-emitted. Infrared and millimetre emission probe solid material, while spectral lines probe gas; neither observable alone supplies a model-free inventory of the whole disk.

Dust settling in an astrophysical disk makes larger, less tightly coupled particles more concentrated toward the midplane, while turbulence and mixing oppose that settling. For an aerodynamic stopping time $t_s$ short compared with ${\Omega }_K^{-1}$, terminal vertical balance gives $v_z\simeq -{\Omega }_K^2{t}_{s}z$. Small grains remain better coupled to the gas. Radial drift also changes the distribution of solids and their access to snow lines and embryos.

The disk evolves through accretion and angular momentum transport, potentially involving turbulent stresses, magnetorotational instability or magnetically driven winds where conditions permit. Protoplanetary disk dispersal eventually removes gas through accretion and mass loss, including photoevaporation. In standard evolutionary pictures this occurs on a scale of a few million years, with substantial system-to-system variation. That finite reservoir lifetime is central to whether an embryo acquires a massive gaseous envelope.^[ref]

The essential structure is a nearly Keplerian gas disk with vertical pressure support, radial and vertical thermal gradients, and an evolving solid population concentrated by settling, growth and drift. Its composition is set jointly by the original elemental inventory, local condensation conditions and transport.

Sub-Keplerian rotation means that the gas rotates more slowly than a pressure-free particle on a circular orbit at the same cylindrical radius. Let $v_K=(G{M}_{\star }/r{)}^{1/2}$. Neglecting radial acceleration, radial force balance in the gas isAn outwardly decreasing pressure, $d{P}_g/dr<0$, supplies part of the inward-gravity balance through an outward pressure force. Less centrifugal support is needed, so $v_{g\varphi }<v_K$. Using $c_s^2=P_g/{\rho }_g$, define the pressure-gradient parameter of a protoplanetary disk byThenFor a thin disk and a smooth radial pressure profile, $\eta$ is small, of order $(H/r{)}^2$. The factor of two is part of this explicit convention; changing the convention for $\eta$ changes the drift formula correspondingly.

A solid particle feels essentially no continuum gas-pressure support. Left alone, it would orbit at $v_K$, so in the gas it feels a headwind. Drag removes angular momentum, causing radial drift of solids in a gas disk. Define Stokes number $\mathrm{St}={\Omega }_K{t}_s$ using the aerodynamic stopping time $t_s$. In a dilute test-particle limit with linear drag, negligible radial gas flow and small departures from a circular Keplerian orbit, let $u_{\varphi }=v_{p\varphi }-v_K$. The steady local radial and azimuthal balances areSolving givesThe sign is inward for the usual negative pressure gradient. Very small particles, $\mathrm{St}\ll 1$, closely follow the gas and have a small relative drift $v_{pr}\simeq -2\eta v_K\mathrm{St}$. Very weakly coupled large bodies, $\mathrm{St}\gg 1$, also drift slowly, with $v_{pr}\simeq -2\eta v_K/\mathrm{St}$. The fastest pressure-driven inward drift occurs at $\mathrm{St}=1$, where $|v_{pr}|=\eta v_K$. The corresponding physical grain or aggregate size depends on gas density, orbital radius and the drag regime; “metre-size barrier” is a conventional description of some disk models, not a universal size.

The drift timescale in this model isIt can be much shorter than the disk gas lifetime, threatening loss of the material needed to make planetesimals. Growth through the vulnerable coupling range must compete with this loss, while differential drift generates collisions that may stick, bounce or fragment. Radial redistribution also changes solid-to-gas ratios and moves material across condensation fronts. These are consequences for planet formation, rather than merely a small correction to an orbital frequency.

The same mechanism can help concentrate solids. At a local radial pressure maximum, $d{P}_g/dr=0$, so this component of drift vanishes. On the inside, a positive pressure gradient gives outward drift; on the outside, a negative gradient gives inward drift. Particles therefore converge toward a dust trap. Turbulent diffusion and inward gas advection can limit retention. With a nonzero radial gas speed $v_{gr}$, the test-particle result generalizes to $v_{pr}=[v_{gr}-2\eta v_K\mathrm{St}]/(1+{\mathrm{St}}^2)$, so gas advection need not vanish even at a pressure maximum.

Finally, if local solid loading becomes large, backreaction on the gas can no longer be neglected. Coupled gas-particle drift can support streaming instability and strong solid concentrations, potentially enabling gravitational collapse into planetesimals. The simple test-particle drift formula is a useful starting point, but not a description of that nonlinear high-loading regime. Pressure support creates both a drift obstacle to growth and opportunities for concentration where the pressure structure changes.

Three research directions natural in the 2017 setting are:

Small-planet atmospheres, formation through composition, and atmospheric dynamics/variability provide three concrete emerging directions. The examples describe research aims as of the paper's date, rather than treating later discoveries or later operating missions as established in 2017.

Three major research directions, appropriate to the scientific questions posed in 2018, are:

These are research goals rather than claims that the relevant mechanisms or habitable atmospheres have already been established.

Philip J. Armitage develops the theory of planet formation and the early evolution of planetary systems. His lecture notes follow the sequence from the evolution of protoplanetary disks through planetesimal growth to terrestrial and gas giant formation.

An important physical connection is that formation and orbital evolution occur together. Planets exchange angular momentum with their surrounding gas, scatter smaller bodies and interact gravitationally with other planets. The resulting planetary migration means that an observed orbit need not identify the location where a planet originally acquired its mass.^[ref]

A planet is a large astronomical body whose structure is shaped by its own gravity and whose usual setting is an orbit around a star. Terrestrial planets such as Earth and Venus are dominated by rocky material, while a gas giant such as Jupiter has a large hydrogen-helium envelope. Neptune illustrates the ice giant category.

The label combines physical structure with orbital classification; simply measuring a large radius is insufficient to distinguish a giant planet from a brown dwarf or a stellar companion. Mass, radius, composition and thermal history are separate constraints. Comparing Solar-system bodies with exoplanets tests how planet formation, subsequent cooling and interactions can produce different structures from initially related material.

Planetary science explains the formation, structure and evolution of planets, their natural satellites and smaller bodies by connecting observations with physical and chemical models. A body's mass and radius constrain its mean density, but several mixtures and interior structures can fit the same pair of measurements.

Spectroscopy probes accessible surface or atmospheric composition, gravity measurements constrain internal mass distribution, and seismic waves probe elastic layers. Comparing these independent observations tests models of differentiation, cooling, planet formation and atmospheric evolution. Nearby Solar-system bodies also provide resolved examples for interpreting the limited signals available from an exoplanet.

A planetesimal is a solid body participating in the growth and collisional evolution of a planetary system. Its collisions can build larger bodies or fragment existing ones; the result depends on relative velocity, material strength and self-gravity. Gravitational encounters can increase random motion, while gas drag and dynamical friction can reduce it.

The category marks a physical role rather than one sharply defined diameter. A population can feed planetary embryos during planet formation or act as the parent bodies of a later debris disk. The same measured total mass can have very different collision behaviour when distributed among many small bodies or a few large ones.

Planet formation is the growth of planets from material around a young star. Solid grains in a protoplanetary disk can collide and stick through protoplanetary dust coagulation, concentrate into planetesimals, and build planetary embryos. Their gravity modifies further encounters through gravitational focusing, while collisions can merge or fragment the bodies.

In core accretion, a growing solid core can acquire a gas envelope if the gas can cool and remain bound. Direct formation from a self-gravitating gas concentration is a distinct route involving gravitational instability of an astrophysical disk. Planetary migration and planetary scattering can subsequently change the orbits, so present location alone does not identify formation location. Planetary differentiation then reorganizes the material inside an assembled body.

A protoplanetary disk is a gas-and-solid astrophysical disk around a young star in which planet formation occurs. Its evolution combines accretion, radiation, gas dynamics and the growth of solid material. A minimum-mass solar nebula is one reconstruction of the material needed to form the Solar System.