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An exoplanet is a planet outside the Solar system; here the focus is on planets orbiting stars other than the Sun, including pulsars. Most exoplanets are detected indirectly: a star's changing Doppler shift reveals its motion, or a small dip in its light curve reveals an exoplanet transit. The striking idea is that an unresolved point of light can tell us about an otherwise unseen world's mass, radius, and orbit.

The measurements answer different questions. Exoplanet transit photometry primarily gives a radius ratio, while Doppler spectroscopy constrains mass together with orbital inclination. Combining them with the host star's properties gives a mean mass density, ${\bar{\rho }}_p=3M_p/(4\pi R_p^3)$, useful for exoplanet interior models. Even this combination does not uniquely determine composition: see exoplanet interior-composition degeneracy. Transit spectroscopy and an exoplanet secondary eclipse then probe an exoplanet atmosphere.

The discovery history has two distinct milestones. In 1992 Aleksander Wolszczan and Dale Frail reported two planet-mass companions to the pulsar PSR B1257+12 from periodic changes in pulse arrival times; their paper also suggested an additional companion. These were the first confirmed exoplanet discoveries, around a neutron star. ^[ref]. Wolszczan's contemporary conference account, archived by NASA, likewise distinguishes the two reported companions from a possible third. ^[ref].

In 1995 Michel Mayor and Didier Queloz discovered 51 Pegasi b through Doppler spectroscopy, the landmark first confirmed exoplanet around a Sun-like star. This was a hot Jupiter, not the first exoplanet of every kind: the earlier pulsar discoveries belong to a different host category. ^[ref]. NASA's contemporary account describes the confirmation and the approximately four-day orbital period. ^[ref].

An exoplanet bulk-composition ternary diagram represents three component mass fractions by one point inside a triangle. If the components have fractions $x_1,x_2,x_3$, then $x_i\ge 0$ and $x_1+x_2+x_3=1$. Each vertex is a pure component, an edge excludes the opposite component, and the centre corresponds to equal thirds. The fractions are barycentric coordinates relative to the vertices.

For an illustrative iron-rock-water model, measured mass and radius restrict the allowed part of the triangle through an exoplanet interior calculation. They need not select a unique point, making exoplanet interior-composition degeneracy visible. The diagram is conditional on the chosen three components: an unmodelled hydrogen-helium envelope cannot automatically be interpreted as more of one of those solid components.

An exoplanet interior is the structure and composition beneath the layers directly sampled by observations. Mass and radius constrain the mean mass density, but a physical model also needs hydrostatic equilibrium, an equation of state, and assumptions about composition and thermal history. Dense material can be concentrated in a core or mixed into surrounding layers.

Solid-planet calculations show how iron-, silicate- and water-rich bodies occupy different broad mass-radius relations, with compression preventing one constant-density scaling from applying to every mass.^[ref] A light gas envelope can substantially alter the observed radius. Exoplanet interior-composition degeneracy therefore remains even when both mass and radius are measured precisely.

HD 149026 b is a transiting, roughly Saturn-mass exoplanet whose small measured radius made it an early test of giant-planet composition. The discovery combined Doppler spectroscopy at two observatories with ground-based exoplanet transit photometry: the first constrains mass, the second radius and orbital inclination. The original exoplanet interior models required a substantial inventory of elements heavier than hydrogen and helium to reproduce the measured compactness.^[ref]

The inferred concentration of heavy material is a model result, rather than an image of a planetary core. Its partition between a central core and the surrounding envelope depends on the assumed equation of state, age, and temperature; this illustrates exoplanet interior-composition degeneracy.

A super-Earth is a planet more massive than Earth but below the usual ice-giant scale, conventionally about $1$--$10M_{\oplus }$. The term does not guarantee an Earth-like or rocky composition; water-rich planets and objects with small H/He envelopes can lie in this range.

The most direct constraints are transit radius and a mass from radial velocities or transit-timing variations, giving $\bar{\rho }=3M/(4\pi R^3)$. Comparing these with interior models constrains the possible iron core, silicate mantle, water layer and gas envelope, but many mixtures share one mass and radius.

Additional observables include atmospheric spectra/mean molecular weight and escape signatures, which test whether a low-density envelope is H/He or heavier volatiles; stellar Fe/Mg/Si abundances as priors on refractory composition; and age, irradiation and orbital history, which control thermal expansion and envelope survival. Where measurable, a Love number or related apsidal response constrains central concentration. Mass and radius constrain families of interiors, not a unique composition; atmospheric and dynamical information can reduce this exoplanet interior-composition degeneracy.

At fixed mass, three broad controls of a super-Earth-sized body's observed radius are:

Bulk composition, envelope fraction/composition, and thermal/irradiation history are three independent controls. If mass is not fixed, the mass itself is an additional major variable. The label super-Earth does not ensure a rocky composition or an Earth-like atmosphere, and a radius alone does not determine which of these effects dominates.

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.

The location between pure-silicate and pure-water planetary mass-radius curves does not determine a unique composition. Possibilities include a rocky iron-silicate interior with a deep water layer, a rock-water mixture, or a mostly rocky planet whose small hydrogen-helium envelope inflates its radius. The atmosphere could accordingly be hydrogen-helium, steam-rich, carbon-dioxide-rich, or secondary gas released from the interior. Residence in the circumstellar habitable zone constrains stellar flux but does not by itself distinguish these models or guarantee surface liquid water.

For an atmosphere spanning fixed base and photospheric pressures,Thus two compositions haveAt equal temperature, a hydrogen-helium atmosphere with $\mu \simeq 2.3$ is abouttimes thicker than a steam atmosphere with $\mu \simeq 18$. This large difference is one reason atmospheric composition contributes strongly to the exoplanet interior-composition degeneracy.

The measured mass and radius illustrate the exoplanet interior-composition degeneracy. Three plausible structures are:

An exoplanet transmission spectrum can distinguish a large-scale-height hydrogen atmosphere from compact steam or carbon-dioxide atmospheres. Molecular abundances, clouds, atmospheric mean molecular mass, and atmospheric escape constrain the envelope mass and evolution. An exoplanet emission spectrum constrains temperature, albedo, and internal cooling. A detected hydrogen-rich atmosphere favors the first scenario, a water-dominated high-molecular-weight spectrum supports the second, and a thin secondary atmosphere supports the third, although clouds can preserve degeneracies.