= Solution
Three research directions natural in the 2017 setting are:
* Atmospheric characterization of small planets, extending beyond bright <hot Jupiters> to super-Earth-sized and terrestrial bodies. The aim is to determine whether they retain light envelopes or heavier secondary atmospheres and to understand <exoplanet habitability>. Potential <exoplanet biosignatures> require a chemical and stellar context because abiotic processes can mimic individual molecules; future infrared observatories such as the then-planned <James Webb Space Telescope> offered a route to better spectra, without guaranteeing a biological interpretation.
* Connecting measured composition to <planet formation> and <planetary migration>. <Atmospheric metallicity of a giant planet>, <atmospheric carbon-to-oxygen ratio>, orbital architecture and bulk <mass>-<radius> measurements can constrain accretion of gas and solids. <Atmospheric condensate rainout>, <disequilibrium chemistry in an exoplanet atmosphere> and <exoplanet interior-composition degeneracy> make a one-to-one mapping from a measured ratio to a birth location unreliable; comparative samples and coupled models are needed.
* Resolving atmospheric structure in space and time. <Exoplanet thermal phase curves>, repeated spectra, high-resolution planetary spectroscopy and <exoplanet direct imaging> can test <day-night heat redistribution>, winds, patchy <exoplanet clouds>, <atmospheric escape> and chemical heterogeneity. The challenge is to distinguish actual variability or circulation from changes in stellar light and instrumental calibration.
\b[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.
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