Observational constraints on the formation and evolution of Neptune-class exoplanets

TL;DR

This paper reviews the properties and formation of Neptune-sized exoplanets, providing observational constraints that help understand ice giant formation beyond our solar system.

Contribution

It offers a comprehensive overview of small exoplanets with radii 1.6-4.0 Earth radii, highlighting their role in constraining planet formation models.

Findings

Exoplanets with radii 1.6-4.0 Earth radii are common in multi-planet systems.

Current observations lack true Neptune/Uranus analogues.

These planets help refine models of ice giant formation.

Abstract

Among exoplanets, the small-size population constitutes the dominant one, with a diversity of properties and compositions ranging from rocky to gas dominated envelope. While a large fraction of them have masses and radii similar to or smaller than Neptune, yet none share common properties in term of orbital period and insulation with our ice giants. These exoplanets belong to multi-planet systems where planets are closely packed within the first tenth of AU and often exposed to strong irradiation from their host star. Their formation process, subsequent evolution, and fate are still debated and trigger new developments of planet formation models. This paper reviews the characteristics and properties of this extended sample of planets with radii between $\sim$ 1.6 and 4.0$_\oplus$. Even though we still lack real Neptune/Uranus analogues, these exoplanets provide us with key observational constraints that allow the formation of our ice giants to be placed in a more general framework than the sole example of our solar system.