Oceans and the interaction between landmasses and water were crucial driving forces during the evolution of life on Earth, so in the classical sense of habitability, it is worth searching for 'water worlds' outside our Solar System, to find possible habitats for extraterrestrial life.

Exoplanet databases proved to be valuable tools for the characterization of extrasolar systems. Some of the parameters define a certain range around the star, which determines the fate of the possible surface ocean on a planet: vanishing, evaporating, and staying in a gaseous state, or freezing and may hide under an icy crust. Thus, the term “habitable zone” was created, to define the domains around stars, where liquid water is possible to exist. Various data of 5187 confirmed exoplanets was used to find the constraints of the possibility of a global, surface water ocean.

The goal of this study was twofold: i) to apply previous models and calculations to the current database, in order to define the inner and outer edges of the circumstellar habitable zone, and ii) to characterize and classify the 239 planets, which fell into the habitable zone, regarding the appearance of the surface water ocean.

In this study, some of the parameters of the stars and the orbiting exoplanets (such as the spectral type of the system’s star, planetary mass, atmospheric composition, eccentricity, and inclination) were selected, and they were used as a proxy which may indicate a liquid surface water ocean. The selected proxies were examined with statistical tools (e.g., PCA analysis) to reveal a connection between planetary characteristics and the possible appearance of a surface ocean.