Sub-Neptunes, with radii between those of Earth and Neptune, dominate the exoplanet population. In particular, sub-Neptunes orbiting M-dwarfs present a promising avenue for their detection and atmospheric characterisation, including those in the habitable zones of their host stars. Traditionally, notions of planetary habitability have focused on predominantly rocky exoplanets and ocean worlds with terrestrial-like atmospheric compositions. In this talk, I will discuss the possibility of habitable conditions on a subset of temperate sub-Neptunes with water-rich interiors and hydrogen-rich atmospheres, called Hycean worlds. Such planets can be larger and hotter than terrestrial-like habitable planets, significantly expanding the habitable zone, and are substantially more abundant and observable in the solar neighbourhood. I will discuss the theoretical developments underlying this new paradigm and their observational implications.
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