A new family of planets is considered which is in between the rocky terrestrial planets and the gaseous giants, "Ocean-Planets." We present the possible formation, composition and internal structure of these putative planets. We consider their oceans, as well as their possible Exobiology interest. These exoplanets should be detectable by Space missions such as Eddington, Kepler, and possibly COROT (launch scheduled in 2006). They have a density lower than that of rocky planets. Their rather large radius would make them attractive targets for exoplanet spectroscopic missions such as Darwin/TPF, all the more because a robust biosignature appears to exist.