This paper reviews results of Solar System explorations and telescope observations of geologically-active icy bodies (Europa, Enceladus, and Ceres) and Mars, where liquid waters are, or were, present. In particular, we focus on the availability of both reductants and oxidants as well as water chemistry on these planetary bodies. This paper also proposes an idea of “planetary redox”—a planetary-scale system that can generated a redox and pH gradient through geohydrological cycles. Based on the understandings of planetary redox on Europa, Enceladus, and Mars, we discuss about bioavailable energy on these bodies and science targets in future missions. Knowledge on the presence or non-presence of life in these habitable worlds would shed light on the conditions where life could emerge on early Earth.

Exoplanet census revealed that Earth-sized planets are abundant and ubiquitous in the Galaxy. Upcoming space missions such as JWST and the development of 30-m class telescopes as well as the recent discovery of water plumes on Enceladus and possibly, on Europa have encouraged us to search for life beyond Earth; e.g., biosignatures such as the red edge and biotic gases. In the context of astronomy, both liquid water on the planetary surface and greenhouse effects in the atmosphere are indispensable to the habitability of terrestrial planets.

Technological advances in exoplanet observations are capable of exploring atmospheres of Earth-sized planets outside the Solar System. Here we review the current status of exoplanet characterization and our understating of the habitability of terrestrial planets, including the Earth.