Does life exist only on the Earth?

Abstract

Does life exist only on the Earth? Are there extraterrestrial living things in the universe? This is a long-standing question. Although no extraterrestrial living things have been found yet, the space explorations and study of meteorites do reveal the existence of habitable environments on other planets or their satellites today or in the past. In the solar system, Mars is a planet most favoring life other than the Earth. According to the numerous explorations and studies of Martian meteorites, Mars had a wet environment in its early history, with water flowing in rivers, lakes and probably oceans. Life could survive and/or emerge under such conditions. In-situ analysis of water contents and H isotopes of Martian meteorites indicated the presence of underground water ~180 Ma ago, via melting subsurface glacier by magma. Moreover, high special resolution topographic images revealed recent activities of underground water on Mars. Other important targets for future exploration missions are Europa and other satellites of the giant planets, they likely have oceans under the ice crusts. On the other hand, a large number of exoplanets have been found, especially by Kepler space telescope. A part of the exoplanets probably have rocky surface and locate within the habitability zones. These discoveries further confirm the common presence of habitable planets in the universe.