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. Organic compounds are the building blocks of living things, and they have been found common in various primitive chondrites, carbonaceous asteroids, comets, and interstellar dust particles. The extraterrestrial organic matter is a bridge connecting inorganic matter and the origin of life. The extraterrestrial organic matter has experienced cosmic-ray and UV radiation, thermal and hydrothermal metamorphism in the solar nebula and/or in the parent bodies, and the evolution processes can be constrained by isotopic ratios of C and other elements. Organic compounds have also been found in Martian meteorites and detected in situ on Mars. However, their origins are still controversial, including adding by impact of carbon-rich asteroids or comets, Fisher-Tropsch-type reactions and bioenic activity. Organic carbon has been found in the recently fell Tissint Martian meteorite, with light C isotopes typical for biogenic origins. The origin of life and the presence of extraterrestrial living things are the basic questions of science. Exploration for extraterrestrial life has the top priority in the future missions. Besides studies of paleoclimate, search for living things or possible fossils on Mars or satellites of the giant planets is the ultimate goal.