Not too long ago, it was widely believed that the origin of life was an extremely unlikely event that might never have occurred a second time in the universe. Today, however, we know that countless stars in the Milky Way alone are surrounded by planetary systems, and new “exoplanets” are constantly being discovered.
Nevertheless, although we may speculate that life has arisen or is still arising beyond our planet, Earth—the “Blue Planet,” consisting of about two-thirds water—is the only known celestial body on which life exists.
Earth has only one moon. It orbits Earth at an average distance of 384,400 kilometers (85 mm in the model). The Moon is the most thoroughly explored celestial body other than Earth—and so far the only one that humans have set foot on."
