The scientific discipline that studies the origin, evolution, distribution, and future of life in the universe. Astrobiology is an interdisciplinary science integrating contributions from biology, geology, astronomy, paleontology, and planetary science, among others. The first use of the term appears to have been in 1941 when it was defined as the subject of life in the universe other than on Earth (Fig. 1). The U.S. National Aeronautics and Space Administration (NASA) adopted the term in 1996 and expanded the meaning to include the origin and history of life on Earth, and the future of life in the universe. See also: Astronomy; Biology; Geology; Paleontology; Science; Universe

A telescope or telescopes, their protective enclosures (if any), support and headquarters buildings, and the staff of astronomers, engineers, technicians, and other support personnel. The telescopes can be optical or infrared (reflecting or refracting) inside a corotating dome, or radio dishes without enclosures.

The use of spectroscopy (the analysis of light as a function of wavelength) as a tool for obtaining observational data on the chemical compositions, physical conditions, and radial velocities of astronomical objects. Astronomical applications of optical spectroscopy from ground-based observatories cover the electromagnetic spectrum from the near-ultraviolet [wavelengths around 0.3 micrometers (μm)] through the visible (0.4–0.7 μm) and into the near-infrared (2 μm). Space-based observatories, such as the Hubble Space Telescope and the Herschel Space Observatory, extend spectroscopic observations from the far-ultraviolet (0.1 μm) to the far-infrared (200 μm). Work at shorter wavelengths (x-ray and gamma-ray spectroscopy) and longer wavelengths (submillimeter and radio wavelengths) requires techniques other than those discussed here. See also: Gamma-ray astronomy; Hubble Space Telescope; Radio astronomy; Submillimeter astronomy; Ultraviolet astronomy; X-ray astronomy

The sixth-largest moon of Saturn, known for its bright icy surface, active geysers, and astrobiological potential. Enceladus is the most reflective astronomical body in the solar system because it is covered in fresh, smooth ice. The sources of this ice are the geyser-like jets spewing from Enceladus's south polar region (Fig. 1). These geysers send various gases including water vapor, ice particles, and organic material into space, creating Saturn's E ring, with some of the material depositing back onto Enceladus's surface. The icy, gassy plumes emanate from relatively warm surface fissures known as tiger stripes, which are vents in the moon's crust connecting to a global subsurface saltwater ocean. Tidal interactions with Saturn, amplified by an orbital resonance with the moon Dione, primarily generate the internal heat that powers Enceladus' geological activity. See also: Geology; Geyser; Heat; Satellite (astronomy); Saturn; Tide; Water

Planets beyond our solar system. For millennia, humankind has known about most of the major planets in our solar system because these worlds are visible to the naked eye. Exoplanets, however, were not indisputably known until 1995, when astronomers discovered the first planet orbiting a typical star like the Sun. Since then, and using a variety of detection techniques, scientists have uncovered thousands of exoplanets. The current tally of confirmed exoplanets exceeds 5300, while many thousands of other candidate planets are awaiting confirmation through corroborative measurements by additional instruments. Because almost all stars seem to form and host planets, the running tally is but a small fraction of the hundreds of billions of exoplanets expected to exist just in the Milky Way Galaxy alone (Fig. 1). See also: Astronomy; Astrophysics; Planet; Solar system; Star

Postulated entities beyond Earth with a level of intelligence and comprehension at least equal to that of present-day humans. Whether alien life—and especially intelligent alien life—exists is one of the profoundest mysteries in science. Extraterrestrial intelligence is usually envisioned as an advanced civilization, populated by creatures that evolved via Darwinian evolution on a planet vaguely similar to Earth. However, extraterrestrial intelligence could conceivably be artificial in nature, having been initially created by biological beings. The search for extraterrestrial intelligence is a subset of astrobiology, which encompasses all aspects of the existence of, and search for, extraterrestrial life. Considerations of technology, engineering, and the limits of physics also figure prominently in the efforts to detect evidence of extraterrestrial intelligence (Fig. 1). See also: Artificial intelligence; Astrobiology; Evolution

The first orbiting science satellite and atmospheric probe sent to study Jupiter. The Galileo spacecraft arrived at Jupiter on December 7, 1995 and operated until September 21, 2003 (Fig. 1). It consisted of two main components: an atmospheric entry probe and a planetary orbiter. The probe plunged into the atmosphere of Jupiter on December 7, accomplishing the first direct sampling of the atmosphere of one of the outer giant planets and surviving the most difficult atmospheric entry ever attempted. On the same date, the orbiter was the first ever to be placed in orbit about Jupiter. Before reaching Jupiter, the spacecraft took the first closeup pictures of asteroids, discovering that one of them had a small satellite (about 1.6 km or 1 mi in diameter), and was the only platform that had a direct view of the impact of the comet Shoemaker-Levy 9 with Jupiter in July 1994. The Jovian system would not have another dedicated science vessel visit it until the arrival of Juno in 2016. See also: Asteroid; Atmosphere; Comet; Jupiter; Planet; Solar system

The field of astronomical observations specializing in detecting photons from the infrared portion of the electromagnetic spectrum. Infrared astronomy plays an important role particularly in the study of cooler astrophysical objects, ranging from asteroids and comets to exoplanets and developing stars. These objects emit brightly in the infrared, although they may be veiled by clouds of dust that conceal their presence at shorter, optical wavelengths. Infrared astronomy thus affords researchers an opportunity to study phenomena that would go unseen if relying on just visible light (Fig. 1). See also: Asteroid; Comet; Exoplanets; Galaxy; Light; Star

A space telescope mission dedicated to detecting exoplanets. The tenth mission launched under the National Aeronautics and Space Administration's (NASA's) Discovery Program, Kepler was the agency's first mission dedicated to the search for planets beyond our solar system. Its scientific goal was to describe the diversity of planetary systems by utilizing exoplanet detection statistics to estimate the distribution of planet sizes, orbital parameters, host-star properties, and multiple-planet systems. Of particular interest was the identification of Earth-sized planets orbiting in the "habitable zone" of Sun-like stars, wherein a rocky planet with a sufficient atmosphere could contain liquid water on its surface, permitting the possibility of alien life (Fig. 1). See also: Exoplanet; Solar system; Telescope

The passage of one astronomical object across another. The term "transit" implies that the foreground object is smaller than the background object (Fig. 1). A related term, "eclipse," implies that the foreground object hides a background object that is extended in size, or that an object passes into another object's shadow. A third term, "occultation," implies that the foreground object extinguishes the background object, as when the disk of a solar-system object blocks a star. See also: Eclipse; Occultation; Solar system; Star