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The region around a star where conditions may be just right to allow the presence of liquid water on a planet's surface.

A planet that orbits a star outside our solar system.

Search for Extraterrestrial Intelligence, the collective name for scientific searches for intelligent extraterrestrial life, for example, monitoring electromagnetic radiation for signs of transmissions from civilizations on other planets.

An equation used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy.

Evidence that may indicate the presence of past or present life, potentially identified in the geological record or in the atmosphere of a planet.

The hypothesis that life exists throughout the Universe, distributed by meteoroids, asteroids, comets, planetoids, and potentially by spacecraft in the form of unintended contamination by microorganisms.

The total number of fossils that have been discovered, as well as the information derived from them, which provides evidence about the history of life on Earth.

A property of asymmetry in molecules, where a molecule cannot be superimposed on its mirror image. This concept is significant in astrobiology because of its potential implications for the origin of life.

The study of the origin, evolution, distribution, and future of life in the universe.

An organism that lives inside rock, coral, animal shells, or in the pores between mineral grains of a rock.

A chemical experiment that simulated the conditions thought at the time to be present on the early Earth and tested the chemical origin of life under those conditions.

The ratio of the carbon isotope $^{13}C$ to $^{12}C$, often used in the study of ecological and biological processes since different processes affect the ratios differently.

The study of the processes of decay, preservation, and the formation of fossils in the geological record.

The study of the relationships between alien life forms and their space environments.

A moon of Saturn that has been of particular interest to astrobiologists due to its subsurface ocean and geysers ejecting water vapor which could potentially harbor life.

A hypothetical microbial biosphere of Earth that uses radically different biochemical and molecular processes than currently known life.

The region of space surrounding a planet in which charged particles are manipulated or affected by that planet's magnetic field, important in protecting habitable planets from solar wind.

Organisms that thrive in physically or geochemically extreme conditions that are detrimental to most life on Earth.

Life that currently exists, as opposed to extinct life, which died out.