Astrobiology Glossary L – Space

Term Definition
Last Common Ancestor The last common ancestor of all organisms living today. The root of the tree of life.
Lateral Gene Transfer The transfer of genes between different species. Lateral gene transfer may have been widespread in the early stages of life on Earth and this complicates the interpretation of the tree of life.
Light Curve A plot showing how the light output of a star (or other variable astronomical object) changes with time.
Light-Year The distance that a particle of light (photon) will travel in a year – about 10 trillion kilometers (6 trillion miles). It is a useful unit for measuring distances between stars.
Lithosphere The solid part of a planet’s surface, composed of the crust and upper mantle. On Earth, it includes the continents and the sea floor.
Local Group A small cluster of more than 30 galaxies, including the Andromeda galaxy, the Magellanic Clouds, and the Milky Way galaxy.
Long-Period Comet A comet having an orbital period greater than 200 years and usually moving in a highly elliptical, eccentric orbit. Comets have orbits that take them great distances from the Sun. Most long-period comets pass through the inner solar system only once. Hale-Bopp is an example of a long-period comet.
LUCA Another term used for the Last Common Ancestor of all living organisms. Acronym for Last Universal Common Ancestor.
Luminosity The amount of energy radiated into space every second by a celestial object, such as a star. It is closely related to the absolute brightness of a celestial object.
Lunar Eclipse A darkening of the Moon, as viewed from Earth, caused when our planet passes between the Sun and the Moon.
Lyman Limit A specific wavelength (91.2 nm) that corresponds to the energy needed to ionize a hydrogen atom (13.6 eV). Galactic space is opaque at wavelengths shorter than the Lyman limit. Subsequently, light from cosmic objects at wavelengths less than the Lyman limit is exceedingly difficult to detect.