In astronomy and astrobiology, the habitable zone (HZ), or more precisely the circumstellar habitable zone (CHZ), is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure. The bounds of the HZ are based on Earth's position in the Solar System and the amount of radiant energy it receives from the Sun. Due to the importance of liquid water to Earth's biosphere, the nature of the HZ and the objects within it may be instrumental in determining the scope and distribution of planets capable of supporting Earth-like extraterrestrial life and intelligence.
A diagram depicting the habitable zone boundaries around stars, and how the boundaries are affected by star type. This plot includes Solar System planets (Venus, Earth, and Mars) as well as especially significant exoplanets such as TRAPPIST-1d, Kepler-186f, and our nearest neighbor Proxima Centauri b.
Natural shielding against space weather, such as the magnetosphere depicted in this artistic rendition, may be required for planets to sustain surface water for prolonged periods.
Earth's hydrosphere. Water covers 71% of Earth's surface, with the global ocean accounting for 97.3% of the water distribution on Earth.
Artist's concept of a planet on an eccentric orbit that passes through the HZ for only part of its orbit
A planetary surface is where the solid or liquid material of certain types of astronomical objects contacts the atmosphere or outer space. Planetary surfaces are found on solid objects of planetary mass, including terrestrial planets, dwarf planets, natural satellites, planetesimals and many other small Solar System bodies (SSSBs). The study of planetary surfaces is a field of planetary geology known as surface geology, but also a focus on a number of fields including planetary cartography, topography, geomorphology, atmospheric sciences, and astronomy. Land is the term given to non-liquid planetary surfaces. The term landing is used to describe the collision of an object with a planetary surface and is usually at a velocity in which the object can remain intact and remain attached.
Apollo 11 astronaut Buzz Aldrin walking on the surface of the Moon, which consists of lunar regolith (photographed by Neil Armstrong, July 1969).
Perspective radar view of Titan's Bolsena Lacus (lower right) and other northern hemisphere hydrocarbon lakes
Pluto's Tombaugh Regio (photographed by New Horizons flyby on July 14, 2015) appears to exhibit geomorphological features previously thought to be unique to Earth.
Pebbled plains of Saturn's moon Titan (photographed by Huygens probe, January 14, 2005) composed of heavily compressed states of water ice. This is the only ground-based photograph of an outer Solar System planetary surface
