Earthlike

Earthlike, often written Earth-like, is a qualitative descriptor used in astronomy and astrobiology to indicate planets or other bodies that resemble Earth in certain key features. The term is not a formal scientific category but a heuristic that encompasses a range of criteria rather than a strict threshold. In common usage, Earthlike implies at least a rocky (silicate) composition, a solid surface or surface-like conditions, and a size and mass not far from Earth, often with a location within the star’s habitable zone where liquid water could potentially exist on the surface.

Assessing Earthlikeness involves measurements of radius and mass to infer density, and, when possible, atmospheric composition through spectroscopy. An Earthlike planet may be smaller or larger than Earth, but remains within the general regime of terrestrial worlds. However, many rocky exoplanets differ in atmospheric composition, surface conditions, or other factors, so an Earthlike designation is often provisional. True habitability depends on multiple factors, including atmospheric pressure, greenhouse effects, volatile inventory, magnetic field, and tectonic activity; a planet might be rocky and in the habitable zone yet lack liquid water or a stable climate.

In scientific contexts, more precise terms such as Earth analog or Earth twin are sometimes preferred, whereas Earthlike is commonly used in media and broader discussions. The search for Earthlike worlds is part of the broader effort to identify potentially habitable exoplanets through observations that determine size, mass, orbit, and atmospheric traits.