Earths

Earth is the third planet from the Sun and the only known celestial body to harbor life. In formal writing, the planet is typically referred to by the singular proper name Earth rather than the plural. The form Earths appears mainly when discussing multiple Earth-like worlds or fictional variations of Earth in speculative or literary contexts.

Earth-like planets, or Earth-twins, are rocky worlds with sizes similar to Earth and, in some cases, atmospheres that could allow liquid water. In exoplanet research, “Earth-sized” generally denotes a radius roughly 0.8 to 1.25 times that of Earth and a mass compatible with a rocky composition. Many such planets have been detected, including those orbiting nearby stars, with the habitability of a given world depending on atmosphere, water presence, and stellar radiation. Common discovery methods include transit photometry and radial velocity, with confirmation requiring multiple lines of evidence.

The plural Earths is used when referring to multiple worlds that resemble Earth or to fictional Earth-like worlds in alternate histories or universes. It is not a distinct scientific category, but rather a descriptive term for a class of planets either observed beyond the Solar System or imagined in fiction. In scientific discourse, attention remains on the specific properties of each world rather than on a generalized plural.