Earthtwins

Earthtwins is a term used in astronomy and science fiction to describe Earth-like planets that exist as twin worlds within a single star system. In its broad sense, it refers to exoplanets with similar size, mass, and rocky composition to Earth, often imagined as appearing in the same system and potentially sharing comparable surface conditions. The concept frequently envisions two such planets in related orbits, which may be near each other in distance from their star or even arranged in a co-orbital or resonant configuration.

In a scientific context, Earthtwins typically denotes planets with radii close to Earth’s and masses within a similar range, located in or near the star’s habitable zone where liquid water could persist under suitable atmospheric conditions. The emphasis is on similarity to Earth's fundamental properties, while recognizing that atmospheric composition, tectonics, and surface environments could differ markedly between the two worlds. Observationally, identifying Earthtwin candidates relies on transit photometry and radial-velocity measurements, with atmospheric characterization possible through transmission spectroscopy, reflected light, and thermal emission studies as techniques advance.

In culture and worldbuilding, Earthtwins appear in speculative fiction and theoretical discussions as devices to explore parallel biospheres, evolutionary trajectories, and the implications of two similar worlds facing analogous climatic challenges.

Note: Earthtwins is not a formal astronomical category, and no confirmed pair of Earth-twin planets has been established. It remains a descriptive label used in research summaries and fictional contexts.