How does the transit method reveal an Earth-size planet orbiting a distant star?

The transit method is a powerful technique used to detect extrasolar planets by observing the light from their host stars. When a planet’s orbit is oriented such that it periodically passes directly between its star and a telescope, it creates a transit—a temporary, measurable drop in the star's brightness.12

By monitoring these periodic dips in starlight, astronomers can determine the planet's orbital period and its size relative to the star. When combined with radial velocity data, which measures the star's motion in response to the planet's gravity, scientists can calculate the planet's mass and density. This information is essential for identifying Earth-size worlds, particularly those located within a star's habitable zone, where conditions might support liquid water and potentially life. Missions such as Kepler and the Transiting Exoplanet Survey Satellite (TESS) have utilized this photometry-based approach to discover thousands of planets, including those comparable in size to Earth.123