Habitable-Worlds-Observatory

Directly imaging an exoplanet — seeing the planet itself rather than inferring it from its effect on the star — is the hardest challenge in observational astronomy. A planet is 10⁹ times fainter than its star at visible wavelengths, squeezed within 1 arcsecond of it. Yet direct imaging has been achieved for dozens of young gas giants. The HR 8799 system, with four imaged planets, was the first multi-planet system directly resolved. The ELT's next-generation coronagraphs aim to directly image Earth-like planets.

Astronomers have found thousands of exoplanets. An Earth-sized planet in a habitable zone isn't exotic anymore. But a true Earth twin — similar size, similar temperature, liquid water, an oxygen atmosphere — would be something else entirely. Here's what would happen if we found one.