The planet known as Earth's "twin" is Venus. This nickname arises from their similar size, composition, and proximity to the Sun. Both planets have rocky surfaces, but while Earth supports life, Venus has a thick, toxic atmosphere primarily composed of carbon dioxide with clouds of sulfuric acid. The extreme greenhouse effect on Venus makes it the hottest planet in our solar system. These factors highlight the stark differences between the two planets, making Venus a point of intrigue for scientists studying planetary formation and the potential for habitability elsewhere in the universe.