Stephen R. Kane and colleagues explore why Earth supports life while Venus and Mars do not by examining their atmospheres, geology, and solar influences. Earth’s stability stems from processes that balanced its climate, supporting liquid water and life. Venus, with a runaway greenhouse effect, and Mars, which lost its atmosphere, exemplify extreme planetary conditions. Their findings offer insights into the “habitable zone” and guide the search for life on exoplanets using Venus, Earth, and Mars as models of diverse evolutionary paths.