Carl Sagan made a number of significant discoveries about Venus and Mars, but the discovery that launched him to fame was his proof that Venus's atmosphere drew its heat from the greenhouse effect. This refers to the atmosphere-trapping hat that boosts the surface temperature of the planet.
While Sagan did make a minor error in his discovery by attributing some of the greenhouse effect on Venus to water vapor when the atmosphere contains almost none, Sagan's discovery was a major component of the research that helped scientists figure out why the temperature of the Earth was climbing.
Another contribution that Sagan made to astronomy was his explanation for the different colors on the Martian surface. Some scientists had speculated that vegetation was the cause, but he suggested that shifting dust in wind storms were causing the changes. When the Mariner spacecraft visited Mars in the 1970s, Sagan's theory was confirmed.
As a result of his discoveries, Sagan became a central figure in the missions of the 1970s that NASA established to explore planets with robots. His team directed Mariner 9's imaging mission to the Martian surface, which yielded more than 7,000 images, and he helped to pick the landing spots for the Viking 1 and 2 missions, the first to make successful landings on Mars.