Venus is the second planet from the sun and one of the four inner planets closest to the sun. While Venus has many similarities to Earth like its size, chemistry, gravity and density, it also has some key differences - its atmosphere is far thicker than Earth's and is comprised of carbon dioxide, its day is longer than its year, and it has no moon or ozone layer. Venus is sometimes called Earth's sister planet due to their similarities, though the runaway greenhouse effect has created extreme atmospheric conditions on Venus's surface.