Venus is called Earth's "Twin" because Earth and Venus have almost the same mass, size, similar composition and they are neighboring planets. Some of most notable differences though between the two planets are their color, temperature, Earth's ability to support life and their atmosphere. Venus' atmosphere is about 100x thicker than Earth's, Earth can support life while Venus can not because Earth has water and plant life and Earth is a bright blue and green while Venus is more orange and red.
