Earth is the third planet from the Sun and the only known astronomical object to support life. It has a diverse environment, consisting of land, water, and atmosphere, which sustains various ecosystems. Earth is approximately 4.5 billion years old and has a diameter of about 12,742 kilometers (7,918 miles). Its surface is 71% water, primarily in the form of oceans, and it has a variety of climates and geographical features, including mountains, valleys, and deserts. The planet's atmosphere is composed mainly of nitrogen and oxygen, which are essential for life.