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Science shows that Earth formed around 4.5 billion years ago. The organisms that live on Earth have completely changed its air (atmosphere); this is called a biosphere. 71% of Earth's surface is covered in salt water oceans. Earth is the only place in the Solar System where liquid water is known to exist at present. The other 29% is made of rocky land in the shape of continents and islands. Earth interacts with other objects in the Solar System, particularly the Sun and the Moon. The Earth orbits the Sun roughly once every 365.25 days. One spin is called a day and one orbit around the Sun is called a year. This is why there are 365 days in a year, but a leap day added to the calendar once every 4 years.