The sun is at the center of the solar system and is the primary energy source for life on Earth. It is about 109 times the diameter of Earth and has remained stable for the past 4 billion years and is expected to remain stable for another 4 billion years. The moon is one of the largest natural satellites in the solar system and formed approximately 4.5 billion years ago shortly after Earth. The moon's gravity causes ocean tides on Earth. Earth is the third planet from the sun, formed about 4.54 billion years ago, and is the only known planet that supports life. Life first appeared in Earth's oceans and 71% of Earth's surface is covered with water.