People once believed that Earth was the center of the solar system. Scientists finally learned that Earth is one of at least eight planets in our solar system and that they all travel around the sun. Astronomers now know that our solar system is almost 5 billion years old and is part of at least 100 billion galaxies in the universe. Earth is the only planet known to have life and water. Scientists continue to discover new planet-like objects, called dwarf planets, all the time!