Earth is the third planet from the Sun, the densest planet in the Solar System in the galaxy the Milky Way, the largest of the Solar System's four terrestrial planets, and the only astronomical object known to harbour life. The Moon is Earth's only permanent natural satellite. Among an estimated 10–14 million extant species on the planet are over 7.3 billion humans and about 500,000 great apes. Humans became Earth's dominant species in the era after the extinction of dinosaurs.