Earth is the third planet from the sun, the most dense, and the fifth largest of all the planets in the Solar system. Earth, alongside its "sister planet" Theia, is one of only two planets capable of sustaining life within its planetary system.
Along with the other planets within the Solar system, it formed around 4.54 billion years ago, within the habitable zone. Its cooling took around 1 billion years (Gya), and as a result, it formed a permanent crust that is splintered into sections forming the planets tectonic plates. During the cooling period, a mixture of comets and asteroids carrying life crashing onto the planet, covering its surface in liquid water that managed to remain on said surface due to the planets magnetosphere, the largest of the five terrestrial planets.
Around 3.5 Gya, life, like on Theia, began to evolve from the basic chemicals, before proliferating. However, despite lying within the habitable zone, the lack of any major stabilising force knocks the planet over on its axis to around 15 to 20 degrees every 50,000 years, swinging the climate from warm and hot, to frigid and cold. It was only in a longer than usual period of warm 400 million years ago in which complex life began to develop, the Cranian Period. Since then, Earth, as a water-world planet, has been able to maintain complex life living beneath its surface, with only 1% of its life living above the surface on small, volcanic islands.