The Earth is the third planet in distance from the Sun, the densest and the fifth in size of the eight planets in the Solar System. It is also the largest of the four terrestrial planets in the Solar System. It has only one satellite, the Moon. Sometimes it is called the world or the blue planet. They inhabit millions of species, including humans; and it is the only star where it is known that there is life. The planet was formed approximately 4.6 billion years ago, and life appeared after 1 billion years. Earth's biosphere has significantly altered the atmosphere and other abiotic conditions, allowing the proliferation of aerobic organisms and the formation of the ozone layer, which, along with the Earth's magnetic field, blocks harmful solar radiation. The physical properties of the Earth, as well as its geological history and orbit, have allowed life to persist during this period. The planet is expected to have life for at least another 500 million years. Some astronomers have contributed to know its situation in the Universe and the Solar System, as Aristarchus, Eratosthenes, Nicolaus Copernicus and Johannes Kepler.
