The Earth is a relatively small celestial object. It is the third planet orbiting a medium star, our Sun, located in Orion's arm, sometimes called the Orion Spur; is a minor arm located between the Sagittarius arm and the Perseus arm of the Milky Way, a barred spiral galaxy. Our solar system orbits the central bulge of our galaxy with a radius of 1.7 billion AU, and our galactic revolution period is 230 million years at a tangential velocity of 828,000 km/h about the center. Our solar system has a main sequence star, 4 inner rocky planets, the asteroid belt, 4 large outer gaseous planets, the Kuiper Belt of small icy objects, and the outermost Oort cloud of icy objects composed of water, ammonia and methane. It extends from approximately 2000-5000 AU to an outer distance of up to 100,000 AU. The Earth completes one rotation every 23 hours and 56 minutes. and completes 365.25 rotations (days) during one revolution around the Sun, or 1 Earth year. The Earth's average orbital speed is 29.8 km/s and its rotation speed is approximately 460 m/s. The average radius of Earth's solar orbit is 149,587,817 km, or about 1 AU. About 1,320 Earths would fill the volume of Jupiter and about 1,300,000 would fill that of the Sun. It has a density of 5.5 g/cm3, similar to that of the other rocky inner planets composed of heavy elements, but much greater than that of the outer planets which are mainly made up of hydrogen and helium. Its mass is 5.97×1024 kg. It has an equatorial circumference slightly larger than the polar one, this is due to the rotation of the Earth on its axis and the gravitational attraction of the Sun during the initial phase of the Earth's fusion. Over 4.6 billion years ago, a large molecular cloud located within a branch of the Milky Way. it had been the site of the formation... half of the paper... and the debris orbiting the Earth also accumulated on the Moon. As the number of planetesimal collisions decreased, the Earth began to cool. A solid crust had formed about 3.8 billion years ago. The crust thickened to form a large landmass called Pangea. Subsequently, the crustal plates began to move due to the convective mantle. As they separated, a new, thin ocean floor formed. Earth's atmosphere is formed by outgassing, or the release of gas trapped inside the planet by volcanic activity. Earth's oceans formed when water vapor in the atmosphere condensed and fell as rain to form the oceans. Light elements such as He and H escaped into space, leaving behind an initial atmosphere of volcanic compounds such as NH3, CH4, N and S compounds, water and carbon dioxide. About 3.5 billion years ago, the Earth's surface was destined for the origin of life.