Scientists believe that earth and the other plants formed

The correct answer is - from solar nebula.

The Earth and the other planets in the Solar System are thought to have formed from solar nebula. This solar nebula is believed to have been a left over from the Sun's formation, being consisted of gas and dust in a disc-shape. The material in this disc-shaped gas and dust started to merge gradually, thus little by little forming bigger and bigger objects, and as the objects were getting bigger they had bigger gravitational pull so more and more material was attracted toward them. Because of the very big gravitational pull of the Sun these objects were not able to wander into the space but were instead kept close to the Sun. Because of the orbiting around the Sun, they all started to take a rounded shape. Some planets as the Earth, Venus, Mars, and Mercury became terrestrial planets, while the likes of Jupiter, Saturn, Uranus, and Neptune became gas giants.