Solar system formed in 4.6 billion years. A galaxy is defined as a gravitationally-bound system of stars, stellar remnants, interstellar gas, and dark matter. The solar nebular hyp... Solar system formed in 4.6 billion years. A galaxy is defined as a gravitationally-bound system of stars, stellar remnants, interstellar gas, and dark matter. The solar nebular hypothesis describes the formation of our solar system from a nebula cloud made from dust and gas. It is believed that the sun, planets, moons, and asteroids were formed around 4.5 billion years ago from a nebula. Nebulae form disk-shaped bodies. The protoplanet hypothesis suggests a great cloud of gas and dust of at least 10,000 million kilometers in diameter rotated slowly in space about 5,000 million years ago. As time passed, the cloud shrank under the pull of its own gravitation. Asteroids are made up of rocks and are sometimes referred to as minor planets. The asteroid belt lies between the orbit of Mars and Jupiter. Comets are composed mainly of ice. The Kupiter belt is often called the solar system’s “final frontier.” Inner planets include Mercury, Venus, Earth, and Mars, which are rocky and have a compact surface. The outer planets, known as Jovian planets, include Jupiter, Saturn, Uranus, and Neptune, which are gaseous and much larger compared to Earth. Read more