According to our current understanding, the Universe is about 13.7 billion years old. However, the sun, planets, moons, and other bodies that make up our solar system are significantly younger, about 4.55 billion years old.
According to the Nebular Hypothesis, a cloud of interstellar dust and gas, or nebula, formed when a previous star exploded. As the nebula floated through space in one of the spiral arms of the Milky Way galaxy, a nearby supernova explosion blasted new material and a lot of energy into the cloud. As a result, the cloud began to collapse in on itself, causing most of the material to become condensed in the middle of the cloud to form a new star, our sun. The remaining gas and dust formed a disk around the new star. Over a fairly short time, matter in each of several concentric zones within this disk came together through electrostatic and gravitational attraction to form larger and larger bodies that eventually became the planets.
There are four terrestrial (rocky) planets in the inner part of our solar system, Mercury, Venus, Earth, and Mars. Venus and Earth are about the same size, and Mars and Mercury are significantly smaller. They all formed in the same way. All the rocky planets would have been very hot for a long time, with many active volcanoes. They melted deeply enough to form planetary cores made of iron and to give off gases to form atmospheres. But their similarity ended there, and each inner planet had its own later history. Farther out, the gas giants (or Jovian planets) Jupiter, Saturn, Uranus, and Neptune formed from large quantities of gas that were blown outward by radiation pressure from the sun. Even farther out, icy bodies such as Pluto formed in the coldest regions of the solar system.
Meteorites, most of which reside in the Asteroid Belt between Mars and Jupiter, are leftovers from the process of planetary formation. Because these meteorites never became part of larger planets, they preserve information about the chemical composition and age of the original solar nebula. As such, meteorites have unparalleled scientific importance; they give us key information about the formation of the solar system that can be found nowhere else!
To see this first-hand evidence of the formation of the early solar system, come see the extensive meteorite display at the Limper Geology Museum!