4 The nebula (a cloud of gas and dust) floated in space, until a shock wave disturbed it. The shock wave caused some of the dust particles to bump into each other and stick together. More and more particles began to clump together, forming bigger and bigger lumps. This process is called accretion.
5 As the nebula begins to collapse, it begins to spin and flatten into a disk. Most of the mass accumulates in the center of the disk. The crush of gravity and heat of impacts raises the temperature of the central mass to form a protosun. The central mass will eventually form the sun, once nuclear fusion begins, and the sun begins to shine.
7 Surrounding the protosun, material continues to spin Surrounding the protosun, material continues to spin. This material is referred to as a protoplanetary disk. It is from this material, that our planets, asteroids and comets will form.
8 The larger the mass of the clump, the greater the gravitational pull it has on other objects, so larger lumps pull in more and more material. Eventually these lumps are large enough, that they will form planetessimals. Planetesimals – lumps of materials that will become protoplanets, which will later become planets.
9 As planetesimals continue to bump into material in their orbit around the protosun, they continue to grow larger through the process of accretion. Like a snowball, they grow larger and have cleared their orbit of smaller particles. Eventually, the planetesimals become large enough to be called protoplanets.
11 Planets closer to the sun are rocky and small with few moons Planets closer to the sun are rocky and small with few moons. Planets further from the sun are larger, made of gas, and have many moons.
12 Pluto is an exception, it is hypothesized that it could be a comet, from further out in the solar system, captured by our sun’s gravity. It has been demoted to dwarf planet status because it hasn’t cleared it’s orbit, and it’s moon is more like a twin planet.
13 Today, the sun shines steadily because of nuclear fusion in its core and the protoplanetary disk has condensed to form only 8 planets, numerous moons, comets, an asteroid belt and various dwarf planets.