Inner Solar System
The solar system formed around 4.6 billion years ago when gravity caused a huge molecular cloud to collapse. As the cloud collapsed it began to spin, resulting in a compressed sphere of material at the center of a wide, flat, rotating disk. Eventually, the heat and pressure at the core of the disk grew to the point where hydrogen could be fused into helium, and our Sun was born. Meanwhile the material in the disk also accreted to form the planets, moons, asteroids, comets, and everything else that orbits the Sun.
The Sun is by far the most massive object in the solar system, with over seven hundred times the amount of matter of everything else combined. Indeed our star, sometimes called “Sol”, is how the Solar system takes its name.
Huddled closest to the warmth of the Sun, there are four relatively small planets composed mostly of metal and rock: Mercury, Venus, Earth and Mars. Beyond this lies a ring of rocky fragments called asteroids which, contrary to science fiction, are separated from each-other by the vastness of space, with perhaps a million kilometers between them.
Outer Solar System
At around this distance, an imaginary barrier called the frost line divides the solar system in two. Beyond this point, heat from the Sun has diminished to the point where many volatile compounds freeze, resulting in a vast increase in available planet-building material in the early outer solar system.
Here beyond the asteroid belt, we find the giant planets. The first two of these, Jupiter and Saturn, are the most massive planets in the Solar system. The vast majority of their mass is comprised of their enormous hydrogen and helium atmospheres, and this gives rise to their name, the Gas Giants.
Still farther out we come across Uranus and Neptune. Here, the Sun appears as little more than a bright star, and their distance from Earth meant that unlike the other six planets, their discovery had to wait until the advent of the telescope. The frigid temperature at this distance allows these planets to hold on to a larger proportion of volatile compounds in their atmospheres, giving rise to their name, Ice Giants.
Minor Solar System Bodies
Although Neptune is the outermost planet, the dominion of the Sun extends well beyond the ice giants. A disk of icy bodies known as the Kuiper Belt extends into the darkness, and is home to the dwarf planets such as Pluto, Haumea and Makemake. Due to the difficulty in detecting such objects, it is thought that there may be many more lurking in the darkness, yet to be detected.
Further still lies the Oort Cloud, a hypothesized spherical shell of small, icy bodies that may extend up to one light year from the Sun. It is thought that the gravitational influence of passing stars may disturb some of these objects, causing them to fall inwards towards the Sun, becoming long-period comets.
Although the Sun‘s gravity extends infinitely, the Oort Cloud probably marks the boundary of its effective hold on other bodies. Beyond this point, we enter the interstellar space of the Milky Way Galaxy.
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