Solar System

The Sun and planets of the Solar System. Sizes are to scale. Distances not to scale.

The Solar System consists of the Sun and those celestial objects bound to it by gravity: the eight planets and five dwarf planets, their 173 known natural satellites (usually termed "moons"), and billions of small bodies. The small bodies include asteroids, icy Kuiper belt objects, comets, meteoroids, and interplanetary dust.

The charted regions of the Solar System comprise the Sun, four terrestrial inner planets, the asteroid belt, four gas giant outer planets, and finally the Kuiper belt and the scattered disc. The hypothetical Oort cloud may also exist at a distance roughly a thousand times beyond these regions. The solar wind, a flow of plasma from the Sun, permeates the Solar System, creating a bubble in the interstellar medium known as the heliosphere, which extends out to the middle of the scattered disc.


An orbital period is the time it takes one object to travel around another in a complete circuit. The orbital period of a planet around the Sun is also the length of its year. The rotational period of a planet is how long it takes to make a complete turn on its axis.

Most of the planets, moons, and asteroids travel in almost circular orbits in the same direction (west to east) around the Sun. Most orbits also lie close to the plane of Earth's orbit, called the ecliptic. So if you looked at the solar system side-on, you would see most of the orbits are roughly on the same level. Mercury's and Pluto's orbits are not—they orbit at an angle.


The Solar System formed around 4.5 billion years ago from a huge swirling cloud of dust.

Throughout the Milky Way, and other galaxies like it, are gigantic swirling clouds of dust and gas known as nebula. It is within nebula that stars are born. Our star, the Sun, was created in one such nebula.

Something, perhaps the shock wave from an exploding supernova (dying star) triggered dust particles to be drawn together to form a dense spherical cloud. The accumulation of dust set off a chain reaction. As the core of the cloud attracted more dust, its gravitational pull increased. More and more dust was sucked in and the cloud collapsed in on itself. As this happened, the rotation of the cloud increased in speed, as happens when spinning ice skaters pull in their arms. The rotational forces at the equator of the cloud prevented dust along this plane being drawn in, causing the cloud to flatten into a disc spinning around a dense core.

As more and more mass accumulated at the center of the disc, the temperature increased dramatically. Eventually there was enough energy to set off nuclear reactions. Hydrogen atoms fused to form helium, releasing enormous amounts of energy in vigorous bursts. This marked the birth of the Sun, although it would take between one and 10 million more years for it to settle into the main sequence star recognizable today.

The planets, and other extraterrestrial objects such as asteroids, formed in the flat plane of the spinning disc of dust. Electrostatic forces or sticky carbon coatings made dust particles stick together to form clusters, which in turn stuck together to form rocks. Mutual gravity caused these rocks to come together, eventually to form planets. This "coming together" of material is a process known as accretion.

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