The Solar System is our neighborhood in space. Itís made up of the Sun and everything that orbits it. Here is a quick tour.
Our star system is named for the Sun, which is sol in Latin. The Sun formed 4.6 billion years from the collapse of part of a giant cloud of gas and dust. It contains 99.8% of the mass of the Solar System, so everything else in the system was made from the tiny bit left over. Since the force of gravity is dependent on mass, you can see why itís the Sunís gravity that holds the system together.
In the Sunís core mass is converted into energy in thermonuclear reactions. This is what provides the energy we receive as light and heat.
Planets, dwarf planets and moons
The definition of a planet was established by the International Astronomical Union in 2006. Itís a body that orbits the Sun, has enough mass to collapse into a nearly round shape, and has cleared the region around its orbit.
Dwarf planets are similar to planets, but they havenít cleared their orbits. So asteroids and Kuiper Belt objects such as Pluto arenít considered planets.
Moons are bodies that orbit planets or dwarf planets.
Inner Solar System
The inner Solar System contains four planets Ė Mercury, Venus, Earth and Mars Ė and three moons, i.e., our Moon and two Martian ones. There is one dwarf planet, Ceres, and lots of asteroids. Most of the asteroids are in the asteroid belt between Mars and Jupiter, but you can also find them in other parts of the Solar System.
Inner Solar System objects are small rocky bodies. The Earth is the largest of them and the asteroids are the smallest. When the Solar System was forming, Jupiterís gravity disrupted the formation of a planet where the asteroid belt is, so these rocks are examples of material from the early Solar System.
The habitable zone of a star system is the region around the star where water could be liquid on the surface. Earth is, of course, in the habitable zone of the Solar System, but so is Mars. Mars is smaller and colder than Earth and doesnít have liquid water on its surface, though it probably did at some time in the past.
The inner Solar System is a very small part of the whole, as you can see in this diagram of the Solar System. Yet the distances are still large. Earth is 150 million km (93 million miles) from the Sun and the asteroid belt is two to four times as far away. For convenience, astronomers use astronomical units (AU) for Solar System distances - 1 AU is the average distance from the Earth to the Sun.
Outer Solar System: planets and moons
The outer planets are giant planets, Jupiter being by far the most massive. It has two and a half times the mass of all the other planets put together. Itís also very big. You could fit 765 Earths into a Saturn-sized hollow, but nearly twice that number would fit in a hollow Jupiter. Jupiter and Saturn are known as gas giants, because they are primarily made of hydrogen and helium.
Uranus and Neptune may be grouped with the gas giants, but they are also called ice giants because although they contain a substantial amount of hydrogen, the bulk of each planet consists of heavier elements in the form of ices. Although much smaller than Jupiter and Saturn, the ice giants are still big Ė each has a volume equal to about sixty Earths.
All the giants have ring systems, varying from Saturnís spectacular ones to the dim, dusty Jovian rings. They also have large numbers of moons. As of January 2013, Jupiter had 67 known moons, Saturn 62, Uranus 27 and Neptune had 13. Two of these moons, Jupiterís Ganymede and Saturnís Titan, are bigger than Mercury.
Outer Solar System: Kuiper Belt and scattered disk
Neptune lies at about 30 AU from the Sun. Thatís thirty times our distance - the Sun would just look like a particularly bright star from there. Beyond Neptuneís orbit the Kuiper Belt begins. Itís much bigger than the asteroid belt, for it stretches from 30 AU to at least 50 AU. Rather than rocky remnants of the early Solar System, Kuiper Belt objects are icy leftovers from that early time.
The region known as the classical Kuiper Belt is at 42-48 AU. Two-thirds of the Kuiper Belt objects discovered so far were found there. These objects are in stable orbits, unlike the objects of the scattered disk. Scattered-disk objects (SDOs) orbit at very steep angles to the ecliptic in highly elongated orbits. Eris, discovered in 2005 is an SDO. On this diagram you can see the orbits of Pluto and Eris. Erisís orbit is at angle of more than forty degrees to the ecliptic. The classical KBOs are shown as white dots.
There is no agreement yet about whether the scattered disk is part of the Kuiper Belt or a separate region.
The Oort Cloud
Astronomers think there is a sphere of icy debris surrounding the Solar System at a great distance, tens of thousands of AU. The Sunís gravity is weak there and other stars can affect the comet-like objects, sometimes kicking them into an orbit that takes them into the inner Solar System. Although some comets originate in the Kuiper Belt or scattered disk, the Oort Cloud is probably the main source of comets.
The outer edge of the Oort Cloud is the ultimate boundary to the Solar System, for beyond this the Sunís gravity no longer dominates.