What Is The Solar System?The Solar System is made up of all the planets that orbit our Sun. In addition to planets, the Solar System also consists of moons, comets, asteroids, minor planets, and dust and gas.

Everything in the Solar System orbits or revolves around the Sun. The Sun contains around 98% of all the material in the Solar System. The larger an object is, the more gravity it has. Because the Sun is so large, its powerful gravity attracts all the other objects in the Solar System towards it. At the same time, these objects, which are moving very rapidly, try to fly away from the Sun, outward into the emptiness of outer space. The result of the planets trying to fly away, at the same time that the Sun is trying to pull them inward is that they become trapped half-way in between. Balanced between flying towards the Sun, and escaping into space, they spend eternity orbiting around their parent star.

Solar System – OverviewStructure

At the heart of the solar system is our sun. The four planets nearest it are rocky, terrestrial worlds— Mercury, Venus, Earth and Mars. After that are four gas giants — Jupiter, Saturn, Uranus andNeptune. Between the orbits of Mars and Jupiter lies the asteroid belt, which includes the dwarf planet Ceres. Beyond the orbit of Neptune one finds the disk-shaped Kuiper belt, in which dwarf planet Pluto resides, and far beyond that is the giant, spherical Oort Cloud and the teardrop-shaped heliopause. See the structure of the solar system (infographic).

Our solar system is a vast place, with lots of mostly empty space between planets. But out there are comets, asteroids and more rocky, frozen objects (including dwarf planets) yet to be discovered in the Kuiper Belt and Oort Cloud.CREDIT: NASA

Discovery

For millennia, astronomers have followed points of light that seemed to move among the stars. The ancient

Greeks named these planets, meaning wanderers. Mercury, Venus, Mars, Jupiter and Saturn were known in antiquity, and the invention of the telescope added the asteroid belt, Uranus and Neptune, Pluto and many of these worlds' moons. The dawn of the space age saw dozens of probes launched to explore our system, an adventure that continues today. The discovery of Eris kicked off a rash of new discoveries of dwarf planets, and more than 100 could remain to be found.

Sun

The sun is by far the largest object in our solar system, containing 99.8 percent of the solar system's mass. It sheds most of the heat and light that makes life possible on Earth and possibly elsewhere. Planets orbit the sun in oval-shaped paths called ellipses, and the sun is slightly off to the side of the center of each ellipse.

Inner Solar System Inner Planets - The inner four

planets — Mercury, Venus, Earth and Mars — are made up mostly of iron and rock. They are known as terrestrial or earthlike planets because of their similar size and composition.

Asteroid belt - Asteroids are minor planets, most of which circle the sun in a region known as the asteroid belt, between the orbits of Mars and Jupiter. Scientists estimate that there are more than 750,000 asteroids in the belt with diameters larger than three-fifths of a mile (1 kilometer), and there are millions of smaller asteroids. A number have orbits that take them closer into the solar system that sometimes lead them to collide with Earth or the other inner planets. 

Outer Solar System Outer Planets - The outer planets

are giant worlds with thick outer layers of gas. Nearly all their mass is made up of  hydrogen and helium, giving them compositions like that of the sun. Beneath these outer layers, they have no solid surfaces — the pressure from their thick atmospheres liquefy their insides, although they might have rocky cores. Rings of dust, rock, and ice encircle all these giants, with Saturn's being the most famous.

Comets - Comets are often known as dirty snowballs, and consist mainly of ice and rock. When a comet's orbit takes it close to the sun, some of the ice in its central nucleus turns into gas that

shoots out of the comet's sunlit side, which the solar wind carries outward to form into a long tail. Short-period comets that complete their orbits in less than 200 years are thought to originate from the the disk-shaped Kuiper belt, while long-period comets that take more than 200 years to return are thought to come from the spherical Oort cloud. 

FormationMany scientists think our solar system formed from a giant, rotating cloud of gas and dust known as the solar nebula. As the nebula collapsed because of its gravity, it spun faster and flattened into a disk. Most of the material was pulled toward the center to form the sun. Other particles within the disk collided and stuck together to form asteroid-sized objects named as planetesimals, some of which combined to become the asteroids, comets, moons and planets. The solar wind from the sun was so powerful that it swept away most of the lighter elements, such as hydrogen and helium, from the innermost planets, leaving behind mostly small, rocky worlds. The solar wind was much weaker in the outer regions, however, resulting in gas giants made up mostly of hydrogen and helium.

Trans-Neptunian RegionAstronomers had long suspected that a band of icy material known as the Kuiper belt existed past the orbit of Neptune extending from about 30 to 55 times the distance of Earth to the sun, and from the last decade of the 20th century up to now, they have found more than a thousand of such objects. Scientists estimate the Kuiper belt is likely home to hundreds of thousands of icy bodies larger than 60 miles (100 km) wide, as well as an estimated trillion or more comets.

Well past the Kuiper belt is the Oort cloud, which theoretically extends from 5,000 to 100,000 times the distance of Earth to the sun, and is home to up to two trillion icy bodies. Past that is the very edge of the solar system, the heliosphere, a vast, teardrop-shaped region of space containing electrically charged particles given off by the sun. Many astronomers think that the limit of the heliosphere, known as the heliopause, is about 9 billion miles (15 billion kilometers) from the sun.

Pluto is now considered a dwarf planet dwelling in the Kuiper belt. It is not alone — recent additions include Makemake, Haumea and Eris. Another object dubbed Sedna, which is about three-fourths the size of Pluto, might be the first dwarf planet discovered in the Oort cloud.