Our Solar System: Facts, Formation and Discovery

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 and Neptune. 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.
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.
Formation
Many 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 Region
Astronomers 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.