The Solar System is the gravitationally bound planetary system of the Sun and the objects that orbit it either directly or indirectly. Of the objects that orbit the Sun directly the largest are the eight planets with the remainder being smaller objects such as the five dwarf planets and small Solar System bodies.
The Solar System was formed 4.6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud. The vast majority of the system’s mass is in the Sun with the majority of the remaining mass contained in Jupiter. The four smaller inner planets Mercury Venus Earth and Mars are terrestrial planets being primarily composed of rock and metal.
The four outer planets are giant planets being substantially more massive than the terrestrials. The two largest planets Jupiter and Saturn are gas giants being composed mainly of hydrogen and helium. The two outermost planets Uranus and Neptune are ice giants being composed mostly of substances with relatively high melting points compared with hydrogen and helium, called volatiles such as water, ammonia and methane.
All eight planets have almost circular orbits that lie within a nearly flat disc called the ecliptic. Our solar system extends much farther than the eight planets that orbit the Sun. The solar system also includes the Kuiper Belt that lies past Neptune’s orbit. This is a sparsely occupied ring of icy bodies, almost all smaller than the most popular Kuiper Belt Object, dwarf planet Pluto.
And beyond the fringes of the Kuiper belt is the Oort Cloud. This giant spherical shell surrounds our solar system. It has never been directly observed, but its existence is predicted based on mathematical models and observations of comets that likely originate there.
The Oort Cloud is made of icy pieces of space debris the sizes of mountains and sometimes larger, orbiting our Sun as far as 1.6 light years away. This shell of material is thick, extending from 5,000 astronomical units to 100,000 astronomical units. One astronomical unit (or AU) is the distance from the Sun to Earth, or about 93 million miles (150 million kilometers). The Oort Cloud is the boundary of the Sun’s gravitational influence, where orbiting objects can turn around and return closer to our Sun.