The Solar System is the gravitationally bound 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. Of the objects that orbit the Sun indirectly—the moons—two are larger than the smallest planet, Mercury.
The Solar System 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, 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.
The Solar System also contains smaller objects. The asteroid belt, which lies between the orbits of Mars and Jupiter, mostly contains objects composed, like the terrestrial planets, of rock and metal. Beyond Neptune's orbit lie the Kuiper belt and scattered disc, which are populations of trans-Neptunian objects composed mostly of ices, and beyond them a newly discovered population of sednoids. Within these populations are several dozen to possibly tens of thousands of objects large enough that they have been rounded by their own gravity. Such objects are categorized as dwarf planets. Identified dwarf planets include the asteroid Ceres and the trans-Neptunian objects Pluto and Eris. In addition to these two regions, various other small-body populations, including comets, centaurs and interplanetary dust clouds, freely travel between regions. Six of the planets, at least four of the dwarf planets, and many of the smaller bodies are orbited by natural satellites, usually termed "moons" after the Moon. Each of the outer planets is encircled by planetary rings of dust and other small objects.
The solar wind, a stream of charged particles flowing outwards from the Sun, creates a bubble-like region in the interstellar medium known as the heliosphere. The heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the interstellar medium; it extends out to the edge of the scattered disc. The Oort cloud, which is thought to be the source for long-period comets, may also exist at a distance roughly a thousand times further than the heliosphere. The Solar System is located in the Orion Arm, 26,000 light-years from the center of the Milky Way galaxy.
is the seventh planet
from the Sun
; it is the third largest and fourth most massive planet in the solar system
. Uranus was the first planet discovered in modern times. Though it is visible to the naked eye like the five classical planets
, it was never recognised as a planet by ancient observers due to its dimness. Sir William Herschel
announced its discovery on March 13, 1781, expanding the known boundaries of the solar system. Uranus' atmosphere, although similar to Jupiter
in being composed primarily of hydrogen
, contains a higher proportion of "ices" such as water
, along with the usual traces of hydrocarbons
. It has the coldest planetary atmosphere in the solar system, with a minimum temperature of 49 K
, and has a complex layered cloud
structure in which water is thought to make up the lowest clouds, while methane makes up the uppermost layer of clouds. In 1986, images from the Voyager 2 space probe
showed Uranus as a virtually featureless planet in visible light without the cloud bands or storms
associated with the other giants. The wind
speeds on Uranus can reach 250 m/s (560 mph).
Ceres – map of gravity fields: red is high; blue, low.
Orrery showing the motions of the inner four planets. The small spheres represent the position of each planet on every Julian day, beginning July 6 2018 (aphelion) and ending January 3 2019 (perihelion).
Schematic of the hypothetical Oort cloud, with a spherical outer cloud and a disc-shaped inner cloud
Beyond the heliosphere is the interstellar medium, consisting of various clouds of gases. The Solar System currently moves through the Local Interstellar Cloud.
Zooming out the Solar System:
- inner Solar System and Jupiter
- outer Solar System and Pluto
- orbit of Sedna (detached object)
- inner part of the Oort Cloud
Orrery showing the motions of the outer four planets. The small spheres represent the position of each planet on every 100 Julian days, beginning January 21 2023 (Jovian perihelion) and ending December 2 2034 (Jovian perihelion).
Andreas Cellarius's illustration of the Copernican system, from the Harmonia Macrocosmica (1660)
Did you know...
- ...that although NASA originally thought that there was only one scalloped margin dome on the planet Venus (pictured), they have since discovered hundreds of them?
In the news
Things you can do