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 fifth planet
from the Sun
and the largest
planet within the solar system
. It is two and a half times as massive as all of the other planets in our solar system combined. Jupiter, along with Saturn
, and Neptune
, is classified as a gas giant
. When viewed from Earth, Jupiter can reach an apparent magnitude
of -2.8, making it the fourth brightest object in the night sky. The planet was known by astronomers
of ancient times and was associated with the mythology and religious beliefs of many cultures. Jupiter is primarily composed of hydrogen
with only a small proportion of helium
; it may also have a rocky core of heavier elements. The outer atmosphere is visibly segregated into several bands at different latitudes
, resulting in turbulence and storms along their interacting boundaries. A prominent result is the Great Red Spot
, a giant storm that is known to have existed since at least the seventeenth century. Surrounding the planet is a faint planetary ring system and a powerful magnetosphere
. There are also at least 63 moons, including the four large moons called the Galilean moons
that were first discovered by Galileo Galilei
in 1610. Jupiter has been explored on several occasions by robotic spacecraft, most notably during the early Pioneer
fly-by missions and later by the Galileo orbiter
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).
Ceres – map of gravity fields: red is high; blue, low.
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
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.
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 troilite, a form of pyrrhotite, is extremely rare on Earth but is abundant on Mars (pictured)?
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