The Solar System Portal
and planets of the solar system (distances not to scale)
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, the 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, some objects are large enough to have rounded under their own gravity, though there is considerable debate as to how many there will prove to be.
Such objects are categorized as dwarf planets. Identified or accepted 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, the six largest possible 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.
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is the eighth and farthest planet
from the Sun
in the Solar System
. It is the fourth largest planet by diameter, and the third largest by mass. The planet is named after the Roman god of the sea
. Discovered on September 23, 1846, Neptune was the first planet found by mathematical prediction rather than regular observation. Unexpected changes in the orbit of Uranus led astronomers to deduce the gravitational perturbation
of an unknown planet. Neptune was found within a degree of the predicted position. The moon Triton
was found shortly thereafter, but none of the planet's other 12 moons
were discovered before the 20th century. Neptune has been visited by only one spacecraft, Voyager 2
, which flew by the planet on August 25, 1989. Neptune is similar in composition to Uranus
, and both have different compositions from those of the larger gas giants Jupiter
. Traces of methane in the atmosphere
, in part, account for the planet's blue
appearance. At the time of the 1989 Voyager 2 flyby
, its southern hemisphere possessed a Great Dark Spot
comparable to the Great Red Spot
. Neptune has a faint and fragmented ring system
, which may have been detected during the 1960s but was only indisputably confirmed by Voyager 2
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"Earthrise," the first occasion in which humans saw the Earth seemingly rising above the surface of the Moon, taken during the Apollo 8 mission on December 24, 1968. This view was seen by the crew at the beginning of its fourth orbit around the Moon, although the very first photograph taken was in black-and-white. Note that the Earth is in shadow here. A photo of a fully lit Earth would not be taken until the Apollo 17 mission.
Relative size of the Sun as it is now (inset) compared to its estimated future size as a red giant
The geology of the contact binary
(nicknamed Ultima Thule
), the first undisturbed planetesimal
visited by a spacecraft, with comet 67P
to scale. Notable surface features are highlighted at right. The eight subunits of the larger lobe, labeled ma
, are thought to have been its building blocks. The two lobes came together later, forming a contact binary
. Objects such as Arrokoth are believed in turn to have formed protoplanets
Andreas Cellarius's illustration of the Copernican system, from the Harmonia Macrocosmica (1660)
Orrery showing the motions of the inner four planets. The small spheres represent the position of each planet on every Julian day, beginning 6 July 2018 (aphelion) and ending 3 January 2019 (perihelion).
Location of the Solar System within the Milky Way
The Ring nebula, a planetary nebula similar to what the Sun will become
Neptune and its moon Triton, taken by Voyager 2. Triton's orbit will eventually take it within Neptune's Roche limit, tearing it apart and possibly forming a new ring system.
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
Hubble image of protoplanetary discs in the Orion Nebula, a light-years-wide "stellar nursery" probably very similar to the primordial nebula from which the Sun formed
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 21 January 2023 (Jovian perihelion) and ending 2 December 2034 (Jovian perihelion).
Simulation showing outer planets and Kuiper belt:
a) Before Jupiter/Saturn 2:1 resonance
b) Scattering of Kuiper belt objects into the Solar System after the orbital shift of Neptune
c) After ejection of Kuiper belt bodies by Jupiter
Orbit of Jupiter
Orbit of Saturn
Orbit of Uranus
Orbit of Neptune
Meteor Crater in Arizona. Created 50,000 years ago by an impactor about 50 metres (160 ft) across, it shows that the accretion of the Solar System is not over.
Ceres - map of gravity fields: red is high; blue, low.
Schematic of the hypothetical , with a spherical outer cloud and a disc-shaped inner cloud
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