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 innermost and smallest planet
in the solar system
, orbiting the Sun
once every 88 days. It ranges in brightness from about −2.0 to 5.5 in apparent magnitude
, but is not easily seen as its greatest angular separation from the Sun (greatest elongation
) is only 28.3°. It can only be seen in morning or evening twilight
. Comparatively little is known about the planet: the only spacecraft
to approach Mercury was Mariner 10
, which mapped only 40%–45% of the planet’s surface.
Physically, Mercury is similar in appearance to the Moon as it is heavily cratered. It has no natural satellites and no substantial atmosphere. The planet has a large iron core which generates a magnetic field about 0.1% as strong as that of the Earth. Surface temperatures on Mercury range from about 90 to 700 K (−180 to 430°C, −290 to 810°F), with the subsolar point being the hottest and the bottoms of craters near the poles being the coldest.
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).
Schematic of the hypothetical Oort cloud, with a spherical outer cloud and a disc-shaped inner cloud
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).
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
Ceres – map of gravity fields: red is high; blue, low.
The geology of 2014 MU69
("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 MU69
are believed in turn to have formed protoplanets
Beyond the heliosphere is the interstellar medium, consisting of various clouds of gases. The Solar System currently moves through the Local Interstellar Cloud.
Andreas Cellarius's illustration of the Copernican system, from the Harmonia Macrocosmica (1660)
Did you know...
- ...that the passing of the Great Comet of 1577 (pictured) caused almost century-long debate, during which Galileo argued that comets were merely optical illusions?
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