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.
Selected article -
(from the Latin name Ūranus
for the Greek god Οὐρανός
) is the seventh planet
from the Sun
. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System
. Uranus is similar in composition to Neptune
, and both have bulk chemical compositions which differ from that of the larger gas giants Jupiter
. For this reason, scientists often classify Uranus and Neptune as "ice giants
" to distinguish them from the gas giants. Uranus' atmosphere
is similar to Jupiter's and Saturn's in its primary composition of hydrogen
, but it contains more "ices
" such as water, ammonia
, and methane
, along with traces of other hydrocarbons
. It has the coldest planetary atmosphere in the Solar System, with a minimum temperature of 49 K (−224 °C; −371 °F), and has a complex, layered cloud
structure with water thought to make up the lowest clouds and methane the uppermost layer of clouds. The interior of Uranus is mainly composed of ices and rock.
Like the other giant planets, Uranus has a ring system, a magnetosphere, and numerous moons. The Uranian system has a unique configuration because its axis of rotation is tilted sideways, nearly into the plane of its solar orbit. Its north and south poles, therefore, lie where most other planets have their equators. In 1986, images from Voyager 2 showed Uranus as an almost featureless planet in visible light, without the cloud bands or storms associated with the other giant planets. Observations from Earth have shown seasonal change and increased weather activity as Uranus approached its equinox in 2007. Wind speeds can reach 250 metres per second (900 km/h; 560 mph).
Selected picture -
A TRACE image of sunspots on the surface, or photosphere, of the sun from September 2002, is taken in the far ultraviolet on a relatively quiet day for solar activity. However, the image still shows a large sunspot group visible as a bright area near the horizon. Although sunspots are relatively cool regions on the surface of the sun, the bright glowing gas flowing around the sunspots have a temperature of over one million °C (1.8 million °F). The high temperatures are thought to be related to the rapidly changing magnetic field loops that channel solar plasma.
Andreas Cellarius's illustration of the Copernican system, from the Harmonia Macrocosmica (1660)
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.
Location of the Solar System within the Milky Way
Beyond the heliosphere is the interstellar medium, consisting of various clouds of gases. The Solar System currently moves through the Local Interstellar Cloud.
Ceres – map of gravity fields: red is high; blue, low.
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 6 July 2018 (aphelion) and ending 3 January 2019 (perihelion).
The Ring nebula, a planetary nebula similar to what the Sun will become
Relative size of the Sun as it is now (inset) compared to its estimated future size as a red giant
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
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
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
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 21 January 2023 (Jovian perihelion) and ending 2 December 2034 (Jovian perihelion).
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.
In the news
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?
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