Solar System

Solar System
A true-color image of the Solar System with sizes, but not distances, to scale. The order of the planets are from right to left.
The Sun, planets, moons and dwarf planets[a]
(true color, size to scale, distances not to scale)
Age4.568 billion years
System mass1.0014 solar masses[citation needed]
Nearest star
Nearest known planetary system
Proxima Centauri system (4.2441 ly)
Planetary system
Semi-major axis of outer known planet (Neptune)
30.11 AU
(4.5 bill. km; 2.8 bill. mi)
Distance to Kuiper cliff~50 AU
Stars1 (Sun)
Known planets
Known dwarf planets
Known natural satellites
Known minor planets1,199,224[b][2]
Known comets4,402[b][2]
Identified rounded satellites19
Orbit about Galactic Center
Invariable-to-galactic plane inclination60.19° (ecliptic)
Distance to Galactic Center27,000 ± 1,000 ly
Orbital speed220 km/s; 136 mi/s
Orbital period225–250 myr
Star-related properties
Spectral typeG2V
Frost line≈5 AU[3]
Distance to heliopause≈120 AU
Hill sphere radius≈1–3 ly

The Solar System[c] is the gravitationally bound system of the Sun and the objects that orbit it. It formed 4.6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud. The vast majority (99.86%) of the system's mass is in the Sun, with most of the remaining mass contained in the planet Jupiter. The four inner system planets—Mercury, Venus, Earth and Mars—are terrestrial planets, being composed primarily of rock and metal. The four giant planets of the outer system are substantially larger and more massive than the terrestrials. The two largest, Jupiter and Saturn, are gas giants, being composed mainly of hydrogen and helium; the next two, Uranus and Neptune, are ice giants, being composed mostly of volatile substances with relatively high melting points compared with hydrogen and helium, such as water, ammonia, and methane. All eight planets have nearly circular orbits that lie near the plane of Earth's orbit, called the ecliptic.

There are an unknown number of smaller dwarf planets and innumerable small Solar System bodies orbiting the Sun.[d] Six of the major planets, the six largest possible dwarf planets, and many of the smaller bodies are orbited by natural satellites, commonly called "moons" after Earth's Moon. Two natural satellites, Jupiter's moon Ganymede and Saturn's moon Titan, are larger but not more massive than Mercury, the smallest terrestrial planet, and Jupiter's moon Callisto is nearly as large. Each of the giant planets and some smaller bodies are encircled by planetary rings of ice, dust and moonlets. The asteroid belt, which lies between the orbits of Mars and Jupiter, contains objects composed of rock, metal and ice. Beyond Neptune's orbit lie the Kuiper belt and scattered disc, which are populations of objects composed mostly of ice and rock.

In the outer reaches of the Solar System lies a class of minor planets called detached objects. There is considerable debate as to how many such objects there will prove to be.[9] Some of these objects are large enough to have rounded under their own gravity and thus to be categorized as dwarf planets. Astronomers generally accept about nine objects as dwarf planets: the asteroid Ceres, the Kuiper-belt objects Pluto, Orcus, Haumea, Quaoar, and Makemake, and the scattered-disc objects Gonggong, Eris, and Sedna.[d] Various small-body populations, including comets, centaurs and interplanetary dust clouds, freely travel between the regions of the Solar System.

The solar wind, a stream of charged particles flowing outwards from the Sun, creates a bubble-like region of interplanetary medium 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 26,000 light-years from the center of the Milky Way galaxy in the Orion Arm, which contains most of the visible stars in the night sky. The nearest stars are within the so-called Local Bubble, with the closest, Proxima Centauri, at 4.2441 light-years.

Cite error: There are <ref group=lower-alpha> tags or {{efn}} templates on this page, but the references will not show without a {{reflist|group=lower-alpha}} template or {{notelist}} template (see the help page).

  1. ^ "Solar System Objects". NASA/JPL Solar System Dynamics. Archived from the original on 7 July 2021. Retrieved 2 April 2022.
  2. ^ a b "Latest Published Data". The International Astronomical Union Minor Planet Center. Archived from the original on 5 March 2019. Retrieved 2 April 2022.
  3. ^ Mumma, M. J.; Disanti, M. A.; Dello Russo, N.; Magee-Sauer, K.; Gibb, E.; Novak, R. (2003). "Remote infrared observations of parent volatiles in comets: A window on the early solar system". Advances in Space Research. 31 (12): 2563–2575. Bibcode:2003AdSpR..31.2563M. CiteSeerX doi:10.1016/S0273-1177(03)00578-7.
  4. ^ a b c "Resolutions B5 and B6: 'Definition of a Planet in the Solar System' and 'Pluto'" (PDF). Resolutions adopted at the General Assemblies. International Astronomical Union. 2006. Archived (PDF) from the original on 6 January 2018. Retrieved 9 April 2022.
  5. ^ Cite error: The named reference planetarysociety was invoked but never defined (see the help page).
  6. ^ "Pluto and the Developing Landscape of Our Solar System". International Astronomical Union. Archived from the original on 30 January 2016. Retrieved 9 April 2022.
  7. ^ Ekers, Ron. "IAU Planet Definition Committee". International Astronomical Union. Archived from the original on 3 June 2009. Retrieved 13 October 2008.
  8. ^ "Plutoid chosen as name for Solar System objects like Pluto". Paris: International Astronomical Union. 11 June 2008. Archived from the original on 13 June 2008. Retrieved 11 June 2008.
  9. ^ Grundy, W. M.; Noll, K. S.; Buie, M. W.; Benecchi, S. D.; Ragozzine, D.; Roe, H. G. (December 2018). "The Mutual Orbit, Mass, and Density of Transneptunian Binary Gǃkúnǁʼhòmdímà ((229762) 2007 UK126)" (PDF). Icarus. 334: 30–38. doi:10.1016/j.icarus.2018.12.037. S2CID 126574999. Archived from the original on 7 April 2019.

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