The planets of the solar system are believed to have formed out of a collapsed and spinning cloud of gas and dust. The outer parts of this cloud became a disk surrounding the young Sun as it condensed and ignited. From this cloud and its gas and dust, the various planets formed. The inner solar system was too warm for volitile molecules like water and methane to condense, and so what formed there were relatively small and rocky planets. Farther out, the gravitational effects of Jupiter made it impossible for the protoplanetary objects present to come together, leaving behind the asteroid belt. In the outer solar system, Jupiter and Saturn developed as large gas giants, while Uranus and Neptune captured much less gas and are known are ice giants because their cores are believed to be made mostly of ice. A planet is generally considered to be a relatively large mass of accreted matter in orbit around a star. ... Major features of the solar system (not to scale) The solar system comprises the Sun and the retinue of celestial objects gravitationally bound to it: nine planets and their 158 currently known moons, as well as asteroids, meteoroids, planetoids, comets, and interplanetary dust. ... The Sun is the star at the center of our solar system. ... Adjective Jovian Atmospheric characteristics Atmospheric pressure 70 kPa Hydrogen ~86% Helium ~14% Methane 0. ... Image of the main asteroid belt between the orbits of Mars and Jupiter. ... Adjective Jovian Atmospheric characteristics Atmospheric pressure 70 kPa Hydrogen ~86% Helium ~14% Methane 0. ... Note: This article contains special characters. ... Adjective Uranian Atmospheric characteristics Atmospheric pressure 120 kPa (at the cloud level) Hydrogen 83% Helium 15% Methane 1. ... Adjective Neptunian Atmospheric characteristics Surface pressure ≫100 MPa Hydrogen - H2 80% ±3. ...

The planets were originally believed to have formed in or near the orbits at which we see them now. However, this view has been undergoing radical change during the late 20th century and the beginning of the 21st century. Currently, it is believed that the solar system looked very different after its initial formation, with five objects at least as massive as Mercury being present in the inner solar system (instead of the current four), the outer solar system being much more compact than it is now, and the Kuiper belt starting much farther in than it does now. Artists rendering of the Kuiper Belt and hypothetical more distant Oort cloud. ...

According to the currently accepted view, the inner solar system was "completed" by a giant impact in which the young Earth collided with a Mars-sized object (being the "fifth" inner solar system object alluded to above). This impact resulted in the fomation of the Moon. The current speculation is that this Mars-sized object formed at one of the stable Earth-Sun Lagrangian points (either L₄ or L₅) and later drifted away from that position. The Big Splash. ... For the Roman god, see Mars (mythology). ... Bulk composition of the moons mantle and crust estimated, weight percent Oxygen 42. ... A contour plot of the effective potential of a two-body system (the Sun and Earth here), showing the 5 Lagrange points. ...

The outer solar system is now believed to have been shaped by planetary migrations. As the gravity of the planets perturbed the orbits of the Kuiper belt objects, many were scattered inwards by Saturn, Uranus, and Neptune, while Jupiter often kicked those objects out of the solar system altogether. As a result, Jupiter migrated inwards while Saturn, Uranus, and Neptune migrated outwards. A major breakthrough in the understanding of how this lead to the current solar system structure occurred in 2004. Computer models have shown that if Jupiter started out taking fewer than two orbits around the Sun for every time that Saturn orbited the Sun once, this migration pattern would put Jupiter and Saturn into a 2:1 resonance when the orbital period of Jupiter became exactly twice that of Saturn's. This resonance would have put Uranus and Neptume into highly elliptical orbits, with there being a 50% chance that they would have exchanged places. As their orbits got more elliptical, Uranus and neptune would have plowed into the Kuiper belt. The subsequent interaction between the planets and the Kuiper belt after Jupiter and Saturn passed through the 2:1 resonance explains the orbital charactertistics and axial tilts of the giant outer planets. The resultant scattering of Kuiper belt objects also would explain the late bombardment phase of the history of the solar system. Planetary migration is the act of a stellar satellite altering its orbital parameters, especially semi-major axis, through various means during its lifetime. ... The Tacoma Narrows Bridge (shown twisting) in Washington collapsed spectacularly, under moderate wind, in part because of resonance. ...

Imapcts are currently believed to be a regular (if infrequent) part of the development and evolution of the solar system. In addition to the Moon-forming impact noted above, the Pluto-Charon system is believed to be the result of a collision between Kuiper Belt objects. Other cases of moons around asteroids and other Kuiper Belt obejcts are also believed to be the result of coillisions. That collisions continue to happen is evidenced by the collision of Comet Shoemaker-Levy 9 with Jupiter in 1993, and the impact feature Meteor Crater in Arizona. Adjective Plutonian Atmospheric characteristics Atmospheric pressure 0. ... Media:Example. ... An asteroid is a predominantly rocky body that orbits around its star. ... Hubble Space Telescope image of Comet Shoemaker-Levy 9, taken on May 17, 1994. ... Adjective Jovian Atmospheric characteristics Atmospheric pressure 70 kPa Hydrogen ~86% Helium ~14% Methane 0. ... Meteor Crater The Meteor Crater, sometimes known as the Barringer Crater and formerly as the Canyon Diablo crater, is a famous impact crater, located about 35 miles (55 km) east of Flagstaff, near Winslow in the northern Arizona desert (USA). ... Official language(s) None Capital Phoenix Largest city Phoenix Area  Ranked 6th  - Total 113,998 sq mi (295,254 km²)  - Width 310 miles (500 km)  - Length 400 miles (645 km)  - % water 0. ...

History of Solar System Formation Hypotheses

During the late-19th century the Kant-Laplace nebular hypothesis was criticized by James Clerk Maxwell, who showed that if matter of the known planets had once been distributed around the Sun in the form of a disk, forces of differential rotation would have prevented the condensation of individual planets. Another objection was that the Sun possesses less angular momentum than the Kant-Laplace model indicated. For several decades, most astronomers preferred the near-collision hypothesis, in which the planets were considered to have been formed due to the approach of some other star to the Sun. This near-miss would have drawn large amounts of matter out of the Sun and the other star by their mutual tidal forces, which could have then condensed into planets. Alternative meaning: Nineteenth Century (periodical) (18th century — 19th century — 20th century — more centuries) As a means of recording the passage of time, the 19th century was that century which lasted from 1801-1900 in the sense of the Gregorian calendar. ... James Clerk Maxwell (13 June 1831 – 5 November 1879) was a Scottish mathematical physicist, born in Edinburgh. ... The Sun is the star at the center of our solar system. ... Differential rotation is seen if parts of a rotating object move with different angular velocity. ... Gyroscope. ... Comet Shoemaker-Levy 9 after breaking up under the influence of Jupiters tidal forces. ...

Objections to the near-collision hypothesis were also raised and, during the 1940s, the nebular model was improved such that it became broadly accepted. In the modified version, the mass of the original protoplanet was assumed to be larger, and the angular momentum discrepancy was attributed to magnetic forces. That is, the young Sun transferred some angular momentum to the protoplanetary disk and planetesimals through Alven waves, as is understood to occur in T Tauri stars. // Events and trends World War II was a truly global conflict with many facets: immense human suffering, fierce indoctrination, and the use of new, extremely devastating weapons such as the atomic bomb. ... In cosmogony, a protoplanet is a quasi-planetoid which is slightly larger than a planetesimal and orbits within a solar nebulas protoplanetary discs. ... In physics, magnetism is a phenomenon by which materials exert an attractive or repulsive force on other materials. ... Drawing of a T-Tauri star with a circumstellar accretion disk T Tauri stars are a class of variable stars named after their prototype - T Tauri. ...

The refined nebular model was developed based entirely on observations of our own solar system, because it was the only one known until the mid 1990's. It was not confidently assumed to be widely applicable to other planetary systems, although scientists were anxious to test the nebular model by finding or protoplanetary disks or even planets around other stars, so-called extrasolar planets. A planetary system consists of at least one star and various orbiting objects (such as asteroids, comets, moons, and planets). ... A protoplanetary disc (also protoplanetary disk, proplyd) is an accretion disc surrounding a T Tauri star. ... Infrared image of the star GQ Lupi (A) orbited by a brown dwarf or planet (b). ...

Stellar nebula or protoplanetary disks have now been observed in the Orion nebula, and other star-forming regions, by astronomers using the Hubble Space Telescope. Some of these are as large as 1000 AU in diameter. A protoplanetary disc (also protoplanetary disk, proplyd) is an accretion disc surrounding a T Tauri star. ... The entire Orion Nebula in visible light Optical images reveal clouds of gas and dust in the Orion Nebula; an infrared image (right) reveals the new stars shining within. ... The Hubble Space Telescope (HST) is a telescope in orbit around the Earth. ...

And as of January 2006, the discovery of over 180 exoplanets has turned up many surprises, and the nebular model must be revised to account for these discovered planetary systems, or new models considered. There is no consensus on how to explain the observed 'hot Jupiters,' but one leading idea is that of planetary migration. This idea is that planets must be able to migrate from their initial orbit to one nearer their star, by any of several possible physical processes, such as orbital friction while the protoplanetary disk is still full of hydrogen and helium gas. Infrared image of the star GQ Lupi (A) orbited by a brown dwarf or planet (b). ... Planetary migration is the act of a stellar satellite altering its orbital parameters, especially semi-major axis, through various means during its lifetime. ...

In recent years, an alternative model for the formation of the solar system, the Capture Theory, has been developed which has explained features of the solar system not explained by the Solar Nebula Theory. This hypothesis has been published in the following references:

References

• William K. Hartmann and Donald R. Davis, Satellite-sized planetesimals and lunar origin, (International Astronomical Union, Colloquium on Planetary Satellites, Cornell University, Ithaca, N.Y., Aug. 18-21, 1974) Icarus, vol. 24, Apr. 1975, p. 504-515
• Alfred G. W. Cameron and William R. Ward, The Origin of the Moon, Abstracts of the Lunar and Planetary Science Conference, volume 7, page 120, 1976
• K. Tsiganis, R. Gomes, A. Morbidelli and H. F. Levison (May 2005). "Origin of the orbital architecture of the giant planets of the Solar System". Nature 435 (7041): 459-461. DOI:10.1038/nature03539.
• Adrián Brunini (April 2006). "Origin of the obliquities of the giant planets in mutual interactions in the early Solar System". Nature 440 (7088): 1163-1165. DOI:10.1038/nature04577.
• M M Woolfson 1969, Rep. Prog. Phys. 32 135-185
• M M Woolfson 1999, Mon. Not. R. Astr. Soc.304, 195-198.

Dr. William K. Hartmann is a noted planetary scientist, author, and writer, and is currently a senior scientist at the Planetary Science Institute. ... A digital object identifier (or DOI) is a permanent identifier (permalink) given to a World Wide Web file or other Internet document so that if its Internet address changes, users will be redirected to its new address. ... A digital object identifier (or DOI) is a permanent identifier (permalink) given to a World Wide Web file or other Internet document so that if its Internet address changes, users will be redirected to its new address. ...

See Also

• Giant impact hypothesis
• Planetary migration
• Solar System
• Solar nebula