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January 2010

In cosmogony, the nebular hypothesis is the most widely accepted model explaining the formation and evolution of the Solar System. It was first proposed in 1734 by Emanuel Swedenborg. Originally applied only to our own Solar System, this method of planetary system formation is now thought to be at work throughout the universe. The widely accepted modern variant of the nebular hypothesis is Solar Nebular Disk Model (SNDM) or simply Solar Nebular Model.

According to SNDM stars form in massive and dense clouds of molecular hydrogen—giant molecular clouds (GMC). They are gravitationally unstable, and matter coalesces to smaller denser clumps within, which then proceed to collapse and form stars. Star formation is a complex process, which always produces a gaseous protoplanetary disk around the young star. This may give birth to planets in certain circumstances, which are not well known. Thus the formation of planetary systems is thought to be a natural result of star formation. A sun-like star usually takes around 100 million years to form.

The protoplanetary disk is an accretion disk which continues to feed the central star. Initially very hot, the disk later cools in what is known as the T tauri star stage; here, formation of small dust grains made of rocks and ices is possible. The grains may eventually coagulate into kilometer sized planetesimals. If the disk is massive enough the runaway accretions begin, resulting in the rapid—100,000 to 300,000 years—formation of Moon- to Mars-sized planetary embryos. Near the star, the planetary embryos go through a stage of violent mergers, producing a few terrestrial planets. The last stage takes around 100 million to a billion years.

The formation of giant planets is a more complicated process. It is thought to occur beyond the so called snow line, where planetary embryos are mainly made of various ices. As a result they are several times more massive than in the inner part of the protoplanetary disk.

Recently featured: Eris (dwarf planet) – Herbig–Haro object – Big Bang

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Old featured articles

Month	Article
March 2005	Black Hole
April 2005	Io
May 2005	Kreutz Sungrazers
June 2005	Big Bang
July 2005	Cat's Eye Nebula
August 2005	Transit of Venus
September 2005	Apollo 8
October 2005	Silverpit crater
November 2005	Astrophysics Data System
December 2005	Comet Shoemaker-Levy 9
January 2006	Hubble Space Telescope
February 2006	H II region
March 2006	Carl Edward Sagan
April 2006	Galileo Galilei
May 2006	Comet Hale-Bopp
June 2006	Planetary nebula
July 2006	Sun
August 2006	Definition of planet
September 2006	Fermi paradox
October 2006	Mercury (planet)
November 2006	Crab Nebula
December 2006	Hubble Deep Field
January 2007	Barnard's Star
February 2007	Binary star
March 2007	Globular cluster
April 2007	Solar System
May 2007	Mars
June 2007	Galaxy
July 2007	Enceladus
August 2007	Extrasolar planet
September 2007	Herbig-Haro object
October 2007	Kuiper belt
November 2007	Johannes Kepler
December 2007	Star
January 2008	Saturn
February 2008	Supernova
April 2008	Comet
May 2008	IK Pegasi
June 2008	Callisto
July 2008	White dwarf
September 2008	Venus
October 2008	Scattered disc
January 2009	Radio Astronomy
February 2009	General relativity
March 2009	Open cluster
April 2009	Redshift
May 2009	Rings of Uranus
June 2009	Tau Ceti
July 2009	Gamma-ray burst
August 2009	Atmosphere of Jupiter
September 2009	Zhang Heng
October 2009	Main sequence
November 2009	Big Bang
December 2009	Herbig–Haro object
January 2010	Eris