Solar System: Difference between revisions

Revision as of 22:31, 9 April 2007

Revision as of 22:31, 9 April 2007

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Informally, the Solar System is sometimes divided into separate zones. The '''inner Solar System''' includes the four [[terrestrial planet]]s and the main asteroid belt. Some define the '''outer Solar System''' as comprising everything beyond the asteroids.<ref>{{cite web|title=An Overview of the Solar System|author=nineplanets.org|url=http://www.nineplanets.org/overview.html|accessdate=2007-02-15}}</ref> Others define it as the region beyond Neptune, with the four [[gas giant]]s considered a separate "middle zone".<ref>{{cite web| title= New Horizons Set to Launch on 9-Year Voyage to Pluto and the Kuiper Belt|author=Amir Alexander

|work=The Planetary Society|year=2006|url=http://www.planetary.org/news/2006/0116_New_Horizons_Set_to_Launch_on_9_Year.html|accessdate=2006-11-08}}</ref>

==Layout and structure==

[[Image:solarsystem.jpg|thumb|200 px|The ecliptic viewed in sunlight from behind the Moon in this [[Clementine mission|Clementine]] image. From left to right: Mercury, Mars, Saturn]]

The principal component of the Solar System is the Sun, a [[main sequence]] [[stellar classification|G2]] [[star]] that contains 99.86% of the system's known [[mass]] and dominates it [[gravitation]]ally.<ref>{{cite web | author= M Woolfson| title=The origin and evolution of the solar system| work=University of York| url=http://www.oso.chalmers.se/~michael/astrobiologi-2003/j.1468-4004.2000.00012.x.pdf

| accessdate=2006-07-22}}</ref> Jupiter and Saturn, the Sun's two largest orbiting bodies, account for more than 90% of the system's remaining mass.{{Ref_label|B|b|none}} The currently hypothetical [[Oort cloud]] would also hold a substantial percentage were its existence confirmed.<ref>{{cite web| title=Estimates of mass and angular momentum in the Oort cloud|author=Marochnik, Leonid S.; Mukhin, Lev M.; Sagdeev, Roal'd. Z.| work=Institut Kosmicheskikh Issledovanii, Moscow|url=http://adsabs.harvard.edu/abs/1988Sci...242..547M|accessdate=2006-07-23}}</ref>

Most objects in orbit around the Sun lie near the [[ecliptic]], a shallow plane parallel to that of Earth's orbit. The planets are very close to the ecliptic while [[comet]]s and [[Kuiper belt]] objects are usually at significantly greater angles to it.

All of the planets and most other objects also orbit with the Sun's rotation in a counter-clockwise direction as viewed from a point above the Sun's north pole. There are exceptions, such as [[Halley's Comet]].

[[Image:Oort_cloud_Sedna_orbit.jpg|thumb|200 px|The orbits of the bodies in the solar system to scale (clockwise from top left)]]

Objects travel around the Sun following [[Kepler's laws of planetary motion]]. Each object orbits along an ellipse with the Sun at one focus of the ellipse. The closer an object is to the Sun, the faster it moves. The orbits of the planets are nearly circular, but many comets, asteroids and objects of the Kuiper belt follow highly elliptical orbits.

To cope with the vast distances involved, many representations of the Solar System show orbits the same distance apart. In reality, with a few exceptions, the farther a planet or belt is from the Sun, the larger the distance between it and the previous orbit. For example, Venus is approximately 0.33 AU farther out than Mercury, while Saturn is 4.3 AU out from Jupiter, and Neptune lies 10.5 AU out from Uranus. Attempts have been made to determine a correlation between these orbital distances (see [[Titius-Bode law]]), but no such theory has been accepted.

==Formation==

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Calculations of the ratios of hydrogen and [[helium]] within the Sun suggest it is halfway through its life cycle. It will eventually move off the main sequence and become larger, brighter, cooler and redder, becoming a [[red giant]] in about five billion years.<ref>{{cite web |year=1997| author=Richard W. Pogge| title= The Once and Future Sun| work=Perkins Observatory|url=http://www-astronomy.mps.ohio-state.edu/~pogge/Lectures/vistas97.html| accessdate=2006-06-23}}</ref>

The Sun is a [[metallicity|population I star]]; it was born in the later stages of the [[Earth's universe#Evolution|universe's evolution]]. It contains more elements heavier than hydrogen and helium ("[[metallicity|metals]]" in astronomical parlance) than older population II stars.<ref>{{cite journal | author=T. S. van Albada, Norman Baker | title=On the Two Oosterhoff Groups of Globular Clusters | journal=Astrophysical Journal | volume=185 | year=1973 | pages=477–498 }}</ref> Elements heavier than hydrogen and helium were formed in the [[solar core|cores]] of ancient and exploding stars, so the first generation of stars had to die before the universe could be enriched with these atoms. The oldest stars contain few metals, while stars born later have more. This high [[metallicity]] is thought to have been crucial to the Sun's developing a [[planetary system]], because planets form from accretion of metals.<ref> {{cite web| title=An Estimate of the Age Distribution of Terrestrial Planets in the Universe: Quantifying Metallicity as a Selection Effect|author=Charles H. Lineweaver |work=University of New South Wales|date=2000|url=http://arxiv.org/abs/astro-ph/0012399

| accessdate=2006-07-23}}</ref>