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Line 1: − + − This loss of material means that they must have relatively short lives (calculated to be 11 million years at the most<ref>Faulkner, 1997</ref>), which means that they could not have been travelling past the sun for the supposed 4,600-million-year age of the Solar System. − Most cosmologists therefore believe that there must be a source supplying fresh comets, and this is believed to be a giant sphere surrounding the solar system. This sphere is called the '''Oort cloud''' after Jan Oort who postulated its existence in 1945. Thus most comets are said to come from the Oort cloud, with the rest coming to the solar system from the nearer [[Kuiper Belt]], which is located past the orbit of [[Pluto]]. Kuiper belt objects are suspected by those who believe the universe to be billions of years old to be the source of the [[Jupiter]] family, a group of comets whose orbits take them between Jupiter and the sun in a short period of time (3-5 years). Comet Shoemaker-Levy 9 may have been one of those.+ + − [[Young Earth Creationist|Young-Earth creation]] scientists point out that there is no hard evidence for the existence of the Oort comet cloud, and that it is nothing more than an ''ad hoc'' explanation for the existence of "young" comets in an "old" Solar System, and that the actual evidence is more consistent with a biblical age of the universe.<ref>Faulkner, 1997 and Faulkner, 2001</ref>+ + − + − * Faulkner, Danny, [http://www.creationontheweb.com/content/view/1528 Comets and the age of the solar system], ''Journal of Creation'' 11(3):264–273, December 1997.+ − * Faulkner, Danny, [http://www.creationontheweb.com/content/view/1830 More problems for the ‘Oort comet cloud’], ''Journal of Creation'', 15(2):11, August 2001.+ + + + + + + + + + + + + + + + + − <references />+
Expansion
[[Comet]]s are observed to come into the [[Solar System]] from all directions{{fact}}, and on each orbit as they pass near the sun they lose material, which forms the comet's "tail".
The '''Oort cloud''' is the hypothetical region, said to be 50,000 to 100,000 AU in radius, that contains many billions of [[comet]]s in orbit around the [[Sun]]. It is the supposed source of all observed comets in the [[solar system]].<ref name=Weisstein>Weisstein, Eric W. "[http://scienceworld.wolfram.com/astronomy/OortCloud.html Oort Cloud]." ''Eric Weisstein's World of Astronomy'', 1996-2007. Accessed June 2, 2008</ref>
== Background ==
Comets are divided into two groups, the long-period and the short-. A comet having a period longer than 200 years is considered long-period.<ref name=Weisstein/><ref name=Faulkner1>[[Danny Faulkner|Faulkner, Danny]]. "[http://www.answersingenesis.org/docs/4108.asp Comets and the Age of the Solar System]." TJ, 11(3):264-273, December 1997. Accessed June 2, 2008.</ref> The two groups have the following characteristics, with occasional exceptions:
# Short-period comets orbit prograde around the Sun and with little or no inclination with respect to the ecliptic.
# Long-period comets are as likely to be in retrograde as in prograde orbits, and have been observed to have any inclination with respect to the ecliptic.<ref name=Weisstein/>
== Sources ==
Weisstein observes that the [[apsis|aphelia]] of long-period comets are distributed about two modes: 10,000 AU and 50,000 AU.<ref name=Weisstein/>
Faulkner observes that [[Comet Halley]], a short-period comet, is highly inclined and in a retrograde orbit. Comet Halley is, therefore, an important exception to the distinction. Faulkner ''also'' observes that short-period comets ''do not'' differ from long-period comets in composition.<ref name=Faulkner1/>
Cometary orbits are highly elliptical<ref name=Faulkner1/>, and some of these orbits are almost parabolic.</ref name=Weisstein/> ''Hyperbolic'' cometary orbits have never been observed.<ref name=Faulkner1/> This last observation is credited to Jan Oort himself.<ref name=Arnett>Arnett, Bill. "[http://www.nineplanets.org/kboc.html The Kuiper Belt and the Oort Cloud]." ''The <s>Nine</s> 8 Planets'', March 18, 2007. Accessed June 2, 2008.</ref>
The more important point is that comets have a limited life span. After a few hundred perihelion passes at the Sun, a comet loses all of its tail-forming substance.<ref name=Arnett2>Arnett, Bill. "[http://www.nineplanets.org/comets.html Comets]." ''The <s>Nine</s> 8 Planets'', May 1, 2003. Accessed June 2, 2008.</ref> In addition, comets often destroy themselves in collision with planets and other bodies, or are ejected from the solar system entirely after making close flybys of [[Jupiter]] and other [[gas giant]]s.<ref name=Arnett2/><ref name=Faulkner1/><ref name=Faulkner2>[[Danny Faulkner|Faulkner, Danny]]. "[http://www.creationontheweb.com/content/view/1830 More problems for the 'Oort comet cloud']." ''Journal of Creation'' 15(2):11, August 2001. Accessed June 2, 2008.</ref>
For these reasons, astronomers realized that the supply of comets would require replenishment over the supposed great age of the solar system. Comets cannot be interstellar, else they would have hyperbolic orbits. Some astronomers have proposed cryovulcanism as the source of comets, but comets are not sufficiently dissimilar in composition to support this.<ref name=Faulkner1/>
In 1950, astronomer [[Jan Oort]] first proposed the existence of a "cloud of comets" in the outer fringes of the solar system.<ref name=Sonoma>Tenn, Joseph S. "[http://www.phys-astro.sonoma.edu/BruceMedalists/Oort/ The Bruce Medalists: Jan Hendrik Oort]." Department of Physics and Astronomy, Sonoma State University, October 11, 2006. Accessed June 2, 2008.</ref><ref name=Oort>Oort, J.H., 1950. The structure of the cometary cloud surrounding the solar system and a hypothesis concerning its origin. Bulletin of Astronomy of the Netherlands, 11:91–110.</ref> Oort calculated that this cloud might contain a trillion comets.<ref name=Arnett/> Short-period comets are supposed to come from the [[Kuiper belt]].<ref name=Weisstein/><ref name=Faulkner1/><ref name=Arnett/><ref name=Harvey>Harvey, Samantha. "[http://solarsystem.nasa.gov/planets/profile.cfm?Object=OortCloud&Display=OverviewLong Solar System Exploration: Oort Cloud]." [[National Aeronautics and Space Administration|NASA]], March 29, 2006. Accessed June 2, 2008.</ref> Oort originally speculated that the total mass of the Oort cloud would be as much as the mass of [[Jupiter]]. More recent studies have suggested that the total mass might be forty times the mass of the earth.<ref name=Faulkner2/>
== Problems with the Oort Cloud Hypothesis ==
# The Oort cloud has never been directly observed. Most astronomers believe that individual comets would be unobservable so far away.<ref name=Faulkner1/><ref name=Arnett/>
# Collisions between comets, over the course of billions of years, would have destroyed most of them by now, leaving only a combined mass for the entire Oort cloud of 1 to 3.5 times the total mass of the earth.<ref name=Faulkner2/> Such a low mass is far less likely to have supplied the inner solar system with comets over billions of years. For that reason, astronomers are now speculating that the Oort cloud must have its ''own'' even more-distant supply.<ref name=Faulkner2/>
# Comets have twenty times the concentration of [[deuterium]] (heavy [[hydrogen]]) found in the inner solar system. If, according to current models, the material in the Oort cloud came from inside the solar system, this would not be the case.<ref name=Brown>Brown, Walt. "[http://creationscience.com/onlinebook/Comets.html The Origin of Comets]." ''In the Beginning: Compelling Evidence for Creation and the Flood'', May 28, 2008. Accessed June 2, 2008.</ref>
# The suggested formation mechanism for comets is the accretion of water vapor from the solar nebula, according to the [[nebula hypothesis]]. Such accretion would likely be stopped by simple Brownian motion.<ref name=Brown/>
== An alternative hypothesis ==
[[Young earth creationism|Young earth creationists]] generally propose that the solar system is much younger than billions of years. In the six thousand years usually proposed, attrition of comets would not have destroyed many of them. Therefore, no source of replacement comets would be necessary.
== References ==
== References ==
{{reflist|2}}
{{Solarsystem}}
{{Solarsystem}}
[[Category:Astronomy]]
[[Category:Astronomy]]