Changes

The [[nebular hypothesis]] would not even have allowed Neptune and its twin, [[Uranus]], to form at its tremendous distance from the [[Sun]] in the time generally allotted. As the journal ''Astronomy'' stated the problem:{{cquote|Pssst … astronomers who model the formation of the solar system have kept a dirty little secret: Uranus and Neptune don’t exist. Or at least computer simulations have never explained how planets as big as the two gas giants could form so far from the sun. Bodies orbited so slowly in the outer parts of the sun’s protoplanetary disk that the slow process of gravitational accretion would need more time than the age of the solar system to form bodies with 14.5 and 17.1 times the mass of Earth.<ref>R.N., Birth of Uranus and Neptune, ''Astronomy'' '28'(4):30, 2000</ref>}}

The [[nebular hypothesis]] would not even have allowed Neptune and its twin, [[Uranus]], to form at its tremendous distance from the [[Sun]] in the time generally allotted. As the journal ''Astronomy'' stated the problem:{{cquote|Pssst … astronomers who model the formation of the solar system have kept a dirty little secret: Uranus and Neptune don’t exist. Or at least computer simulations have never explained how planets as big as the two gas giants could form so far from the sun. Bodies orbited so slowly in the outer parts of the sun’s protoplanetary disk that the slow process of gravitational accretion would need more time than the age of the solar system to form bodies with 14.5 and 17.1 times the mass of Earth.<ref>R.N., Birth of Uranus and Neptune, ''Astronomy'' '28'(4):30, 2000</ref>}}

Of course, the time allotted for the formation of the [[solar system]] is generally taken to be the alloted age of the [[Earth]]. The problem is that Uranus and Neptune would require more than twice the supposed age of the earth to accrete their present masses. In theory, the solar system could be older than the earth itself, but not much older. The solar system has other bodies in it that are much "younger." Nor does the above account for the [[young solar system evidence|body of evidence]] that the solar system is in fact quite young.

Of course, the time allotted for the formation of the [[solar system]] is generally taken to be the alloted age of the [[Earth]]. The problem is that Uranus and Neptune would require more than twice the supposed age of the earth to accrete their present masses. In theory, the solar system could be older than the earth itself, but not much older. The solar system has other bodies in it that are much "younger." Nor does the above account for the body of evidence that the solar system is in fact quite young.

Boss, in 2002, suggested reviving the old disk-instability hypothesis of planet formation, in which Uranus and Neptune formed through gravitational instability in a gaseous protoplanetary disk, after which a passing star blasted away much of the gaseous envelopes of the two worlds, leaving them much as they are today.<ref name=Boss>Boss, Alan P. "[http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V61-46SVX4B-1&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=8356c9b57117437b185f44aca5ae71d0 Formation of gas and ice-giant planets]." ''Earth and Planetary Science Letters'' 202(3-4):513-523, September 30, 2002. <doi:10.1016/S0012-821X(02)00808-7> Accessed May 7, 2008.</ref> Thommes ''et al.'' suggested another theory: that Uranus and Neptune formed at the same general distance from the Sun as [[Jupiter]] and [[Saturn]] and then ''migrated'' to their present orbits.<ref name=Thommes>Thommes, Edward W., Duncan, Martin J., and Levison, Harold F. "[http://arxiv.org/abs/astro-ph/0111290 The formation of Uranus and Neptune among Jupiter and Saturn]." ''Astron. J.'' 123:2862, 2002. <arXiv:astro-ph/0111290v1> Accessed May 7, 2008.</ref>

Boss, in 2002, suggested reviving the old disk-instability hypothesis of planet formation, in which Uranus and Neptune formed through gravitational instability in a gaseous protoplanetary disk, after which a passing star blasted away much of the gaseous envelopes of the two worlds, leaving them much as they are today.<ref name=Boss>Boss, Alan P. "[http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V61-46SVX4B-1&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=8356c9b57117437b185f44aca5ae71d0 Formation of gas and ice-giant planets]." ''Earth and Planetary Science Letters'' 202(3-4):513-523, September 30, 2002. <doi:10.1016/S0012-821X(02)00808-7> Accessed May 7, 2008.</ref> Thommes ''et al.'' suggested another theory: that Uranus and Neptune formed at the same general distance from the Sun as [[Jupiter]] and [[Saturn]] and then ''migrated'' to their present orbits.<ref name=Thommes>Thommes, Edward W., Duncan, Martin J., and Levison, Harold F. "[http://arxiv.org/abs/astro-ph/0111290 The formation of Uranus and Neptune among Jupiter and Saturn]." ''Astron. J.'' 123:2862, 2002. <arXiv:astro-ph/0111290v1> Accessed May 7, 2008.</ref>