Our Solar System
seems well planned, with a neat set of four little inner planets fenced in by
the Sun and the asteroid belt, with an outer region populated by a stately
progression four gas giants and bounded on the outside by the Kuiper Belt.
The largest gas giant, Jupiter, with a mass of 318 Earth masses, is 5.2
AU from the Sun (where 1 AU is Earth’s average orbital radius).
Next comes Saturn with a mass of 95 Earth masses, which orbits at 9.5 AU.
Uranus, the lightweight of the gas giants with a mass of 14 Earth masses,
orbits at 19.6 AU. The outermost gas giant
The regularity of
the Solar System’s planetary orbits was noted as early as 1715, and in 1768
Johann Bode proposed a “law” stating that the orbits of the planets in AU
could be predicted by a mathematical relation, which we can now write as
Rn = [4 + (3/2) 2n]/10, where n=1, 2, 3, … for the planets from Venus to Neptune, with n=4 corresponding to a “missing planet” at roughly the orbital radius of the asteroid belt.. The orbit of Mercury (n=0) doesn’t quite fit the systematics of the model and has a value of R0=4/10. But is this regularity an accident or an indication of some systematic natural process? It now looks like an accident, in that a new model of planetary orbits suggests that a chaotic catastrophe in the early Solar System produced the present orbits of the outer planets.
astrophysical models that assume the planets formed by the accretion of
proto–planetary material in their present orbits are beset with great
difficulties in explaining the origin of the great quantity of matter far from
the Sun that was needed to form Uranus and Neptune and why Uranus is less
dynamics of the Solar System seems deceptively simple.
There is a massive central body, the Sun, and the other objects, being
much smaller in mass and widely separated in space, have regular roughly
circular orbits that are easily described by applying
radical new departure from previous models of Solar System formation, the Nice
model, offers to provide an explanation for the mysteries of planet and moon
formation and bombardment. The Nice
model is the work of an international team consisting of Alessandro Morbidelli (
Basically, the Nice model starts with the four major planets ordered as Jupiter, Saturn, Neptune, and Uranus, with Jupiter in more or less its present orbit and the other planets in concentric orbits with radii increasing outward is steps of about 4.3 AU, with Neptune and Uranus inverted from their present ordering. In orbit outside Uranus initially are a cloud of thousands of “planetesimals”, icy bodies comparable in mass to the present asteroids but much more numerous. Roughly speaking, in the Nice model the primitive Solar System looks like a “bull’s eye” target board with four concentric rings (the gas giants) surrounded by a dense outer cloud of dots.
system is somewhat unstable, and as time progresses the major planets interact
and alter their orbits a bit, while the outer cloud squeezes together a bit and
develops gaps like the
Then, at about 878 million years after formation of the system, all hell breaks loose. This occurs because the wanderings of the orbits of Saturn and Jupiter have temporarily come into a 2:1 orbital resonance. This exerts cumulative forces on the other planets, disrupting the orbits of Neptune and Uranus, and flinging them outward into the cloud of planetesimals, which scatter in all directions. It takes about another 100 million years for the system to calm down and reach a new state of equilibrium, which is very different from the previous one.
A 7 megabyte QuickTime movie spanning 1.2 billion years of Solar System evolution according to the Nice model is available here. The movie has been purposely slowed down at 878 million years so that the violent disruptions of the system, which happen fast, can be viewed more closely. It’s fun to watch and to try to imagine the planetary violence that must have occurred when the planetesimals start flying off in all directions.
Thus, the Nice model provides a neat explanation of the present configuration and violent past of the Solar System. It also suggests that, like many other aspects of our present environment, the Solar System is “special”. When we are able to explore the planetary systems of other stars, we are not likely to find any twins of the Solar System, since its origins are the chaotic result of a particular set of circumstances. We might, however, find a system like the early Solar System, where a Late Heavy Bombardment is about to begin and the inner planets are about to be subjected to a cosmic 100 million year rock concert. Space travelers take note. That would be a system to avoid.
R. Gomes, H. F.Levison, K.Tsiganis, and A.Morbidelli,, Nature 435, 466 (2005).
A. Morbidelli, K. Tsiganis, A. Crida, H. F. Levison, and R. Gomes, Astronomical Journal 134 1790-1798 (2007), arXiv preprint 0706.1713 [astro-ph].
SF Novels by John Cramer: my two hard SF novels, Twistor and Einstein's Bridge, are newly released as eBooks by Book View Cafe and are available at : http://bookviewcafe.com/bookstore/?s=Cramer .
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