Analog SFF, March 2010 by Dell Magazine Authors

Author:Dell Magazine Authors
Language: eng
Format: epub
Publisher: Dell Magazines

Reader's Department: THE ALTERNATE VIEW: THE NICE WAY TO MAKE A SOLAR SYSTEM by John G. Cramer

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 of 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 Neptune, with a mass of 17 Earth masses, orbits at 30 AU.

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.

Planetary 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 massive than Neptune. There are also problems in explaining the origins of the asteroid belt, the Trojan satellites of Jupiter, the irregular satellites of the outer planets, and the formation of the Kuiper Belt beyond Neptune. Further, the many large craters of the Moon, along with similar evidence of significant cratering on Mercury, Earth, Mars, and the large asteroid Vesta, tell a story of interplanetary violence that occurred about 900 million years after the formation of the Solar System. Planetary astronomers call this the Late Heavy Bombardment (LHB) period, a succession of collision events that up to now has lacked a plausible explanation.

The orbital dynamics of the Solar System seem 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 Newton's Laws of gravitation and mechanics. The Newtonian two-body problem is easily solved, allowing us to understand these orbits. However, the complications are in the details.

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