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Uranus, and Pluto ignored the possible effects of the inner planets. The planet Mercury revolves around the sun in 88 days whereas Pluto requires a little more than 1,000 times longer. Any direct calculation has to proceed in increments small enough to follow each planet and because nothing much happens in mere thousands of years, the evolution of orbits has to be tracked for many millions of years to yield meaningful insights. In principle, a sufficiently powerful computer could step through the necessary calculations but in practice, numerical integrations of the whole solar system required such enormous amounts of computer time that simplification and approximation became essential. Up to 1983, no one had traveled more than five million years into the solar system's future, and those who had traveled this far had seen no signs of irregularity.
A change in technology that occurred in the early eighties was primarily due to Gerard Sussman who seized upon the idea of designing a computer specifically for the task of doing the calculations required in celestial mechanics. This was followed up by collaborative work with astronomer, Jack Wisdom, to design mathematical techniques for exploiting the new technology.
Along with advancing technology, there arose more and more evidence for the occurrence of chaos (in the mathematical sense) in the celestial mechanics of the solar system. Wisdom and Sussman were able to identify elements of chaos in the orbit of Pluto, and Jacques Laskar, of the Bureau des Longitudes, Paris, came up with a study of the whole solar system (except Pluto) and showed that starting with as little a difference as 100 meters in the Earth's position at a given moment, it would be impossible to specify where it would be in its orbit 100 million years later. At the University of Toronto in Canada, Tremaine, Duncan, and Quinn carried out work of importance to Urantia Book readers who have taken an interest in the "missing' planet problem. In their studies of planetary orbits lying between Uranus and Neptune, they found that in roughly half of the cases studied, the orbit became sufficiently chaotic to ensure that, during some part of a five billion year period, any planetary body in that orbit would be likely to be ejected from the solar system.
In 1992, Wisdom and Sussman re-entered the fray with their second custom-built computer with which they were able to trace the evolution of the entire solar system over 100 million years intervals. In doing so, they confirmed the earlier work indicating the chaotic motion of Pluto and Laskar's more general result that the solar system, as a whole, displays elements of chaotic behavior. Models made by Gerald Quinlan of a hypothetical solar system containing just the four planets, Jupiter, Saturn, Uranus, and Neptune, showed that in his simulations of more than fifty randomly adjusted "solar systems," a majority gave evidence for the development of chaotic behavior.
This recent evidence raises the question of whether the particular distribution of planets in our solar system is one of only a limited number capable of enduring because of having exceptional relative stability. Opinion differs about its having room for an additional planet without becoming de-stabilized. Some researchers suspect that any additional planet would be at risk of being ejected from the system. Others speculate that, several billion years ago, the solar system did actually hold additional planets, perhaps the size of our moon or Mars, that were subsequently ejected.
One of the more dramatic consequences of the chaotic evolution of planetary orbits is the effect it may impose upon the angle of tilt of a planet's axis. For the earth, the moon acts as a stabilizing influence. However with inner planets like Mars and Venus, simulations indicate the tilt angles may have evolved chaotically. If so, an alternative explanation is offered for the peculiarity of Venus which rotates about its axis in an opposite direction to its orbital motion. Laskar and his colleagues argue that, because of chaotic effects, the axis of rotation of Venus could have suffered severe tilting to the point that it actually flipped over, thus giving a spin contrary to its orbital motion. If so, Venus is simply "upside down."
Among the concluding remarks in his recent book, Ivars Peterson asks: "How much of a role did chaos play in the formation of the solar system? Did the solar system settle down into its present configuration (with well-spaced planets following nearly circular orbits lying roughly in the same plane) within its first few million years? Or has it gradually evolved to its present configuration over the last five billion years? Were there other planets that have since been ejected? What is the Earth's true trajectory? Is it gradually nearing the sun, eventually to be swallowed up, or is it slowly drifting away into the depths of interstellar space?...What seems clear now is that the solar system is, on astronomical scales, no simple, well-regulated clock."
The Urantia Book (656) indicates that there were actually twelve planets present soon after the birth of the solar system. If chaos in the solar system is a reality, then the number of planets may have decreased through some being ejected. But perhaps it is still possible for new planets to be discovered. An alternative hypothesis is that what were once planets in their own right have since become moons of other planets--or the reverse. Our moon is larger than the planet Pluto. Presumably if it escaped into a stable orbit, it would then be classified as a planet.
As bigger and better computers come on line, perhaps some of the problems arising from The Urantia Book's account of the solar system's evolution will be clarified. One of the more interesting suggestions coming from those who support the concept of chaos is that the "text book" account of its present configuration system having become stabilized in the first few
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