No. 63: May-Jun 1989
It was only about 40 years ago when astronomers, aghast at Velikovsky's vision of worlds in collision, stated very firmly that the solar system was presently stable and had been so for eons. Now it seems that they may have been a bit hasty and all-encompassing. Not that the Velikovsky scenario is correct or that Mars might at any moment depart from its present orbit, but rather that astronomers must now admit an inability to predict planetary motion over billions of years - even tens of millions of years! For the solar system, if those ubiquitous computers are correct, is not a well-behaved family of planets. J. Laskar concludes from extensive numerical experiments:
"The motion of the Solar System is thus shown to be chaotic, not quasiperiodic. In particular, predictability of the orbits of the inner planets, including the Earth, is lost with a few tens of millions of years."
(Laskar, J.; "A Numerical Experiment on the Chaotic Behavior of the Solar System," Nature, 338:237, 1989.)
Comment. Laskar's comments are directed toward the future, but the same conclusions should apply if we ran the solar system backwards in time. From this very narrow perspective of celestial mechanics, one cannot say positively that the planets, the inner ones especially, could not have radically altered their orbits within the past few millions of years.
Reference. See ABB1 in our catalog: The Sun and Solar System Debris for more on the solar system's instability. This book is described here.