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In fact nobody knows, and there is no way to find
out. No software program can tell us, since the necessary data does not
exist. Far from being the precise clockwork construction that Isaac
Newton and his followers long supposed it to be, the solar system is
wholly chaotic. Long-term forecasts about the future orbits of the
planets are wild guesses - the longer the term the wilder the guess -
since Newton's laws of motion are mere approximations.
This represents a vast change in attitude. These laws
used to be considered almost divine for their powers of accuracy. The
mathematician Pierre Laplace speculated two centuries ago about an
omniscient being
``...which knew at a given instant all of the forces
by which nature is animated, and the relative positions of all the
objects, and which, if it were sufficiently powerful to analyse all this
information, would include in one formula [my italics] the
movements of the most massive objects in the universe and those of the
lightest atom. Nothing would be uncertain to it. The future, as the
past, would be present to its eyes.''
But no such omniscient being can exist. The positions
of objects change constantly as if manipulated by an unseen power. For
in addition to being influenced by the Sun, the planets are also
influenced by each other. And slightly but inexorably, they feel the tug
of moons, asteroids, comets and passing stars. Over extremely long
periods, this makes their motions utterly unpredictable.
Because the Moon is a quarter of a million miles
away, we imagine that it was always at that distance. But it is believed
to have formed only 20,000 miles away, and to have been migrating
outwards ever since.
Neptune is thought to have moved 30 per cent further
away from us since its formation - over 700 million miles! - and it may
be only a matter of time before it collides with Pluto. The other giant
planets have also moved by significant amounts. Our solar system
probably bears little resemblance to what it looked like three billion
years ago. It is even possible that a planet could be ejected entirely
from the Sun's neighbourhood!
I do not mean to imply that any such event is likely
to happen tomorrow morning. Many people will remember the ridiculous
1950 book Worlds in Collision by Immanuel Velikovsky, who
theorised that during Biblical times a comet from Jupiter's orbit came
close to crashing into Earth before becoming the planet Venus!
But over eons of time truly bizarre events may be
expected. A star cluster was recently discovered far out in
intergalactic space. Who knows what intricate combinations of causes
threw it so far from the galaxy where it must have formed.
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