Was there ever a planet between the orbits of Mars and Jupiter?
Some astronomers have argued that
material in this part of the solar system would not have had the chance to
assemble itself into much of a planet because of the tidal influence
of Jupiter. Even if you added up all of the material in the asteroid belt,
you would not get more mass than we find in Saturn's little moon Rhea
( about 2.3 x 10^24 grams) which is a moon only 1200 km in diameter.
We are not talking about a 'real' planet like Mars or even one of the
large satellites
of Jupiter like Callisto or Ganymede. Once upon a time, there
were LOTS of bodies a few hundred to a few thousand kilometers across, but
they eventually were captured as moons, or crashed into the growing bulks
of the existing planets leaving behind large craters. Perhaps in
what is now the asteroid belt, two or more of these bodies collided.
We know that asteroids come in 'stony' and 'iron/nickel' varieties. Also
microscopic studies of their crystalline structure shows that many of
the asteroids come from 5 - 6 distinct families; perhaps the original
5-6 bodies that collided billions of years ago
to form the rubble we now see.
Also, the crystals in the meteorites from the asteroid
belt are big enough that the rock must have cooled slowly as though it were
part of larger bodies once upon a time. There is plenty of evidence
that the asteroids were once a smaller number of bodies that collided.
But again, these bodies were MUCH smaller than what we now call planets.
Copyright 1997 Dr. Sten Odenwald
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