QUESTION: 
Could the asteroids be the remains of a destroyed planet?

ANSWER from Dick Shaw on MAy 28, 1996:
Your question about the formation of the asteroids (or minor planets) 
has interested astronomers for centuries!  According to a mathematical 
relation first worked out by Daniel Titius, if you take the sequence 
of numbers 0, 3, 6, 12,..., add 4 to each number, then divide them all 
by 10, the progression gives the distance to each of the planets (in 
units of the Earth's distance from the sun).  All of the planets 
except Neptune and Pluto, that is -- but these outermost planets were 
undiscovered at the time the sequence was first worked out.  

This sequence was popularized by the German astronomer Johann Bode, who 
was very impressed that it matched beautifully with the distance of the 
then newly-discovered planet Uranus, which occurred almost a decade 
after Titius' work.  It has been known as "Bode's Law" ever since.  

A mystery of the time, however, was why there was no known planet at a 
distance of 2.8 times the distance of Earth from the sun.  That lead to 
an intensive search (begining in the year 1800) for a planet between the 
orbits of Mars and Jupiter.  After a short time, one was found.  In fact, 
many "minor planets" were found in the following decades, in a location 
known today as the asteroid belt.  Today, estimates of the total number 
of asteroids exceed 100,000.  But where did they come from?  

To understand the answer to this question, it is helpful to study the 
asteroid orbits as a group.  (In fact, a lot of what we know about 
asteroids comes from studies of their orbits.)  I encourage you to read 
other astronomy books to learn more about them.  (I am drawing some of 
the material in this answer from a classic book by George O. Abell called 
"Exploration of the Universe")  We know, for example, that most of the 
orbits are fairly close to circular, and they are seldom inclined from 
the plane that describes the average of all the asteroid orbits.  We 
also know from such studies, and from orbital mechanics, that asteroid 
orbits are greatly perturbed by Jupiter's gravity.  And we know the 
masses of at least some of the largest asteroids.  From that it is 
possible to estimate that the combined mass of all the asteroids 
probably does not exceed 0.05% that of the Earth.  That's awfully small, 
much smaller even than the mass of Pluto.  

From these and other asteroid properties, it seems more likely that the 
asteroid belt is a region of space where, because of Jupiter's gravity, 
it would have been difficult to form a planet in the first place.  
The asteroids there now may be the remnants of material that, under 
more favorable circumstances, might have coalesced into a planet when 
the solar system was formed.