Ask A Scientist© Astronomy Archive

Author:      kris
Are planets made out of dirt or rock?  Could another planet hit Earth?

Response #:  1 of 1
Author:      asmith
Well, dirt or rock is pretty much the same thing, is not it?  Dirt is just
ground up rock.  Actually, only the inner 4 planets of our solar system
(Mercury, Venus, Earth, Mars) are mostly made of rock, like the Earth is.


The outer planets (Jupiter, Saturn, Uranus, Neptune, Pluto) are made of
"lighter" things, like hydrogen, helium, ammonia, and probably some water. 
However, these planets (except Pluto) have so much of these "lighter"
elements that they are much heavier than the Earth is.
In answer to your second question:  The planets right now all orbit the Sun
in very nice almost circular orbits.  The diameters of those orbits differ
by 10 million miles or more, and the planets them selves are only a few
thousand miles in diameter, so there is no way one planet could come close
to hitting another.  The only exception to this is Pluto, that has a much
more non-circular (elliptical) orbit than any of the others, and which
sometimes crosses Neptune's orbit.  Even so, they have been doing this for
billions of years and have not collided yet, it is not likely to happen
soon.  So, there is no chance of another planet hitting the Earth.  In fact,
we know exactly how the planets are going to move in the future and can use
computers to calculate where they will be, and nothing very interesting at
all happens for the next 10 millions years at least.  But there are only 9
planets, and there are probably millions of smaller objects - asteroids and
comets - in our solar system, and they will collide with the Earth once in a
while.  But they will not be quite as damaging as an entire planet would be.