Date: 1993 - 1999
Are planets made out of dirt or rock? Could another planet hit earth?
Well, dirt or rock is pretty much the same thing, isn't 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 much heavier than the Earth is.
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's 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've been doing this for billions of years
and haven't collided yet, it's not likely to happen soon.
So, there's 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
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 won't be quite as damaging as an entire planet
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Update: June 2012