Planets are formed from the dust and particles
left over from an exploding star. Our solar
system was formed the same way. We can seen the
same thing happen in other solar systems far away
Right after the explosion of an old star, most
of the left-over particles are much smaller than
planets. These particles will "float" around in
space until gravity pulls some of the biggest
pieces into the center of a dust cloud.
Eventually, a lot of big pieces are pulled into
the center and form a star.
The other floating pieces travel around the new
star as a dust cloud, but they also begin to crash
into each other because of gravity. As they crash
and smash into each other, they build up and
create planet-sized pieces. That is how a planet
is formed. Our solar system had enough pieces
in its original dust cloud to form 8 planets:
Mercury, Venus, Earth, Mars, Jupiter, Saturn,
Uranus, and Neptune.
Exactly how and when the dust cloud pieces
smashed into each other to form the 8 planets is
still being studied, but it appears that there are
many, many ways for dust particles to collide to
form planet-sized objects.
Some pieces of the dust cloud don't get smashed
into a planet, and they keep traveling around the
new star in the center. For example, there are
a lot of asteroids in our solar system which are
not big enough to be called planets. We have
an asteroid belt between Mars and Jupiter. Other
objects in our solar system are bigger than
asteroids, like Pluto, but they are not big enough
to be called planets.
Scientists didn't always think the planets
formed this way. It took many years and lots of
experiments and observations to develop these
ideas. We make new instruments every year as
technology improves. Our ability to study
space gets better each day, and sometimes we
change our ideas based on scientific
observation. Perhaps one day all our current
ideas will change to better ideas about the
formation of planets.