It is the force of gravity which makes
planets into spheres. Everything gets pulled
closer to the center of the planet, so the planet
becomes nearly a perfect sphere, if it is big
Pluto used to be a planet, but now scientists
say it isn't a planet, because it's not round
enough. Pluto isn't big enough to have enough
gravity to pull it into a sphere. Asteroids
are even smaller than Pluto, and they have shapes
that aren't even as nearly spherical as Pluto.
Earth's moon looks like a sphere. Do you
think the moon is larger or smaller than Pluto?
Gravity is not always easy to understand. You
might want to read about gravity
Now, for your question of why are orbits
also round: Actually orbits are not all round,
but also elliptical, spiral, and
hyperbolic. However for planets, the orbits
are almost circular. In order for planets to
have "orbits", we have to consider again the
existence of gravity and the fact that
two objects with mass will be attracted to each
other, and this will affect their movement
through space. We also have to consider that
an object in motion will stay in motion unless
something pushes or pulls on it.
Then, in order to analyze the reasons why an
orbit is circular, or elliptical, let's consider
the following cases:
1) If a planet moves too fast toward the sun, it
will have too much speed to be pulled back around
by gravity. Instead of orbiting, it will be flung
out of the solar system.
2) If the planet moves too slowly toward the sun,
it willn’t have enough speed to stop it from
falling toward the Sun. The orbit will continue to
get closer and closer to the sun until it fell in.
3) If the planet has just the right amount of
speed and comes in at the right angle, it will
move in a circular orbit, not moving closer or
farther from the Sun.
4) If a planet has a speed and approach that is
somewhere in between, it’s orbit will be
elliptical (a circle that isn’t perfectly round).
When it’s closer to the sun, the planet like Earth
is going faster but the force pulling it closer is
stronger. When it’s farther from the sun, it’s
slower but the force is also slower. Over time,
these forces will make the planet's elliptical
orbit more and more round until it is a perfect
circle, but there may be other forces (from the
other planets) that will pull it out of this
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