Answer 1:
When the solar system formed, gravity pushed a
cloud of dust and rock together to form the Earth.
Let’s look at all the options for orbits:
1) If the new Earth moved too fast toward the
sun, it would have too much speed to be pulled
back around by gravity. Instead of orbiting, it
would be flung out of the solar system.
2) If the new Earth moved too slowly toward the
sun, it wouldn’t have enough speed to stop it from
falling toward the Sun. The orbit would continue
to get closer and closer to the sun until it fell
in.
3) If the Earth had just the right amount of
speed and came in at the right angle, it would
move in a circular orbit, not moving closer or
farther from the Sun. This is very unlikely,
because there are so many speeds/angles that
wouldn’t lead to a circular orbit.
4) If the Earth had a speed and approach that
was somewhere in between, it’s orbit would be
elliptical (a circle that isn’t perfectly round).
When it’s closer to the sun, the 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 Earth’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
alignment.
This is what happened to the Earth.
Scientists define zero “eccentricity” as a perfect
circle and one “eccentricity” as no longer in
orbit – Earth’s eccentricity is 0.0167, the
most circular orbit of any planet in our solar
system. | 
