It just depends on the energy and momentum the planets possessed when they formed.
But they actually all have orbits which are very close to being circular, except for Mercury.
And yes, the expanse of the Universe *should* be slowing down if we only consider ordinary matter.
But there is also...
To visualise it try something like this:
place something heavy on your bed so that it "sinks" a little.
Take a little ball and let it go in a straight path near the heavy object. It will start orbiting the heavy object (some trial-and-error might be ended to give the ball the right velocity so...
Black holes won't shine. They are black. They emit no light.
The only way to "see" a black hole is to take a picture of the same patch of sky at two different times.
If in the second picture something seems to have "disappeared", it's very likely that a black hole is hidden there.
Light is composed by tiny particles called photons, which are massless.
Only something with a definite mass can feel a gravitational attraction towards another body, as the gravitation force is given by:
where G is just constant, m and M are the masses of the two interacting...