If I make a object enter a planet's gravity it hits the surface with huge velocity, where is it gaining k.e. from? If I make a object enter a planet's gravity range,it hits the surface with huge velocity,where is it gaining k.e. from?
is it from the potential energy that it gained when it is separated from the planet  at the big bang?
 A: Total energy is conserved. Let $E$ stand for total energy. Let $T$ stand for kinetic energy. Let $U% stand for potential energy.
$E = T + U$
If $T$ increases and $E$ is constant (which it is in a closed system due to conservation of energy), then $U$ must decrease. Simply put, the kinetic comes from the potential energy.

Potential energy comes from configuration or position. In this case, it comes from the position of the ball in  gravitational field.
Since, $E$ is constant, you could get energy from having things fall to earth. This energy is not unlimited, however. Once the object is falling, all of its energy is becoming kinetic. To lift it back up, you must counter act the potential energy, requiring energy.
A: The energy is not entirely from from the Big Bang since a lot of the material had to come from supernovas. This would mean that some of the energy came from the supernovas separating the dust that makes up the objects.
Yes, you can harvest energy from the falling objects, if they fall onto the targets.
