Why do planets go in an elipce?

Answer 1

All planets in the solar system have an elliptical orbit to some degree - Venus is the most circular or least elliptical. The imperfection in the circularity of the orbit is simply a reflection of the initial conditions of the planet's formation, if you consider the rotational inertia of the components of the accretion disk from which planets formed, it's almost certain that they will not have precisely the exact amount to form a circular orbit, there would always be a slight irregularity no matter how small.

The reason for the shape of the ellipse reflects this irregularity in terms of the gravitational force between the planet and the sun; laws of physics do the rest. You might say that because gravity's force follows an inverse-square law, the mathematics of a stable orbit are such that an ellipse is consequential. This same observation appears in other guises; Kepler's laws indicate that a line joining the planet to the sun sweeps out equal areas in equal times; when a planet is closer to the sun and feels more force it moves faster, further away, it moves more slowly.

Finally, note that the orbits' ellipses themselves also rotate around the sun (apsidal precession), making the actual orbit a complex-looking spiral shape; as if you had slightly rotated the entire ellipse about the focus that the sun occupies with each orbit. Newton made some calculations to explain this motion; Einstein refined this somewhat in general relativity.