Because that's the way gravity works.
Any more detailed answer requires a lot of calculus and geometry.
I don't know what "elliptical planet" is supposed to mean, but by any reasonable definition I can think of, no, all planets are "elliptical planets".
All planets have elliptical orbits.
Planets travel in elliptical orbits.
As an elliptical orbit is any orbit that isn't perfectly circular, everything has an elliptical orbit. The planets Mercury and Pluto have the most elliptical orbits of the planets, and are easily seen to be oval shaped. Comets also have highly elliptical orbits.
Elliptical orbits of the planets around the sun actually match what we observe. Newton's Theory of Universal Gravitation states that planets will move around the sun in elliptical orbits.
No, because all planets known have elliptical orbits.
All the planets move with an elliptical orbit, but with a very low eccentricity.
Planets circle in an elliptical orbit around the Sun.
Kepler
Johannes Kepler.
Yes.
Kepler discovered that the planets orbit the Sun in elliptical shapes. This means that their orbits are not perfect circles, but instead are stretched out ovals with the Sun located at one of the foci of the ellipse.