Question

why planets move in elliptical orbits only, not in circle nor in straight line?

Answer 1

Actually if you solve the newtons equation of motion all the paths, circular, elliptical, straight line, hyperbola, and parabola all are possible. If it is a straight line then the it will move towards sun and crushed to sun. So a planets moving in straight will seas to exist after some time. Also for hyperbola and parabola the curve is open so it will not be a stable system, that means the planet will be close to sun for very sort period of time. So for stable system elliptic and circular. Circle is a special case of ellipse when eccentricity is zero. So for all possible eccentricities from 0 to 1, it is very un likely that naturally eccentricity of any curve will be zero. That's why the planets moves in elliptical orbits, not circular. By the way, eccentricity of earth's orbit is very small, almost close to circle.