Why do planets orbit stars in ellipses and not perfect cirlces?
Two reasons, really: 1. A circular orbit requires that the body start with exactly the right speed for its distance from the primary. The alternative is that its orbital eccentricity be damped by e.g. tidal effects. None of this is going on in a big way in the Sol system. The combination of inverse-square gravity and conservation of momentum forces the orbit to be a conic section: circle, ellipse, parabola or hyperbola in order of increasing eccentricity. Ellipse is next on the list. 2. The planets disturb each others orbits, both the period and eccentricity. There is a tendency for them to be driven to resonances of integer multiples of the orbital period, and for the apogees and perigees to align so that the orbits are furthest apart when the planets align with respect to the Sun.
This is not a scientific answer, but merely a common sense guess. My guess is that gravity is pulling a planet to move in a forward motion, toward the star, which has a gravitational pull. The planet speeds up as the gravity increases, or gets closer to the star. As that planet passes the star, at that point inertia takes over and in essence the planet is being “flung”. As it passes a certain point, and the planet’s velocity gradually slows, the object’s (star’s) gravity recaptures or regains control over the planet, pulling the planet again towards the star (using gravity) and the process starts all over again. This motion would naturally be an elliptical pattern and not a perfect circle that you are referring to.
We don’t know of any other earthlike stars yet, only hot Jupiters, that orbit very close to stars. Ok, about orbits, it is because Earth and other planets have a slight gravitational pull on Sun as well. Since the speeds of planets are not constant, they cannot sustain a perfect circular orbit, and since that is the case, when they are close, they are pulled faster as compared to when they are far away. Think of a yo-yo example.
