A number between 0 and 1, known as the eccentricity of the ellipse. They are quantities used to define an orbit. A circle is also an ellipse. M is an angle that defines the satellite's position along that ellipse and 3 additional angles define the orientation of the orbital ellipse in 3-dimensional space. Then as e approaches 1, the semimajor axist grows to be larger and larger.
Planets are too dim and too close to their suns to be seen by today's telescopes.
However, each such sun also moves, around the joint center of gravity of it and of its planets. For instance, with just one planet, the Sun is always on the opposite side of the center of gravity. Therefore, if astronomers see a slight wobble in the position of a distant star, they may deduce from it the existence and motion of its planets.