The orbits of planets being elliptical was one of the planetary laws developed by Johannes Kepler.

He formulated three laws of planetary motion:

1. Law of Ellipses: Planets move in elliptical orbits with the Sun at one focus.

2. Law of Areas: A line joining a planet to the Sun sweeps out equal areas in equal times.

3. Law of Periods: The square of a planet's orbital period is proportional to the cube of the semi-major axis of its orbit.