An ellipse is the locus of points on a plane whose sum of distances to two fixed points is a constant. It can also be defined as the locus of points whose ratio of distance to a fixed point to distance to a fixed line is a constant less than 1. It is a type of conic section, i.e., a section of a cone and a plane. An ellipse can be expressed in the standard form x^2/a^2+y^2/b^2=1, and it has other forms of expression, such as parametric equations. Ellipses play an important role in Kepler's three laws of planetary motion, i.e., planetary orbits are ellipses with stars as foci.