-- Planets are all very nearly spherical in shape.
-- Planets orbit the Sun, or whatever other star they belong to, in
an orbit that is an ellipse, with the star at one focus of the ellipse.
-- Planets rotate, with their axis of rotation usually, but not always,
being pretty close to perpendicular to the plane of their orbit.
Some other characteristics are:
-- whether the planet has an atmosphere or not
-- the temperature at the surface of the planet
-- the planet's composition and density
