What are three characteristics of planets

-- 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

Answer:   [a]:   "slate" .