The line is called an axis.
An imaginary line that runs through the center of a planet and its poles is called its axis.Or possibly a diameter, in regards to the planet's dimensions.
It is called a diameter.
The Axis
An imaginary line which passes through the poles of the planet.
It is an imaginary planet with imaginary moons. It does not exist
A constellation is an imaginary pattern of stars visible from our vantage point. A planet is an object orbiting the Sun. They are two completely different things.
The equator is an imaginary line around the middle of the planet. We crossed the equator this morning.
Orbit doesn't have a function, as it's not an object, certainly not one created by anyone with a purpose in mind. Orbit is just an approximate trajectory of a planet hurling through space, an imaginary line if you will, not an actual cord that keeps planet afloat.
It would be good for an imaginary planet project to try and emulate an existing planet. This will make your planet more believable.
A planetary axis is the imaginary line or pole through the center of the planet which defines the point around which the planet rotates, or spins.
A planet's axis is the imaginary line that the planet rotates around, like spinning top.
The orbit of a planet is the path it takes on its journey around the sun. The axis is an imaginary line through the planet from pole to pole about which the planet revolves or spins.