Are moons always smaller than the planets they orbit?

Question

I'm not a physicist, asking for knowledge. Is there any moon orbiting a planet, but bigger than that planet? If not, is it mathematically possible for a bigger object to orbit around a smaller object in space?

Answer 1

A moon does not orbit a planet. They both orbit around their centre of mass. This is true regardless of the masses of the bodies. It's even true for the sun and the earth: the earth really does make the sun move.

For a small moon, the centre of mass is inside the planet. If moon and planet were of equal mass, they would both orbit around a point mid-way between them.

And, of course, if the moon is much larger than the planet, the moon would be called the planet and vice versa.

Answer 2

As pointed out in earlier answers: in celestial mechanics the shape of the (orbital) motion is determined by one factor only: the mass of the respective objects.

In our solar system the motion of the celestial bodies is dominated by the Sun, because the Sun has so much more mass than the second most massive object: the planet Jupiter. Incidentally, Jupiter does have so much mass that the common center of mass of the Sun-Jupiter system is a bit outside the Sun. In that sense the Sun and Jupiter are both orbiting; they are orbiting the common center of mass of the Sun-Jupiter system.

More generally, the celestial bodies of the solar system all are orbiting the common center of mass of the celestial bodies of the solar system.

Of course, as a first approximation we treat the motions of the celestial bodies of the solar system as orbits around the Sun.

The planet-moon system with the mass ratio closest to 1:1 is the Pluto-Charon system. Charon is about 0.12 the mass of Pluto. This mass ratio is inferred from the orbital motion.

The common center of mass of the Pluto-Charon system is orbiting the Sun. So the motion of Pluto is a superposition of motion around the Sun and motion relative to the common center of mass of the Pluto-Charon system. (The common center of mass of the Pluto-Charon system is slightly outside Pluto.) The motion of Charon relative to the common center of mass is larger than that of Pluto. From that ratio the the mass ratio of Pluto and Charon is inferred.

In the case of two celestial bodies that form a gravitationally bound system: the one with the least amount of mass is regarded as a satellite of the other.

As pointed out by hdhondt, there is a scenario where there is a system of two celestial bodies where one is much denser than the other. For instance, the case of a celestial body that consists in large part of iron (which is the case for some objects in the asteroid belt), and a celestial body that consists for the most part of ice (some comets are like that, if memory serves me).

With such a difference in density: the comet can be less massive and yet have more volume. So yeah, that would be a case of a larger celestial body (by volume) being a satellite of a smaller (by volume) celestial body

The odds of actual formation of such a combination are very, very small. Those would be celestial bodies formed in very different circumstances. Comets can have very eccentric orbits, but the odds of a comet becoming a moon of another celestial body are extremely small.

When we send a spacecraft to the Moon then in order to enter orbit around the Moon the spacecraft needs a change of velocity. Without that change of velocity you would just get a flyby. In any interaction between two objects: the two exchange momentum but in that interaction only the direction of the respective velocities is changed.

Answer 3

Mathematically, a moon can probably be bigger than its planet if the former has significantly lower density than the latter.