Does a moon revolve around a planet anyway?

Answer

Today we usually reserve the word ‘moon’ for ‘our’ Moon. We generally speak of satellites. These are objects that do not revolve directly around the Sun, but that orbit another body that does move around the Sun. That other body can be a planet, but also a dwarf planet or a planetoid.

The definition of a planet is now as follows. It’s a body that

– revolves around the Sun (remarkable, and actually a bit crazy: around other stars they are therefore exoplanets rather than planets);

– has sufficient mass to be ‘approximately round’ as a result of the attraction in itself;

– does not tolerate other objects moving around the Sun orbiting about the same distance from the Sun. In other words, any such objects were all swept up by the planet.

A dwarf planet meets the first two criteria, but not the third. Three are known: Ceres in the belt of “minor planets” between Mars and Jupiter, and Pluto and Metamake in the belt of minor planets beyond Neptune.

Asteroids or “minor planets” are objects of those belts or others that also revolve around the sun, and are too small to be “more or less round.” The symmetrical round shape is assumed from a radius of about 500 km: then the mass is large enough to flatten mountains.

Six of the eight planets have (natural) satellites, but the dwarf planet Pluto also has them. And there are several small planets known with a satellite, it is suspected that there are a lot in that case. Also note that many satellites of planets (and even one of Pluto) are large enough to be “more or less round” as well.

Finally, a meteoroid is a rather small little planet. In most cases, it is a fragment of an earlier collision of two small planets that has slipped into an orbit that brings it to Earth. It’s not very likely that such a thing has a satellite.

Not to mention comets and other objects that move beyond Neptune but don’t really belong to a belt. For everything beyond Neptune, another name has been invented: trans-Neptunian objects or TNOs. Also a bit strange: because all stars and galaxies are beyond Neptune.

So all quite complicated, and only partially consistent. It has already been mentioned above that it is outdated to limit planets to our solar system. Also, according to the (third condition of the) definition of ‘planet’ Jupiter is strictly speaking none, because precisely in Jupiter’s orbit a number of small planets are trapped on a stable orbit!

At the heart of all this is the current view that planets grew out of the merging of smaller debris. In that sense, the two belts of minor planets are an unfinished product, they never made it into a full-fledged planet. What also plays a role is that there are quite a few fairly large objects in the outer orbits of the solar system, which could well be ‘more or less round’. And they are hard to find, so we may not know them all yet. It is unlikely that we will find any really large planets there, so a definition that allows us to list them all is easier.

Pluto is therefore a planet. But that doesn’t make him any less interesting. On the contrary, those dwarf planets and minor planets can teach us a lot about the process of how planets are formed, and it is mainly that which interests us.

As a consolation the following. It has been a mess with the definition of planets from the beginning. Strictly speaking, they are ‘wandering stars’, bright objects that move noticeably between the stars. Now we know that no star stands still, so ‘all planets’. Okay, then let’s narrow it down to objects that move faster than stars. In ancient times, the Sun and Moon were classified as planets. When they were declassified (or upgraded?), Copernicus came, and since then Earth has also become a planet. And now say for yourself: there is much movement in the sky, but not the Earth, because that is where we stand.

Conclusion: conclusive definitions are difficult to draw up as long as we don’t get everything right, and they don’t really matter that much.