What is a planet?

Planets are celestial bodies that are in orbit around a star (in the case of Earth, this is our sun), without being stars themselves. Additionally, they have to have a nearly spherical shape and they must have mostly cleared their orbital path around their star of smaller bodies such as asteroids. Planets can consist of rock, like e.g. Mercury, Venus, Earth, and Mars, or of gas, like Jupiter, Saturn, Uranus, and Neptune.

Here, planets are differentiated from stars by not having enough mass to initiate nuclear fusion processes in their centre, which is the source of the energy a star radiates away. On a mass scale between stars and planets lie the so-called brown dwarfs, which have enough mass to feature a less efficient form of fusion (the so-called deuterium fusion), but do not have the about 80 Jupiter-masses necessary to start the sustained hydrogen fusion that is usual for stars. Brown dwarfs start at about 13 times the mass of Jupiter, which is the highest-mass planet in the Solar System. The exact boundary depends on the objects’ composition and the research field’s perspective.

To lower masses, however, planets are distinguished from so-called dwarf planets, such as Pluto, much-mourned in public perception. These dwarf planets fulfil the first two criteria of the planet definition – the orbit around a star and the near-spherical shape – but due to their small mass they have not cleared their orbit of similar-sized bodies. In the case of Pluto, the slightly heavier Eris should be mentioned here. Physically, planets and dwarf planets are very similar, so this definition continues to be discussed intensively. In the Solar System, the dwarf planets include, besides Pluto and Eris, Makemake, Haumea, and Ceres.

Rocky bodies of even lower mass, usually don’t have a spherical shape any more due to their low gravity, and so they are distinguished from dwarf planet and, depending on location, size, and orbit, are be called asteroids, meteoroids, or comets.

Finally, we have to talk about moons. Those are primarily distinguished by not orbiting a star, but rather a planet. The corresponding host planet usually has a much larger mass, but there can still be moons that are larger and more massive than many a small planet. An example here is the Jupiter moon Ganymed (the largest moon in our Solar System), that has a larger radius by about 200 km than Mercury (the smallest planet in our Solar System). However, due to its very high density, Mercury has about double the mass of Ganymed.

Today, we don’t only know planets in our Solar System, but also more than 5000 planets orbiting stars other than the Sun. These are called extrasolar planets, or exoplanets for short.