Petite Mercury

 Mercury is the smallest planet in our Solar System. But it is actually smaller than some if the moons in our Solar System.

 

Here is a graphic which shows Mercury's size in comparison to some of the larger moons.

 

So why is Mercury a planet even though it is smaller than Ganymede and Titan (and barely larger than Callisto)?

 

We need to define what makes an object a planet.

1.       It orbits around a central star.

2.       It falls into a well-defined category of planets: terrestrial or Jovian.

Obviously, Mercury fits criteria one. It orbits the Sun.  But so does Pluto. Why isn’t it a planet anymore?

Let’s just look at what defines a terrestrial planet. From its name, terrestrial planets are ”Earth-like”, i.e. similar to Earth. Mercury is a terrestrial planet.

Hold on, you might say. Mercury is nothing like Earth. It’s hot and rocky, has no atmosphere, and there is no liquid water on its surface. You would be correct, in that sense.  But what we mean by terrestrial planets is that they have a defined set of parameters.

1.       They are smaller than Jovian planets.

2.       They have a few or no natural satellites.

3.       Their densities (mass contained in a specific volume) are large, i.e. rocky.

So let’s compare Mercury and Pluto.

1.       They both are much smaller than Jovian planets.

2.       Mercury has no moons, while it is believed that Pluto has at least five known moons. Compare this to Neptune which has at least 14 moons, the fewest of the Jovian planets.

3.       Mercury’s density is about 5400 kg/m³and that of Pluto is 2000 kg/m³. Earth’s density is 5515 kg/m³.

So the third reason is why Pluto is not a planet. More discussion on why Pluto is not a planet will follow later on.

Another unique thing about Mercury is that it does not have an atmosphere.  Mercury might have once had an atmosphere, but because of its proximity to the Sun, the solar wind probably stripped Mercury of an atmosphere.

Come back next time and we will discuss Mercury being lopsided.