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Comment by kristianc | Hacker News Reader

Moon size is completely relative of course. Ganymede is larger than Mercury.

For what it's worth (nothing, just interesting to note): Ganymede is larger, and less dense. Mercury's mass is more than twice Ganymede's.

Interesting. Is that because of different material makeup, or is it somehow related to tidal forces?

My understanding is that the composition of each body is different. There's the obvious: More ices and liquids comprise Ganymede, possible since Ganymede is so distant from the Sun. And the not-so-obvious: Mercury is anomalously dense due in part to (likely) having a disproportionally large core of iron and nickel.

I find Wikipedia's discussion about Mercury's density in contrast to Earth's fascinating [0]:

> If the effect of gravitational compression were to be factored out from both planets, the materials of which Mercury is made would be denser than those of Earth, with an uncompressed density of 5.3 g/cm3 versus Earth's 4.4 g/cm3.

[0] https://en.wikipedia.org/wiki/Mercury_(planet)#Internal_stru...

Mercury is basically a big ball of iron.

That is a fun fact! Gave me a smile to think about our solar system’s variety. Thank you!

They define a large moon in terms of its size compared to the host planet.

Titan as well.

By size, the Solar System has 4 Mercury-sized bodies: Mercury, Ganymede and Callisto (the 2 bigger of the 4 big satellites of Jupiter), and Titan (satellite of Saturn).

There are also 4 Moon-sized bodies: the Moon, Io and Europa (the 2 smaller of the 4 big satellites of Jupiter), and Triton (satellite of Neptune).

Pluto is smaller than Triton, but not much smaller. It is the next in size after these bodies and it could be considered to belong to the same size class as the Moon-sized bodies, because the next smaller bodies are significantly smaller. While Pluto is not a satellite of Neptune, like the satellites it does not have an independent movement, but one that is connected to the movement of Neptune through an orbital resonance.

The Neptune-Pluto gravitational resonance is pretty neat. Wikipedia has a nice listing of other orbital resonances in the solar system:

Likely gravitional:

https://en.wikipedia.org/wiki/Orbital_resonance#Mean-motion_...

Likely coincidental:

https://en.wikipedia.org/wiki/Orbital_resonance#Coincidental...

Eris is slightly smaller than Pluto in volume -- but more massive; it might also be worth considering in that same class.

You are right, Eris is close enough in size to Pluto and to the other 4 Moon-sized objects to be considered in the same class size.

In the past, for a long time the uncertainties about the sizes of Pluto and of Eris were great, so it was not clear how close in size they are to the other much better known Moon-sized bodies.

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