Question 1

How big do moons get?

Accepted Answer

Ganymede and Titan are both larger than Mercury, and seven moons are larger than Pluto. Size is set by where a moon came from: the large ones formed in a disc around their planet, and the small ones were captured.

Question 2

Which planets have the large ones?

Accepted Answer

Jupiter and Saturn hold most of them, because both had enough material around them to build a system in miniature. The Earth is the exception on this list — the Moon is very large relative to its planet, and it was made by an impact rather than a disc.

Question 3

What does the size column actually measure?

Accepted Answer

Not always the same thing, and the site says which. A round body has a mean radius; an oblate planet may have an equatorial one; an irregular asteroid has an effective radius, meaning the radius of a sphere that would show the same disc. Each object's page names the kind it holds.

Question 4

How firm is the order?

Accepted Answer

Firm at the top and soft further down. The largest bodies have been resolved or visited and their sizes are known to a per cent or two; the smaller ones are often sized from brightness plus an assumed reflectivity, which can be wrong by a factor of two in either direction.

Question 5

Why are some moons missing?

Accepted Answer

Because no size is recorded for them here. Measuring one needs a resolved image, an occultation, a transit or a radar echo, and most catalogued bodies have had none of those.

Question 6

Which is the largest moon in the Solar System?

Accepted Answer

Ganymede, at 5,268 km across — larger than Mercury, though less than half its mass because it is mostly ice. It is also the only moon known to generate a magnetic field of its own.

Question 7

Are any of them worlds in their own right?

Accepted Answer

Several. Titan has a thick nitrogen atmosphere, rivers and lakes of liquid methane, and weather; Europa and Enceladus hold liquid water oceans under ice; Io is the most volcanically active body known.

Question 8

Could a large moon be called a planet?

Accepted Answer

Not under the current definition, which requires a direct orbit around the Sun — so Ganymede and Titan are excluded while much smaller bodies qualify. Several of them would be uncontroversial planets if they orbited the Sun instead.

The largest moons in the Solar System

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