Question 1

How much bigger are the giants?

Accepted Answer

Eleven Earths would fit across Jupiter, and about 1,300 would fit inside it. The four giant planets between them hold more than 99 per cent of the mass orbiting the Sun, and the four rocky planets share what is left.

Question 2

Why are the big ones all far out?

Accepted Answer

Because of where ice could survive. Beyond the line where water froze in the young Solar System there was several times more solid material available, so cores there grew fast enough to capture hydrogen and helium before the gas dispersed.

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 planets 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 planet?

Accepted Answer

Jupiter, at 139,820 km across the equator — about a tenth the diameter of the Sun and more massive than every other planet put together. It spins fast enough that the equator bulges by some 4,600 km more than the poles.

Question 7

Is the largest planet also the densest?

Accepted Answer

The opposite. Saturn is the least dense planet in the Solar System, less dense than water, while little Earth is the densest of all — density tracks composition and compression, not size.

Question 8

Why is Pluto not on this list?

Accepted Answer

Because it is a dwarf planet, not a planet: it has not cleared its orbital neighbourhood, sharing the Kuiper belt with a great many similar bodies. It is also smaller than seven moons.

The largest planets in the Solar System

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