Rock, gas, ice: three kinds of planet

The eight planets fall into three groups.

Rock. Mercury, Venus, the Earth and Mars are small and made of rock and metal. They have solid ground you could stand on.

Gas. Jupiter and Saturn are enormous and made mostly of hydrogen and helium, the same stuff as the Sun. They have no ground at all. The gas simply gets thicker and hotter the deeper you go.

Ice. Uranus and Neptune are in between. Under a deep atmosphere they are mostly water, ammonia and methane, crushed into a hot, dense fluid.

The groups are in that order outward from the Sun, and that is no accident. Close to the young Sun it was too warm for ice, so there was only a little rock and metal to build with, and the planets there stayed small. Further out there was ice as well, and far more of it. Planets there grew big enough to pull in gas, and became giants.

Nobody has drilled into another planet, so how is it known what they are made of? Mostly from one number: density, which is mass divided by volume. The mass comes from timing a planet’s moons or passing spacecraft, and the volume from its size.

Planet	Mass (Earths)	Width (Earths)	Density (water = 1)
Mercury	0.055	0.38	5.43
Venus	0.815	0.95	5.24
Earth	1	1	5.51
Mars	0.107	0.53	3.93
Jupiter	318	10.97	1.33
Saturn	95.2	9.14	0.69
Uranus	14.5	3.98	1.27
Neptune	17.1	3.86	1.64

Rock is about 3 times as dense as water and iron about 8. The four inner planets sit between the two, so they must be rock around an iron core. Jupiter and Saturn are far too light for that, even though their own weight squeezes them hard, and only hydrogen and helium fit. Saturn is less dense than water. Uranus and Neptune are smaller than Saturn yet denser, so they must hold something heavier than gas: mostly water, ammonia and methane, with hydrogen and helium making up only the outer tenth or so of their mass.

Why the order. The planets grew in a disc of gas and dust round the young Sun. Close in it was hot, and only rock and metal could form solid grains. Those are rare materials, so the inner planets had little to build with. Beyond a certain distance, called the frost line, water froze as well. Ice was plentiful, and the solid cores that grew there reached perhaps ten times the mass of the Earth, heavy enough to pull in the hydrogen and helium around them. Jupiter and Saturn took a great deal. Uranus and Neptune, growing more slowly further out, got much less before the gas was blown away.

Jupiter is in a class of its own. It has more than twice the mass of all the other planets put together. Deep inside, the pressure turns hydrogen into a liquid that conducts electricity like a metal, and that is the source of its vast magnetic field.

Some of this is less tidy than it sounds. The Juno spacecraft found that Jupiter’s core is not a neat ball but is smeared out through much of the planet. And among the thousands of planets found round other stars, the commonest size lies between the Earth and Neptune, a kind our own Solar System does not have at all.