Half the planets in our solar system are relatively small, rocky and found near our sun. Those of the other half are all significantly bigger, covered in giant atmospheres, and orbit far away. Explaining this trend in size and orbits is simple. If the planets formed from a disc of gas and dust (we have actually observed such discs around young stars) then planets forming farther from the sun are colder. If they are far enough from the Sun that water in the gas freezes into ice, they will jump up in size – a gas cloud has twice as much water as rocky material to snowball into a planet. And once a planet reaches a critical size, it captures gas from the nebula to make a thick atmosphere. So, inner rocky planets stay small; but once the icy outer planets get big enough, they jump up to