When the Sun was born about 4.6 billion years ago, it was encircled by a disc of matter, left over from the parent cloud. Such a cloud would consist of gas, mostly hydrogen and helium, mixed with solid grains. In the cooler outer regions of the cloud, the grains would consist of rock matter and of –“ices’-frozen water, carbon dioxide, methane and ammonia. The ices could relatively easily be turned into vapour, so in the inner regions of the cloud only the rock material condensed as it was warmed by the newly born star.
Grains of dust in the disc collided, accumulated and built up into small bodies called planetoids. These rapidly swept the surrounding space clear of the remaining dust. The inner planetoids consisted of rock, and were fairly small: in the outer system, the planetoids also included the icy materials. Because they were more massive, the outer planetoids gathered large atmospheres of hydrogen and helium around them. The smaller inner planetoids could not retain atmospheres: the warmth from the central star also helped to drive off these light gases. The process of planetoid formation was repeated in miniature around the larger outer planets, which formed systems of numerous satellites, or moons.For hundreds of millions of years the young planets and moons were bombarded by leftover debris. Finally the planetary system became established: in our case, with four small, rocky inner planets, and four giant outer planets, mostly consisting of hydrogen and helium, and their systems of moons; an odd ball rocky planet, Pluto, in the “wrong’ place at the edge of the system; and assorted debris of rock and ice- the asteroids, comets and meteoric bodies.