When Did the Universe Have the Right Stuff for Planets?
To build a planet you need lots of rubble and that means lots of heavy elements – stuff more massive than atoms of hydrogen and helium. The elemental composition of the collapsing nebula that gave birth to the Sun and the planets of the Solar System included things like iron, silicon and magnesium that form the bulk of rocky planets, and carbon, oxygen, nitrogen, potassium and other such elements that are essential for life. However, these materials were present in just trace quantities, amounting to no more than two percent of the entire nebula that was otherwise dominated by hydrogen (74%) and helium (24%). Yet this gaseous cloud was huge; it is estimated that it harbored enough heavy elements to build at least thirty planets like Earth.
These heavy elements – ‘metals’ in astronomer-speak – don’t just materialize out of nothing. They are the products of fusion power within stars, subsequently spewed out across the cosmos on the blast waves of supernovae, lacing the interstellar medium with the raw ingredients for planets. To build up enough of these materials, many stars must first live and die, each one contributing to the evolving chemistry of the universe, but how much material is really required to build a planet and how quickly did the universe accrue a sufficient level to do so?