CATCHING A COMET BY THE TAIL

The story of life in the universe is a story of stars. As the first clouds of gas formed stars in the infant universe, more than 13 billion years ago, the universe contained only hydrogen, helium, and a few other trace light elements. The nuclei of these light elements were forged in the intense heat a few minutes after the big bang, when the entire universe was as hot as the core of the Sun is now. As the universe rapidly expanded, radiation eased its grip and a scant half million years after the big bang, it had cooled enough for electrons to mate with nuclei and for hydrogen and helium atoms to form. Chemistry was now possible, but a universe made of the two simplest elements is singularly dull—hydrogen atoms can only join to form a hydrogen molecule, while helium is inert.

As the first stars congealed out of the expanding gas, there were no planets because there was nothing to make them out of. There was no life because there was no carbon and no nitrogen and no oxygen.1 Our existence on a rocky planet depends on generation after generation of stars fusing heavy elements in their cores and ejecting them into space to become the raw material for solar sys – tems.2 The fireworks couldn’t start until gravity had used its long reach to gather matter into concentrations dense enough to coun­ter the omnipresent cosmic expansion. This took several hundred million years. But then the pockets of spherical collapse ignited le­gions of stars that could slam atomic nuclei together hard enough for them to fuse and populate the periodic table for the first time.

Every carbon atom in our bodies was once in a star in a remote region of space more than 4.5 billion years ago. Some atoms have cycled through multiple generations of stars; their myriad stories played out over eons until they were co-opted and incorporated into our fleeting human story. We are made of stardust.

Understanding the way in which the products of stellar fusion enriched the nebula that formed the Sun and planets requires finding primordial material in the Solar System. The most pris­tine samples available are certain types of meteorites and comets. There may be as many as a trillion comets and they spend most of their time far from the Sun and Earth in the deep freeze of space. Material from the outer Solar System has been radioactively dated back to 4.567 billion years, which is taken to be the formation epoch. The spherical comet cloud extends to 100,000 Earth-Sun distances and it’s a tenuous relic of the time when the Sun switched on for the first time. In the outer part of their orbits, comets are dark and dead, but they become lively and visible when they ap­proach the Sun. This diaphanous shroud of frozen worlds holds important clues to our origins.