Water in the Cosmos - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 4

hydrogen supply of any star, large or small, begins to be exhausted.
Helium and larger nuclei then fuse to produce nuclei up to the size of
iron, releasing yet more energy. In a large star, that process ends in the
catastrophic explosion of a supernova, the source of all elements
heavier than iron. Depending on the star, and where it is in its history,
there are different pathways that lead to different elements.
Oxygen happens to be at the end of some of the most common of
these nuclear fusion pathways, and so has, over time, become—by
far—the next most common element after hydrogen and helium.
Indeed, its total amount in the universe is probably equal to the total
amount of all of the other elements (other than hydrogen or helium)
put together. The universe, hence, is oxygen-rich. It makes up, for
example, almost a third of the whole of the Earth by mass, almost all
of it bound up in minerals, particularly in the silicates of the mantle
and crust. It is, however, a ferociously reactive element, which is why
free oxygen is a rarity in planetary atmospheres—the only 5 exception
we know of being the Earth.
When, in the history of the universe, did that oxygen begin to
link with what is left of the dying star's hydrogen to form water, in
the outrushing material of the nova or supernova? This process is
usually assumed to have begun hundreds of millions of years into
the history of the universe, with the emergence of the first stars in
the billowing gas clouds. But it might, perhaps, have started much
earlier.
The Harvard astrophysicist Abraham Loeb, in 2013, produced a
provocative calculation that suggested that the first stars and planets
might have begun forming just 15 million years into the history of the
universe. 6 At this time, the temperature of the afterglow of the Big
Bang would have cooled to somewhere between 0 and 30 degrees Cel-
sius (it has now cooled to just 3 degrees above absolute zero at −273.15
degrees Celsius). Therefore, Loeb suggested, any planets that might