Is the water on Earth older than the Sun?

Water carries a signature. A small fraction of it is made with deuterium, a heavier form of hydrogen, instead of the ordinary kind, and the proportion depends on where and how cold the water was when it formed. Water that froze in the deep cold of interstellar space ends up unusually rich in deuterium.

Astrochemist Ilsedore Cleeves and colleagues modelled the disc of gas and dust that became the solar system and asked whether it could have made water with as much deuterium as we find in Earth's oceans and in comets today. It could not. The disc was too warm and too bright. The only way to get the numbers to match was for a large fraction of the water to have arrived already made, as ice inherited from the cloud the Sun formed out of.

The Sun is about 4.6 billion years old. Some of the water in every ocean, every cloud, and every cup of tea is older than that, and has been water the whole time. It was ice drifting between the stars before there was a star here to orbit.