You might assume that there has always been water on Earth — that water was there from the very beginning when our planet formed. But scientists increasingly think that water on Earth may have originated elsewhere, and been carried here by comets. However, the water in the comets had to come from somewhere, and astronomers recently made a discovery which could shed light on how that water was found in the solar system.
They were able to compare two different types of water, the one that has one oxygen atom and two hydrogen atoms that we know well, and the other, which is a heavier type of water that has one oxygen, one hydrogen, and one deuterium atom. These types of water form differently, so scientists can use the ratio of normal to heavy water as a fingerprint to see how old the water is and where it comes from.
“The composition of the water in the disc is very similar to that of comets in our own solar system. This is confirmation of the idea that the water in planetary systems formed billions of years ago, before the sun, in interstellar space, and has been inherited by both comets and Earth, relatively unchanged.”