Earth used to be a dry and hostile planet, not suitable for life at all. It is speculated by scientists that water arrived on our planet quite by chance—or should I say, crash-landed?

There is fresh evidence, published in the Monthly Notices of the Royal Astronomical Society and led by the University of Warwick, to suggest that it was water-rich asteroids and comets that acted as our galactic water delivery vehicles. How can scientists possibly know this? Astronomers have a cunning method: They look towards celestial objects that have a very high gravitational attraction that could feasibly draw in a local, watery asteroid—white dwarfs.

A star becomes a white dwarf in the final stages of its life. When a star 'dies,' it ejects its outer layers of gas in a dramatic celestial explosion known as a nova. Only the hot, dense center of the star remains; this is the white dwarf. Its strong gravitational attraction means that a white dwarf could feasibly slurp up a waterlogged asteroid and rip it up to its basic elements. These basic elements will then orbit the white dwarf and slowly become integrated into its core.

By examining the light emitted by white dwarfs, scientists can get a good indication of which elements are trapped inside them. For this particular study, astronomers looked at the white dwarf star SDSS J1242+5226 using the William Herschel Telescope. The light spectrum that this white dwarf emits indicates a large quantity of hydrogen and oxygen: the ingredients for making water (H₂O). The amounts of hydrogen and oxygen within the white dwarf are the right ratios to suggest that they used to be water. A water-bearing asteroid that was sucked into the white dwarf is a good explanation for how these elements wound up in the white dwarf.

For comparison, if we could combine the hydrogen and oxygen in the white dwarf into water, we would have a quantity 30 to 35% the volume of Earth's oceans. In raw size, this would be more than 400 million cubic kilometers (96 million cubic miles) of water.

"Oxygen, which is a relatively heavy element, will sink deep down over time, and hence a while after the disruption event is over, it will no longer be visible," explained Professor Boris Gänsicke from the University of Warwick. 

"In contrast, hydrogen is the lightest element; it will always remain floating near the surface of the white dwarf where it can easily be detected. There are many white dwarfs that hold large amounts of hydrogen in their atmospheres, and this new study suggests that this is evidence that water-rich asteroids or comets are common around other stars than the Sun."

This exciting research hints that it isn't just Earth that received water via asteroids, but other planetary systems as well.