A group of astronomers, including some from Leiden, has found a missing link in the path that water takes before landing on comets, asteroids and planets. Their findings support the hypothesis that water on Earth is even older than the Sun. They published their findings on 8 March in Nature.

Stars form when a cloud of gas and dust in the universe collapses. The centre becomes a star, and around it orbits a disk of gas and dust. This material clumps together over millions of years to form comets, asteroids and eventually planets. Water had already been found in comets, asteroids and planets, but not yet in the rotating dust disk. Astronomers can use the ratio of water (H2O) to heavy water (HDO) to determine where and when water formed.

John J. Tobin and his co-authors have now found water and heavy water in the disk around the star V883 Orionis, about 1,300 light-years from Earth in the constellation Orion. For unknown reasons, the gas disk around this star is warmer than normal, making the water gaseous and allowing astronomers to detect it with telescopes. Using the Atacama Large Millimetre Array (ALMA), they were also able to determine the H2O/HDO ratio, which in this cloud is very similar to the ratio found in cold star-forming clouds and comets in our Solar System.

‘So the ratio of heavy water to water is the same at the beginning and at the end of the water’s journey,’ co-author Margot Leemker said in a Leiden Observatory press release. ’This confirms the idea that water is formed in interstellar space and arrives relatively unchanged on comets and planets. Water has probably also taken this route near our Sun and our Earth.’