Water on Earth is older than the Sun
Astronomers find the missing link for water in our solar system.
Radio astronomers have detected water vapour in a planet-forming disc of matter surrounding the star V883 Orionis, some 1,300 light years from Earth in the constellation of Orion. The chemical signature is said to explain the journey of water from star-forming nebulae to planets, and supports the idea that water on Earth is older than the Sun.
The discovery1 was made using the Atacama Large Millimeter/submillimeter Array (ALMA) radio telescope, a facility operated in large part by the European Southern Observatory (ESO) high on the Chajnantor plateau in the Chilean Andes. ALMA detects photons with wavelengths of around a millimetre, between the infrared and radiofrequency parts of the electromagnetic spectrum.
When a cloud of gas and dust collapses under the influence of gravity, it forms a hydrogen-burning star at its centre. Around this protostar, material from the cloud is shaped into a flat disc, and, over the course of millions of years, solid matter in the disc clumps together to form comets, asteroids, and eventually planets.
Radio astronomer John Tobin and his team used ALMA to measure chemical signatures of the water and its path from the star-forming cloud to planets. The researchers studied a heavy version of water in which one of the two hydrogen atoms is replaced with deuterium - a heavy isotope of hydrogen with a neutron as well as the usual proton and electron.
Given that standard and heavy water form under different conditions, their relative abundance in a planetary nebula can indicate when and where the water was formed. In the case of our solar system, the ratio is similar to that in water here on Earth, which suggests that comets may have delivered water to Earth.
“V883 Orionis is the missing link in this case,” says Tobin. “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.”
The challenge for the astronomers is in observing the water, much of which is frozen in ice, and therefore hidden from view of telescopes that measure the molecular emission spectra of gases. In planetary nebulae, gaseous water is normally found towards the warmer centre of the discs, but there it is partially hidden by dust in regions too small to be detected with current technology.
V883 Orionis has the advantage of being unusually hot as we see it right now, with an energy burst from the star converting the ice into gas. Tobin and his team mapped the composition and distribution of water vapour in the disc, and found that it contains at least 1,200 times the amount of water in all of Earth’s oceans.
The next step is to use ESO’s upcoming Extremely Large Telescope and its infrared detector METIS to better resolve the gas-phase of water in protoplanetary discs, and trace the path of water all the way from star-forming clouds to solar systems.
Tobin et al., “Deuterium enriched water ties planet forming disks to comets and protostars”, Nature (2023); doi:10.1038/s41586-022-05676-z.