Using the Atacama Large Millimeter/submillimeter Array (ALMA), astronomers have detected gaseous water in the planet-forming disk around the star V883 Orionis. This water has a chemical signature that explains water’s journey from star-forming gas clouds to planets, supporting the idea that water on Earth is even older than our sun.

“We can now trace the origin of water in our solar system to before the formation of the sun,” says John J. Tobin, an astronomer at the National Radio Astronomy Observatory, USA and lead author of the study published today in Nature.

This discovery was made by studying the composition of water in V883 Orionis, a planet-forming disc about 1,300 light-years away from Earth. When a cloud of gas and dust collapses, it forms a star in the center. Material from the cloud also forms a disc around the star. Over the course of a few million years, the matter in the disk clumps together to form comets, asteroids and eventually planets. Tobin and his team used ALMA, in which the European Southern Observatory (ESO) is a partner, to measure the chemical characteristics of the water and its path from the star-forming cloud to planets.

Water usually consists of one oxygen atom and two hydrogen atoms. Tobin’s team studied a slightly heavier version of water in which one of the hydrogen atoms has been replaced with deuterium – a heavy isotope of hydrogen. Because simple and heavy water are formed under different conditions, their ratio can be used to trace where and when the water formed. For example, this ratio has been shown to be similar in some solar system comets to that in water on Earth, suggesting that comets may have provided water to Earth.

ALMA images of the planet-forming disc around the star V883 Orionis
ALMA images of the disk around the star V883 Orionis, showing the spatial distribution of water (left, orange), dust (center, green), and carbon monoxide (blue, right). Because water freezes at higher temperatures than carbon monoxide, it can only be detected in gaseous form closer to the star. The apparent gap in the water and carbon monoxide images is actually due to the bright emission of the dust, which attenuates the emission of the gas. Credits: ALMA (ESO/NAOJ/NRAO), J. Tobin, B. Saxton (NRAO/AUI/NSF)

The journey of water from clouds to young stars, and later from comets to planets, has been observed before, but until now the link between the young stars and comets has been missing. “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 confirms 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.”

But observing the water proved difficult. “Most of the water in planet-forming discs is frozen as ice, so it’s mostly hidden from our view,” says co-author Margot Leemker, a PhD candidate at Leiden Observatory in the Netherlands. Gaseous water can be detected thanks to the radiation emitted by molecules as they rotate and vibrate, but this is more complicated when the water is frozen, where the movement of molecules is more restricted. Gaseous water is toward the center of the disks, close to the star, where it is warmer. However, these near regions are hidden by the dust disk itself and are also too small to be imaged with our telescopes.

Fortunately, the V883 Orionis disk was found to be unusually hot in a recent study. A dramatic burst of energy from the star heats the disk, “to a temperature where water is no longer in the form of ice, but in gas, which allows us to detect it”, says Tobin.

From gas clouds to disks to planetary systems (artist's impression)
This diagram illustrates how a cloud of gas collapses to form a star with a disk around it, which will eventually form a planetary system. Credits: ESO/L. Calcada

The team used ALMA, an array of radio telescopes in northern Chile, to observe the gaseous water in V883 Orionis. Thanks to its sensitivity and ability to discern fine details, they were able to detect the water as well as determine its composition, as well as map its distribution within the disk. The observations showed that this disk contains at least 1,200 times the amount of water in all of Earth’s oceans.

In the future, they hope to use ESO’s forthcoming Extremely Large Telescope and its first-generation instrument METIS. This mid-infrared instrument will be able to resolve the gas phase of water in these types of disks, strengthening the connection of water’s path from star-forming clouds to solar systems. “This gives us a much more complete picture of the ice and gas in planet-forming discs,” concludes Leemker.

Magazine reference

  1. John J. Tobin, Merel LR van’t Hoff, Margot Leemker, Ewine F. van Dishoeck (Leiden), Teresa Paneque-Carreño, Kenji Furuya, Daniel Harsono, Magnus V. Persson, L. Ilsedore Cleeves, Patrick D. Sheehan and Luke Ciza. Deuterium-enriched water connects planet-forming discs to comets and protostars. Nature DOI: 10.1038/s41586-022-05676-z