Is our water older than the sun? Astronomers find clue in ice around young star
A team led by Leiden University in the Netherlands and the National Radio Astronomy Observatory have, for the first time, robustly detected semi-heavy water ice around a young sunlike star. In this ice, some of the ordinary hydrogen atoms have been replaced by deuterium, a heavier variant of hydrogen.
One way that astronomers trace the origin of water is through measuring its deuteration ratio. That is the fraction of water that contains one deuterium atom instead of one of the hydrogens. So instead of H2O, it's HDO, which is also called semi-heavy water. A high fraction of semi-heavy water is a sign that the water formed in a very cold place, such as the primitive dark clouds of dust, ice, and gas from which stars are born.
In our oceans and in comets and on icy moons, as high as one out of a couple of thousand water molecules consists of semi-heavy water. This is about ten times higher than expected based on the composition of our sun. Therefore, astronomers hypothesize that some of the water in our solar system originated as ice in dark clouds, hundreds of thousands of years before the birth of our sun. To confirm this hypothesis, they must measure the deuteration ratio of water ice in such star-forming regions.
An international team of astronomers have now detected such a high ratio of semi-heavy water ice in a protostellar envelope. That is the cloud of material that surrounds a star in its embryonic stages. The results strengthen the case that some of the water in our solar system formed before our Sun and the planets.
Beautifully clear signature
The astronomers used the James Webb Space Telescope. Before its launch, the water deuteration ratio in star-forming regions could only be reliably measured in the gas phase, where it can be chemically altered. 'Now, with the unprecedented sensitivity of Webb, we observe a beautifully clear semi-heavy water ice signature toward a protostar,' says Katie Slavicinska, a PhD student from Leiden University (Netherlands) that led the study.
The protostar in question is L1527 IRS, located in the constellation of Taurus about 460 light-years from Earth. 'In several ways, it is similar to what we think our Sun was when it began to form,' says John Tobin, from the National Radio Astronomy Observatory in Virginia (USA) who leads one of the Webb programs responsible for the observations.
The L1527 water deuteration ratio is very similar to the ratio of some comets as well as the protoplanetary disk of a more evolved young star, which suggests similar cold and ancient chemical origins of the water found in all of these objects.
'This finding adds to the mounting evidence that the bulk of water ice makes its journey largely unchanged from the earliest to the latest stages of star formation,' says co-author Ewine van Dishoeck, a professor of astronomy at Leiden University who has spent much of her career tracing the journey of water through space.
30 protostars and clouds on their way
Nevertheless, the measured water ice deuteration ratio in L1527 IRS is slightly higher than the ratios measured in some comets of our solar system and the water ratio on Earth. A variety of factors could cause such a difference. For example, some of the water on those comets and on Earth could have been chemically altered in the disk. Or the dark cloud that formed our Sun may differ from the dark cloud were L1527 IRS formed.
More observations of semi-heavy water ice are planned to investigate possible reasons for these differences. Slavicinska and co-author Tom Megeath, a professor of astronomy at the University of Toledo (USA), lead several Webb programs that will expand the search for HDO ice to 30 new protostars and primitive dark clouds. Meanwhile, Tobin leads complementary observations with the Atacama Large Millimeter/submillimeter Array that will look for HDO gas in several of the same targets.
Scientific paper
HDO ice detected toward an isolated low-mass protostar with JWST. By: K. Slavicinska, Ł. Tychoniec, M. G. Navarro, E. F. van Dishoeck, J. J. Tobin, M. L. van Gelder, Y. Chen, A. C. A. Boogert, W. B. Drechsler, H. Beuther, A. Caratti o Garatti, S. T. Megeath, P. Klaassen, L. W. Looney, P. J. Kavanagh, N. G. C. Brunken, P. Sheehan, W. J. Fischer. In: The Astrophysical Journal Letters, 16 June 2025. [original | preprint]
Webb Reveals the Protostar L1527 IRS — Visualization
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