Detection of ancient water ice suggests interstellar origins predating the Sun

Detection of ancient water ice suggests interstellar origins predating the Sun
by Robert Schreiber
Berlin, Germany (SPX) Jun 17, 2025

A team of astronomers from Leiden University and the National Radio Astronomy Observatory has confirmed the first robust detection of semi-heavy water ice surrounding a young sunlike star, strengthening the hypothesis that some of the solar system's water predates the Sun itself. Their study, based on observations from the James Webb Space Telescope, was published in The Astrophysical Journal Letters.

The researchers focused on the deuteration ratio-the abundance of water molecules where one hydrogen is replaced by deuterium, forming HDO, or semi-heavy water. High deuteration indicates formation in extremely cold environments, such as the primordial clouds of gas and dust where stars originate.

While Earth's oceans, icy moons, and comets exhibit higher HDO levels than the Sun, this discovery of abundant semi-heavy water ice in a protostellar envelope offers direct evidence of its interstellar origins. The findings suggest that much of this water existed long before the solar system began to take shape.

Lead author Katie Slavicinska, a PhD student at Leiden University, highlighted the breakthrough: "Now, with the unprecedented sensitivity of Webb, we observe a beautifully clear semi-heavy water ice signature toward a protostar."

The protostar studied, L1527 IRS, lies 460 light-years away in the Taurus constellation and closely resembles the early Sun. Co-author John Tobin of NRAO noted, "In several ways, it is similar to what we think our Sun was when it began to form."

The measured HDO ratio in L1527 matches levels found in some comets and young protoplanetary disks, suggesting a shared chemical heritage shaped in cold interstellar environments. However, L1527's HDO levels are slightly higher than those found in comets and on Earth, possibly due to later chemical processing or intrinsic differences in star-forming clouds.

To probe these discrepancies, further observations are underway. Slavicinska and Tom Megeath of the University of Toledo are leading Webb programs targeting 30 additional protostars and dark clouds. Tobin is also conducting complementary studies using the Atacama Large Millimeter/submillimeter Array to trace HDO gas in related regions.

Research Report:HDO ice detected toward an isolated low-mass protostar with JWST