In a discovery that could reshape our understanding of the solar system’s origins, astronomers have identified semi-heavy water ice (HDO) surrounding a young protostar, suggesting that some of Earth’s water predates the Sun itself.
Using the James Webb Space Telescope, researchers from Leiden University and the National Radio Astronomy Observatory (NRAO) confirmed the detection of HDO ice around L1527 IRS, a sunlike protostar located about 460 light-years away in the Taurus constellation.
“Now, with the unprecedented sensitivity of Webb, we observe a beautifully clear semi-heavy water ice signature toward a protostar,” said Katie Slavicinska, the study’s lead author and a PhD candidate at Leiden.
HDO: A Molecular Fossil From Interstellar Space
HDO, or semi-heavy water, forms when one hydrogen atom in H₂O is replaced by deuterium—a heavier isotope of hydrogen. Its abundance in cold interstellar environments makes it a key tracer of water’s ancient origins. High HDO levels have previously been found in comets, icy moons, and even Earth’s oceans.
But until now, direct detection around a sunlike forming star was elusive.
“The protostar L1527 is similar to what we think our Sun was like during its formation,” said John Tobin of NRAO, co-author of the paper published in The Astrophysical Journal Letters. “Finding HDO here strengthens the argument that our solar system inherited water from the interstellar medium.”
Tracing the Solar System’s Water Lineage
The study found that HDO levels in L1527 are slightly higher than those in Earth’s oceans and some comets. This raises new questions: Is it due to chemical processing over time? Or are there differences in the clouds where these stars form?
To answer that, Slavicinska and Tom Megeath of the University of Toledo are leading a broader Webb survey of 30 additional protostars and dark clouds. In parallel, Tobin is spearheading a campaign using the Atacama Large Millimeter/submillimeter Array (ALMA) to map HDO in surrounding regions.
These efforts aim to build a timeline of how interstellar water evolves as stars—and eventually, planets—form.
If confirmed on a wider scale, this discovery means that some of the water in your glass may be over 4.6 billion years old, formed long before Earth—or even the Sun—existed.
