JWST Detects Semi‑Heavy Water on Distant Giant Exoplanet Suggesting Unusual Origins

The James Webb Space Telescope has recorded spectral signatures that suggest the presence of semi‑heavy water (HDO) in the atmosphere of the distant gas giant WASP‑39b, a world orbiting a star roughly 700 light‑years from Earth.

JWST Reveals Deuterium‑Enriched Water on a Hot Jupiter

Using its full suite of four infrared instruments, JWST captured an unprecedented transmission spectrum of WASP‑39b, allowing researchers to isolate the faint imprint of HDO—a water molecule in which one hydrogen atom is replaced by deuterium. The deuterium‑to‑hydrogen (D/H) ratio derived from this feature can serve as a tracer of the planet’s water origin and its atmospheric history.

The findings, posted on the preprint server arXiv, stem from a comparison of detailed atmospheric models of WASP‑39b with the actual JWST observations. The models accounted for temperature gradients, cloud coverage, molecular abundances, and the way the atmosphere absorbs and scatters starlight.

The analysis indicates that the D/H ratio in WASP‑39b’s atmosphere is markedly higher than the values measured for the giant planets of our own solar system, hinting at distinct processes that have altered the planet’s water inventory.

Harsh Stellar Irradiation Could Enrich Deuterium

WASP‑39b is a low‑density, intensely heated gas giant that orbits extremely close to its host star. This proximity subjects its atmosphere to strong radiation, which may drive selective escape of lighter water molecules that contain ordinary hydrogen. Over geological timescales, the preferential loss of H‑bearing water could leave behind a reservoir enriched in HDO.

Additional mechanisms such as vertical mixing and photochemical reactions triggered by stellar photons may also modulate the observed D/H ratio, though the precise contribution of each process remains under investigation.

Formation Beyond the Snow Line May Explain the Signature

An alternative hypothesis suggests that WASP‑39b originated farther from its star, in a cooler region beyond the system’s snow line, where icy planetesimals rich in deuterium‑enhanced water could have been incorporated into the planet’s early envelope. Subsequent inward migration would have brought the planet to its current tight orbit while preserving the elevated D/H imprint.

“Observations of protostellar ices from this region that show a degree of deuterium‑enrichment consistent with the D/H ratio inferred from WASP‑39 b,” the authors noted, adding that “determining whether this scenario could produce the observed values requires detailed chemical modeling.”

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Even though WASP‑39b is inhospitable to life, with atmospheric temperatures nearing 1,000 °C (1,832 °F), the detection of semi‑heavy water underscores JWST’s capacity to dissect the chemical makeup of distant worlds. HDO remains a valuable probe because its spectral signature stands out clearly against the background of other atmospheric constituents.