James Webb Space Telescope Detects 'Heavy' Water on Exoplanet WASP-39b

James Webb Space Telescope Detects 'Heavy' Water on Exoplanet WASP-39b

The James Webb Space Telescope (JWST) is revolutionizing our understanding of the universe. With its remarkable infrared sensitivity, the telescope is becoming increasingly pivotal in the study of alien atmospheres, marking what NASA has called a true “new era” in exoplanet research. Among the most intriguing targets of this research is water (including “heavy water”), a crucial element believed to be essential for the emergence of life as we know it.

Discovery on WASP-39b

A team of astronomers has investigated the atmosphere of the exoplanet WASP-39b, a gas giant located nearly 700 light-years from Earth, and has identified traces of semi-heavy water. This molecule resembles regular water, but one of its two hydrogen atoms is replaced by deuterium, an isotope that contains an additional neutron, making it slightly heavier. The researchers focused on the deuterium-hydrogen (D/H) ratio present in the planet's atmosphere, a critical detail that can provide significant insights into planetary formation processes. WASP-39b was selected for study as it is one of the most observed exoplanets, often referred to as a “benchmark exoplanet”, having been studied by all four main instruments of the JWST.

A Surprisingly High D/H Ratio

By comparing theoretical atmospheric models with the actual data collected by the telescope, the team discovered that the D/H ratio of WASP-39b is significantly higher than that of the gas giants in our Solar System, which are the celestial bodies it should resemble most closely.

One possible explanation for this elevated ratio lies in the concept of atmospheric escape: heavy water, with its slightly higher molecular mass, tends to disperse into space more slowly than regular water. Given that WASP-39b is a hot, puffy planet with reduced gravity, it may have lost a significant portion of its “light” water over time. Another hypothesis suggests that the planet formed in the colder, outer regions of its stellar system, beyond the so-called “snow line” of the protoplanetary disk, inheriting deuterium-rich material before migrating closer to its star.

A Potential Indicator of Habitability

According to the study's authors, semi-heavy water remains the most accessible tracer due to its distinctive spectral characteristics, and its detection may soon be possible on temperate rocky planets, where it could serve as a true marker of habitability. While WASP-39b is not a hospitable candidate—temperatures soar to nearly 1,000 degrees Celsius—the technique developed for this extreme world could prove crucial in the search for planets capable of hosting life elsewhere in the universe.

Source:

• https://arxiv.org/abs/2607.19579