Now, astronomers using JWST have modeled the atmospheric properties of the exoplanet WASP-39b and detected the presence of semi-heavy water, a potential habitability marker, on this alien world, located nearly 700 light-years from Earth. The exoplanet WASP-39b being blasted by its Sun-like star. Measuring the water D/H ratio of this planet would offer invaluable constraints on its chemical environment and formation pathway," they explain. A comparison of the D/H ratio and HDO to H 2 O ratios of various celestial bodies and components. Bacon/STScI)Other tracers of the D/H ratio might also provide more insight, but for now, semi-heavy water is the best indicator we've got.