Clouds in the atmosphere of the ultra-hot Neptune reflected starlight.

The authors of a new study led by Louis-Philippe Coulombe from the University of Montreal (Canada) have presented new data about the exoplanet LTT 9779 b — a rare ultra-hot Neptune with a notably high albedo (the ability to reflect light), which is uncommon for such "hellish" worlds, as they typically absorb nearly all incoming light.

Moreover, the temperature on the "day" side of this planet exceeds 2000 °C, and it completes a full orbit around its parent star — the yellow dwarf LTT 9779 — in just 19 hours. Due to its proximity to the star, astronomers have dubbed this exoplanet a resident of the "hot Neptune desert" — a region around the star where such giants are rarely found.

Now, by observing LTT 9779 b with the James Webb Space Telescope, astronomers have been able to "fast-forward" the planet's complete orbit around the star and discovered that the western part of its day hemisphere is significantly brighter than the eastern part due to white, highly reflective clouds. The results of this scientific work are published in the journal Nature Astronomy.

To understand how the high surface temperature of the planet affects its atmospheric conditions, researchers utilized the NIRSpec and NIRISS SOSS spectrographs onboard Webb. The observations lasted nearly 22 hours: during this time, LTT 9779 b completed a full orbit, allowing astronomers to "peek" not only at the transit (the planet passing in front of the star) but also at two "losses to view" (when the planet hid behind the star).

The results indicated that the western half of the "day" side reflected up to 79 percent of the incoming light, while the eastern side reflected approximately 41 percent. The authors of the study suggested that this difference is due to powerful winds that sweep through the atmosphere of the ultra-hot Neptune, carrying heat eastward.

This means that the western part of the planet is slightly "cooler," allowing mineral clouds composed of silicates (such as MgSiO₃ or Mg₂SiO₄), which reflect bright light, to condense, while on the hotter eastern side, they quickly evaporate. Additionally, the temperature on the opposite (night) side of LTT 9779 b is much lower, with its glow being faint.

The discovery of clouds on such a rare exoplanet indicates a complex interaction between the atmosphere, winds, and cloud condensation at extreme temperatures. Further observations of LTT 9779 b will help explain how this ultra-hot Neptune has managed to survive in such a harsh desert.