A Mirror Planet in the Neptunian Desert express
Hundreds of light-years away, there's a planet that looks like a giant mirror. LTT9779b reflects most of the light that hits it — far more than you'd expect from a Neptune-sized body. Astronomers have long debated what makes it so shiny: a thick carbon-rich haze or a layer of clouds.
The James Webb Space Telescope has helped settle the debate. It observed the planet's dayside in the infrared and captured a spectrum with an exceptionally high signal-to-noise ratio. In that spectrum, clear signatures of condensates, tiny solid particles in the atmosphere, emerged. With high confidence (between 3 and 5 sigma), scientists identified these particles as Mg₂SiO₄, magnesium silicate. In other words, on LTT9779b, it rains molten rock.
The cloud layer explains the high reflectivity without invoking an extremely metal-rich atmosphere. Additionally, carbon monoxide (CO) and carbon dioxide (CO₂) were reliably detected in the atmosphere, and the carbon-to-oxygen ratio was estimated. It turned out to be 0.984 ± 0.019—notably higher than the solar value. This links LTT9779b to ultra-hot Jupiters and points to a common mechanism: in scorching atmospheres, oxygen gets locked into refractory particles that sink deeper, while carbon remains in the gas phase. The mirror planet turned out to be a trap for oxygen as well.
Original abstract
as written by the authors arXiv:2606.26075
Discovered deep within the "Neptunian desert", LTT9779b remains the only known ultra-hot Neptune, prompting significant speculation regarding its unique formation and evolutionary history. Its exceptionally high geometric albedo has previously been attributed either to the presence of clouds or to an extremely metal-rich atmosphere. Here, we present a comprehensive panchromatic analysis of its dayside atmosphere using JWST NIRISS and NIRSpec/G395H observations to characterize its atmospheric structure and composition. Leveraging the exceptional signal-to-noise ratio (S/N) in the observed spectra, we report a 3-to-5$$\sigma$$ detection of dayside clouds, with strong evidence for Mg$$_2$$SiO$$_4$$(s) (silicate) condensation. This constitutes the first statistically significant detection of clouds on the dayside of a Neptunian-mass exoplanet. We demonstrate that a highly reflective cloud deck, rather than an extremely high-metallicity atmosphere, is the most likely explanation for the planet's anomalously high optical albedo. Furthermore, our atmospheric retrievals yield robust detections of both CO ($$\sim$$4.88$$\sigma$$) and CO$$_2$$ ($$\sim$$8.76$$\sigma$$), while providing tentative constraints on the H$$_2$$O abundance and upper limits on SiO, TiO, and VO. Finally, our analysis places a robust constraint on the C/O ratio of 0.984 $$\pm$$ 0.019. This aligns LTT9779b with other known ultra-hot Jupiters exhibiting super-solar C/O ratios, suggesting a broader trend driven by the sequestration of oxygen-bearing condensates in ultra-hot atmospheres.
The dayside temperature of LTT9779b reaches about 2000 degrees Celsius — enough to not just melt silicates but vaporize them, after which they condense into clouds, much like water in Earth's atmosphere.
A planet with clouds of molten rock evokes worlds from 'Star Wars' or 'Dune,' only instead of sand and deserts, there's scorching silicate haze.