Ultrahot Neptune LTT 9779 b has clouds in its atmosphere

LTT 9779 b is a Neptune-sized planet that orbits extremely close to its parent star, placing it within the so-called hot Neptune desert - a region where such planets are rarely found due to intense stellar radiation. Observations of this planet are essential to understanding how its atmosphere manages to persist under these extreme conditions. New data from the James Webb Space Telescope provides a broad look at the chemical and physical characteristics of this atmosphere.

The research utilized a wide range of light wavelengths to create a transmission spectrum, which is a method of analyzing starlight as it passes through the outer layers of a planet's atmosphere during a transit. By observing which wavelengths of light are blocked, the composition and physical state of the atmospheric gases can be determined.

The results indicate that the spectral signatures of the atmosphere are muted, a clear indicator that clouds are present in the sky of LTT 9779 b. Atmospheric models suggest that these clouds are likely composed of small, sub-micron particles. The study also calculated a relatively low temperature at the terminator—the line separating the day and night sides of the planet—ranging from approximately 560 to 950 Kelvin depending on the chemical model used.

Further observations in the mid-infrared range are required to identify the specific minerals forming these clouds. Confirming the cloud composition will allow researchers to accurately determine the metallicity, or the abundance of elements heavier than hydrogen and helium, within the planet's atmosphere.

Key findings
- The atmosphere of LTT 9779 b contains clouds that obscure specific chemical signatures.
- The observed spectrum is consistent with small, sub-micron cloud particles.
- The terminator temperature of the planet is estimated between 560 and 950 Kelvin.
- High eddy diffusion, indicating vigorous vertical mixing of gases, is likely present in the atmosphere.
- Future mid-infrared data is needed to distinguish between different cloud species and chemical abundances.

Scientific study

Cool, Cloudy Nights on an Ultrahot Neptune: A Panchromatic NIRISS/NIRSpec Transmission Spectrum of LTT 9779 b (external link)