Atmosphere of giant planet TOI-6894 b reveals ammonia and chemical signatures

Star system: TOI-6894 | Distance from the Sun: 238 light years

TOI-6894 b is a temperate gas giant planet orbiting a red dwarf star. With a temperature of 420 Kelvin, this sub-Saturn sized world is a unique object of study as the only known giant planet transiting a very low-mass red dwarf star. Recent observations have provided a clear look at its chemical composition, showing that its atmospheric structure and metal content closely mirror those of the gas giants within our own solar system.

The planet's atmosphere contains significant levels of methane and carbon disulfide, a chemical produced through photochemistry—a process where starlight drives chemical reactions in the atmosphere. The analysis also confirms the presence of ammonia, which is visually evident for the first time in an exoplanet spectrum, alongside water vapor and carbon dioxide.

To identify these components, researchers used transit spectroscopy, which involves analyzing starlight as it filters through the planet's outer layers during a transit. By comparing these observations against complex computer models that simulate atmospheric chemistry and metal concentrations, the team determined the planet's atmospheric properties.

The results indicate that the planet has a metallicity—the abundance of elements heavier than hydrogen and helium—three to ten times higher than that of the Sun. Despite the low mass of its host star, the planet shows solar-like ratios of carbon, nitrogen, sulfur, and oxygen, reinforcing the idea that it formed in a manner similar to the gas giants in our neighborhood.

Key findings
- Ammonia detected for the first time in an exoplanet atmosphere
- Methane and carbon disulfide identified as major atmospheric components
- Atmospheric composition shows strong similarities to Jupiter and Saturn
- Elemental ratios consistent with solar values
- Metallicity measured at 3 to 10 times solar levels

Scientific study

C, N, O, S, and photochemistry in a temperate giant planet orbiting a late M dwarf (external link)