Water vapor detected in the atmosphere of ultrahot Neptune LTT 9779 b

Researchers have analyzed the atmosphere of the extreme exoplanet LTT 9779 b, an ultrahot Neptune. By using high-resolution spectroscopy, they identified water vapor on the planet's dayside and found no signs of strong atmospheric escape. The data suggest the planet does not have a thermal inversion, meaning its temperature decreases normally with altitude, and align with expectations of a tidally locked world.

Star system: LTT 9779 | Distance from the Sun: 264 light years

The exoplanet LTT 9779 b exists in an environment known as the hot Neptune desert, a region where planets of this size are rarely found due to intense radiation from their host stars. This planet orbits so close to its star that its atmosphere experiences extreme heat, making it a difficult but important target for atmospheric study. By examining light filtered through the planet's atmosphere and thermal light emitted from its dayside, researchers have gained new insights into the chemical makeup and structural properties of this rare world.

To perform this analysis, the team used high-resolution spectroscopy, a method that spreads light into its component colors to detect the chemical signatures of molecules. By identifying specific light patterns corresponding to water vapor and carbon monoxide, they characterized the atmosphere. Additionally, they compared their results with previous observations to look for thermal inversions, which occur when temperatures increase at higher altitudes in an atmosphere, similar to how ozone heats the stratosphere on Earth.

Beyond chemical detection, the team assessed the planet's potential for losing its atmosphere. They looked for signatures of helium escaping from the planet's surface but found no such evidence. This suggests that the planet is not undergoing rapid atmospheric loss. Furthermore, the findings remain consistent with the planet being tidally locked, meaning the same side of the planet always faces its host star, though further data are required to confirm its precise rotation state.

Key findings
Water vapor is confirmed on the dayside of LTT 9779 b.
No evidence of a thermal inversion was found, suggesting a standard temperature profile.
Helium absorption was not detected, which indicates weak atmospheric escape.
The planet's rotational velocity remains consistent with a tidally locked state.
Data analysis shows low water abundance in the upper atmosphere.

Scientific study

Exploring the ultrahot Neptune LTT 9779 b with CRIRES+. Non-detection of helium transmission spectra and detection of H2O on the dayside (external link)