Astronomers have detected signs of water vapor in the atmosphere of a hot Saturn-type exoplanet, designated HD 149026 b and nicknamed Smertrios, thanks to the CARMENES spectrograph at the Calar Alto Observatory. This discovery, reported in a study, could offer new insights into the structure and formation of this exotic world.
Artist's impression of Smertrios.
Credit: NASA.
Smertrios, rich in metals, orbits the star HD 149026, a yellow subgiant located approximately 248.5 light-years from Earth. This planet has a radius of 0.81 times that of Jupiter and a mass three times lower. It completes a full revolution around its star in just 2.876 days at a distance of 0.043 AU, with an equilibrium temperature of 1,693 K (2,600°F).
The team led by Sayyed A. Rafi from the University of Tokyo used the CARMENES spectrograph at the Calar Alto Observatory to obtain high-resolution transmission spectra of Smertrios. Their primary goal was to deepen the understanding of the atmospheric composition of this exoplanet.
Transmission spectra are an observational technique used in astronomy to analyze the composition of exoplanet atmospheres. When starlight passes through the atmosphere of a transiting planet, certain wavelengths of this light are absorbed by molecules present in the atmosphere. By studying the variations of this absorbed light, scientists can identify different molecules present, such as water, carbon dioxide, or methane.
The analysis of the CARMENES data revealed a water vapor signal near the expected location for Smertrios, with a signal-to-noise ratio of 4.8. However, the researchers specify that this signal provides a strong indication of the existence of water vapor, without definitively confirming it.
The study suggests that the presence of water vapor implies a carbon-to-oxygen ratio of less than one, if the atmosphere is homogeneous and in chemical equilibrium, indicating a low abundance of hydrogen cyanide. However, no hydrogen cyanide was detected, possibly due to the low quality of the data set.
The researchers also measured the orbital and rest velocities of Smertrios, found to be 158.17 km/s (98.27 mi/s) and 2.57 km/s (1.6 mi/s), respectively. While the orbital velocity is consistent with expectations, the rest velocity is significantly redshifted, which could be explained by anomalous atmospheric dynamics or an eccentric orbit.