Transmission spectrum of the sub-Neptune HD 191939 b reveals no clear atmospheric signals

Star system: HD 191939 | Distance from the Sun: 174 light years

The sub-Neptune exoplanet HD 191939 b, a world larger than Earth but smaller than Neptune, has been examined to understand its atmospheric composition. Observations were conducted to analyze how light filters through the planet's atmosphere, a method known as transmission spectroscopy, which helps identify the gases present. The results show a flat spectrum with no distinct signatures, suggesting the presence of clouds or hazes that hide the deeper atmospheric layers.

Researchers utilized the Wide Field Camera 3 on the Hubble Space Telescope to measure light across near-infrared wavelengths ranging from 1.1 to 1.7 micrometers. By examining these wavelengths, the aim was to detect chemical absorption features, which act like fingerprints for specific gases.

The collected data provides evidence against a cloud-free atmosphere with a composition similar to the Sun, known as solar metallicity. Instead, the observations point toward the potential presence of hydrocarbon hazes or clouds at the planet's equilibrium temperature of approximately 880 Kelvin (about 607 degrees Celsius), which likely mask any signals from deeper in the atmosphere.

Further analysis suggests that HD 191939 b may possess a super-solar metallicity, meaning it contains a higher proportion of elements heavier than hydrogen and helium compared to the Sun. This finding aligns with established patterns where smaller exoplanets tend to have higher atmospheric metallicities. The presence of these thick clouds or hazes is consistent with what is expected for a planet at this temperature, effectively muting potential absorption features that would otherwise be visible.

Key findings
- No significant spectral features were detected in the transmission spectrum of HD 191939 b.
- Data provides moderate evidence against a cloud-free, solar-metallicity atmosphere.
- Observations suggest the presence of obscuring hydrocarbon hazes or cloud condensation.
- Atmospheric conditions are likely influenced by the planet's equilibrium temperature of 880 Kelvin.

Scientific study

The SPACE Program II: No discernible spectral features in the transmission spectrum of the sub-Neptune HD 191939 b observed with HST/WFC3 (external link)