Optical spectrum of Gliese 436 b appears featureless, with no strong atomic or molecular signals detected

Star system: Gliese 436 | Distance from the Sun: 31.9 light years

GJ 436 b is a warm Neptune-sized planet orbiting a red dwarf star. Scientists used the ESPRESSO spectrograph to observe two transits of the planet, aiming to detect atomic and molecular species in its upper atmosphere. The observations covered a wide range of optical wavelengths and searched for signals from hydrogen, sodium, magnesium, iron, and other elements, as well as molecules like titanium oxide and vanadium oxide.

The analysis found no strong evidence for any of these species, resulting in a featureless optical transmission spectrum. A weak, tentative signal of neutral iron was seen in one transit but not the other, and its low significance suggests it is likely due to noise rather than a real atmospheric feature. Both nights of observation showed post-transit stellar flares, which mainly affected the star's chromospheric lines and could complicate the search for planetary signals.

These results indicate that GJ 436 b's atmosphere is either cloudy, hazy, or depleted in the atomic and molecular species searched for, making it difficult to detect specific features in the optical range. The study highlights the challenges of probing exoplanet atmospheres, especially for planets with muted or featureless spectra.

Key findings
- No strong atomic or molecular signals detected in GJ 436 b's optical spectrum
- A tentative iron signal is likely noise
- Stellar flares were observed during both transits
- The atmosphere may be cloudy, hazy, or depleted in detectable species
- Results highlight the difficulty of studying featureless exoplanet atmospheres

Scientific study

The atmosphere of the warm Neptune GJ 436 b probed with ESPRESSO (external link)