Atmospheric composition of temperate exo-Neptune TOI-1231 b revealed by JWST

Star system: NLTT 24399 | Distance from the Sun: 89.6 light years

The temperate exo-Neptune TOI-1231 b serves as a vital benchmark for understanding planets smaller than Neptune. Because of its density, the planet is known to possess a deep, hydrogen-rich atmosphere. This study provides the first near-infrared transmission spectrum—a technique that analyzes how starlight filters through a planet's atmosphere—to determine its chemical makeup, offering a reference for similar worlds.

Researchers identified a strong presence of methane and found moderate to strong evidence for carbon dioxide in the planet's atmosphere. These findings support the model of a temperate Neptune with a thick, hydrogen-dominated gaseous envelope rather than a distinct solid surface.

The data was collected using the James Webb Space Telescope's advanced instruments to observe the light passing through the planet's atmosphere. While methane and carbon dioxide were detected, the study found no significant evidence for other prominent molecules like ammonia or sulfur-bearing compounds.

These results confirm that TOI-1231 b fits the expected profile of a temperate sub-Neptune with a deep, hydrogen-rich atmosphere. By acting as a keystone reference case, this planet helps clarify the complex nature of temperate sub-Neptunes, which can otherwise be difficult to categorize due to their varied potential interior structures.

Key findings
- Strong detection of methane in the atmosphere of TOI-1231 b.
- Moderate to strong evidence for the presence of carbon dioxide.
- No significant evidence found for ammonia, carbon monoxide, or sulfur-bearing species.
- Findings confirm a deep, hydrogen-rich atmosphere consistent with a temperate exo-Neptune.

Scientific study

Transmission Spectrum of the Benchmark Temperate Exo-Neptune TOI-1231 b (external link)