Formation history of sub-Neptunes TOI-421 b and Gliese 3470 b reveals water-poor environments

Star system: TOI 421 A | Distance from the Sun: 245 light years

Sub-Neptunes—planets larger than Earth but smaller than Neptune—exhibit a wide variety of atmospheric compositions. These characteristics are dictated by the raw materials available during a planet's formation, such as rock, water, and refractory carbon, commonly referred to as soot. By modeling how these materials assemble to form planetary interiors and atmospheres, researchers have clarified the chemical history of several known exoplanetary systems.

Planets formed in water-rich environments naturally develop atmospheres abundant in methane and carbon dioxide. In contrast, those formed from water-poor materials show a significant depletion in these carbon-bearing gases. The presence of soot, a form of carbon that does not melt at low temperatures, further promotes the creation of methane-dominated atmospheres.

Data from the James Webb Space Telescope were used to test these models against observations of specific planets. The findings suggest that while planets like K2-18b and TOI-270d formed in water-rich environments, others like TOI-421b and Gliese 3470b are consistent with a water-poor formation process occurring inside the water ice line—the distance from a star where water cannot freeze into ice.

Researchers identified the ratio of water to methane, combined with the mean molecular weight (the average mass of molecules in the atmosphere), as a critical tool for tracing a planet’s origin. Deviations from these trends can be explained by water condensing in cooler atmospheres or the loss of lighter atmospheric gases over time.

Key findings
- Water-rich formation leads to atmospheres high in methane and carbon dioxide.
- Water-poor formation results in atmospheres strongly depleted of carbon-based gases.
- TOI-421b and Gliese 3470b likely formed in water-poor regions inside the water ice line.
- K2-18b and TOI-270d align with models of water-rich formation.
- The ratio of water to methane serves as a diagnostic for a planet's formation environment.

Scientific study

Atmospheric diversity of sub-Neptunes from formation with rock, water, and soot (external link)