Atmospheric composition of sub-Neptune planets around Red dwarf stars reveals new hybrid class

Researchers have analyzed the atmospheres of eight sub-Neptune planets orbiting Red dwarf stars using the James Webb Space Telescope. By studying the mean molecular weight of their gaseous envelopes, the team identified a new category of intermediate planets, dubbed hybrid worlds, which do not fit the traditional definitions of simple gas-enveloped rocks or water-rich worlds formed in the outer solar system.

Star system: LHS 1140 | Distance from the Sun: 48.8 light years

Sub-Neptune planets orbiting Red dwarf stars are typically categorized into two groups: gas dwarfs, which consist of a rocky core surrounded by a primary hydrogen and helium envelope, and water worlds, which are volatile-rich bodies that formed in the colder regions of their star systems. Distinguishing between these two types based solely on mass and radius is often difficult because their physical characteristics can overlap, making their observed properties appear identical even if their origins and internal structures differ significantly.

To better understand these planets, an analysis was performed on eight sub-Neptune planets using transmission spectra gathered by the James Webb Space Telescope. By measuring the mean molecular weight of the planetary atmospheres—a metric that identifies the average mass of the particles in the gas—the team was able to determine the composition of the outer envelopes. This data was combined with calculations of planetary evolution, including how much atmospheric gas might have escaped into space over the lifetime of each system based on the activity of the host Red dwarf star.

The investigation revealed that four of these planets fall into an intermediate category, which is now classified as hybrid worlds. These planets possess atmospheres that are too heavy to be made primarily of primordial hydrogen and helium, yet they do not show the composition expected of worlds that formed in the outer regions of a star system. These hybrid atmospheres may result from complex processes such as interaction with global magma oceans, supercritical mixing of gases, or the preferential loss of lighter hydrogen atoms over time. While TOI-776 c was identified as a candidate gas dwarf and LHS 1140 b was confirmed as a volatile-rich body, the emergence of the hybrid class suggests that the classification of sub-Neptunes is more complex than previously thought.

Key findings
Four sub-Neptune planets were identified as hybrid worlds with atmospheres too heavy for standard gas-dwarf classifications.
Mean molecular weight serves as a primary tool for distinguishing between primordial gas envelopes and secondary atmospheric origins.
TOI-776 c is categorized as a likely gas dwarf, while LHS 1140 b is classified as a volatile-rich water world.
Atmospheric heaviness in hybrid worlds may stem from magma-ocean interactions or the preferential escape of hydrogen.

Scientific study

Beyond gas dwarfs and water worlds: hybrid envelopes among the M-dwarf sub-Neptunes characterised with JWST (external link)