Internal structure models of K2-18 b and TOI-270 d reveal complex composition gradients

Researchers have re-examined the interiors of sub-Neptune planets K2-18 b and TOI-270 d by moving away from simplified models. By accounting for composition gradients and stable, non-convective regions within the planets, the study reveals a much wider range of possible internal structures than previously thought. This shift in modeling suggests that many sub-Neptunes are more compositionally diverse than traditional assumptions indicate.

Star system: K2-18 | Distance from the Sun: 124 light years

Close

Internal structure models of K2-18 b and TOI-270 d reveal complex composition gradients

Star system: K2-18 | Distance from the Sun: 124 light years

Standard models of planetary interiors often rely on the assumption of purely adiabatic environments - regions where heat moves through the planet via convection - with distinct layers of uniform material. This approach assumes that substances like hydrogen, helium, and rock stay neatly separated in specific zones. However, this simplification may fail to capture the actual complexity of sub-Neptune planets. By testing models that incorporate composition gradients - areas where the chemical makeup changes gradually with depth - and stable, non-convective regions where material does not circulate, researchers have gained a more accurate picture of K2-18 b and TOI-270 d.

To compare these approaches, the researchers calculated planetary structures consistent with known masses, radii, and atmospheric conditions. The results demonstrate that the previous, simpler models significantly underestimated the variety of possible internal compositions. When composition gradients are included, the amount of hydrogen and helium a planet can hold increases dramatically. For K2-18 b, the potential mass fraction of hydrogen and helium increases by a factor of three to ten, depending on the temperature, while for TOI-270 d, this potential increase is over an order of magnitude.

These findings also change how we understand the relationship between a planet's different ingredients. In simpler models, certain chemical components often appeared strongly linked, but these correlations weaken significantly when composition gradients are introduced. While the link between hydrogen-helium abundance and the water-to-rock ratio remains strong, other relationships between iron and lighter gases become less predictable. This indicates that current observations may be less definitive about a planet's bulk composition than previously assumed, highlighting the need for more sophisticated modeling to understand how these worlds formed and evolved over time.

Key findings
Composition gradients significantly broaden the range of viable interior structures for sub-Neptunes.
The maximum potential hydrogen-helium mass fraction for K2-18 b increases by up to ten times at higher temperatures.
For TOI-270 d, the maximum possible hydrogen-helium mass fraction increases by more than an order of magnitude.
Traditional adiabatic models incorrectly overstate the certainty provided by current observational data.
Relationships between hydrogen-helium content and iron abundances are much weaker than simpler models once suggested.

Scientific study

Composition gradients in sub-Neptunes: K2-18 b and TOI-270 d as case studies (external link)