HIP 41378 f

Lore & Background

HIP 41378 f has an anomalously large radius of 9.2 Earth radii for a planet of its size and temperature. Its measured mass of 12 Earth masses suggests a core of at most 3 Earth masses and an envelope fraction of 75% or greater. This envelope fraction exceeds what the core accretion model of planet formation would allow for such a core mass, consistent with the hypothesis that the planet's observed radius is inflated by an optically thick ring system. A 2026 study revised the mass upward, but the planet remains a super-puff with a density of 0.16 g/cm³, keeping the ring hypothesis plausible.

The proposed ring system would have a density roughly equivalent to water, indicating porous rocky material. One proposed origin is an exomoon that migrated outward and had its eccentricity raised until it was tidally disrupted. No atmospheric signatures were found as of 2022, further reinforcing the hypothesis of opaque circumplanetary rings. A 2023 study analyzed the orbital stability and detectability of a hypothetical Mars-sized exomoon, finding such a moon feasible but currently unlikely to be detectable.

Reader's Guide

HIP 41378 f is significant as a candidate for hosting circumplanetary rings, a rare feature among exoplanets. Its anomalously large radius and low density challenge standard core accretion models, suggesting that rings may obscure the planet's true size. The ring hypothesis is supported by the absence of atmospheric signatures and the planet's envelope fraction, which is too large for its core mass under conventional formation theory. The possibility that the rings originated from a disrupted exomoon adds to its importance for understanding moon–planet interactions and ring formation. The planet's location within the optimistic habitable zone also makes it a target for studies of planetary system architecture and potential habitability, though the rings themselves would likely block any view of the planet's surface. The 2023 study on a hypothetical Mars-sized exomoon underscores the system's potential for hosting moons, even if current detection methods cannot confirm them. HIP 41378 f thus serves as a key object for testing models of planetary formation, ring dynamics, and the detectability of exomoons.

Did You Know?

• HIP 41378 f is the outermost planet in its system.
• Its radius of 9.2 Earth radii is anomalously large for its mass and temperature.
• The planet's envelope fraction is 75% or greater, exceeding predictions from core accretion models.
• A 2023 study found that a Mars-sized exomoon could orbit HIP 41378 f stably but is not currently detectable.