Irradiated ocean planets bridge super-Earth and sub-Neptune populations (2002.05243v2)

Abstract: Small planets ($\sim$1--3.9 $\Rearth$) constitute more than half of the inventory of the 4000-plus exoplanets discovered so far. Smaller planets are sufficiently dense to be rocky, but those with radii larger than $\sim$1.6 $\Rearth$ are thought to display in many cases hydrogen/helium gaseous envelopes up to $\sim$30\% of the planetary mass. These low-mass planets are highly irradiated and the question of their origin, evolution, and possible links remains open. Here we show that close-in ocean planets affected by greenhouse effect display hydrospheres in supercritical state, which generate inflated atmospheres without invoking the presence of large hydrogen/helium gaseous envelopes. We present a new set of mass-radius relationships for ocean planets with different compositions and different equilibrium temperatures, which are found to be well adapted to low-density sub-Neptune planets. Our model suggests that super-Earths and water-rich sub-Neptunes could belong to the same family of planets, i.e. hydrogen/helium-free planets, with differences between their interiors simply resulting from the variation in the water content.

An Essay on "Irradiated Ocean Planets Bridge Super-Earth and Sub-Neptune Populations"

The paper by Mousis et al., titled "Irradiated Ocean Planets Bridge Super-Earth and Sub-Neptune Populations," presents a sophisticated exploration into the nature and classification of small exoplanets, specifically focusing on those within the radius range of approximately 1 to 3.9 Earth radii ($R_\oplus$). This research contributes to the ongoing debate surrounding the transition between super-Earths and sub-Neptunes and challenges the conventional perspective that larger planets within this category necessarily possess substantial hydrogen/helium envelopes.

Key Insights from the Paper

1. Supercritical Water-Rich Interiors: The authors propose that some small, low-density planets may possess significant water-rich envelopes in a supercritical state rather than hydrogen/helium gas layers. By modeling the mass-radius relationships of planets under various conditions of temperature and composition, they ascertain that these unique planetary configurations can explain the observed characteristics of certain sub-Neptune-like exoplanets.
2. Mass-Radius Modeling: Utilizing a combination of advanced one-dimensional models for planetary interiors and atmospheres, the researchers present new mass-radius relationships applicable to ocean planets. These models are tuned to reflect the presence of supercritical water envelopes influenced by pronounced greenhouse effects from stellar irradiation.
3. Linking Super-Earths and Sub-Neptunes: In challenging existing paradigms, the paper suggests an intriguing hypothesis: super-Earths and certain sub-Neptunes could be fundamentally similar in nature, differing primarily in their water content rather than the presence of extensive gaseous envelopes. This perspective has significant implications for theories on planetary formation and migration.

Quantitative and Qualitative Implications

The authors provide a critical reassessment of the properties defining super-Earths and sub-Neptunes by examining observational data against their theoretical models. Notably, they provide a clear framework for understanding mass-radius relationships without requiring large hydrogen-helium atmospheres. For example, the sub-Neptune K2-38b, with observed H$_2$O absorption features, is successfully modeled with a supercritical water layer and steam atmosphere — a result not achievable with traditional rocky planet models without significant hydrogen-helium content.

Theoretical and Practical Implications

• Formation Theories: This paper suggests that the diversity in the interiors of close-in exoplanets could result from their formation conditions, particularly from icy building blocks originating beyond the snowline in protoplanetary disks. This hypothesis supports the presence of varied planet formation pathways involving migration scenarios.
• Observational Techniques and Future Research: Improved mass-radius models that incorporate supercritical water envelopes could refine classification and characterization techniques in exoplanet research. Future exploration could benefit from enhanced spectra analysis methods that can differentiate atmospheric compositions beyond just hydrogen and helium.
• Overall Contributions to Exoplanet Classification: By extending mass-radius relationships to accommodate supercritical water-rich interiors and atmospheric conditions, this research broadens the scope of exoplanet investigations, enabling more precise categorization and understanding of small exoplanet populations.

Speculations on Future Developments

Building upon the hypotheses and models presented, future theoretical work may explore the physical properties of supercritical water under varying planetary conditions, potentially requiring the formulation of more comprehensive and robust equations of state (EOS) for planetary modeling. Observational missions might also pivot towards direct investigation of water content in exoplanetary atmospheres, providing a novel insight into planet classification.

In conclusion, the work by Mousis et al. establishes a compelling alternative view to traditional classifications of super-Earths and sub-Neptunes, suggesting a continuum shaped significantly by water content and environmental factors, independent of hydrogen/helium envelope presence. This paper enriches our understanding of planetary diversity and dynamics, casting new light on the intricate nature of these extraterrestrial bodies.