The Enigmas of Uranus and Neptune

Despite being our neighbors in the Solar System, the internal structures of Uranus and Neptune are shrouded in mystery. To compound this intrigue, sub-Neptunes are emerging as the most common type of exoplanets, necessitating a deeper understanding of the physical processes that rule the insides of these celestial bodies.

The Science Behind Water Demixing

Recent research suggests that in cold, water-rich planets, phase separation between hydrogen and water may occur—a phenomenon known as demixing. By examining Uranus, Neptune, K2-18 b, and TOI-270 d, scientists are exploring how this process affects planetary evolution and structure.

A Study of Planetary Evolution Models

By integrating planetary evolution models with cutting-edge calculations of the hydrogen-water phase diagram, researchers can account for temperature shifts linked to uncertainties in miscibility gaps. Their findings reveal a startling possibility: the outer layers of Uranus and Neptune could be completely stripped of water due to demixing.

Consequences for Planetary Mass and Size

Temperature shifts of up to 1100 K can create a water-depleted zone making up 16% of a planet's mass. This depletion also results in a significant increase—nearly 20%—in planetary radius. For instance, models of K2-18 b suggest that hydrogen-water demixing might be happening, which could explain the lack of water features in its spectrum observed by the James Webb Space Telescope (JWST).

Insights from K2-18 b and TOI-270 d

To fully deplete water in K2-18 b's atmosphere, a temperature offset of 500 K is necessary. Meanwhile, TOI-270 d may also show signs of this demixing process; applying a similar temperature offset reveals that it too has experienced a partial depletion of water consistent with JWST's observations.

Shaping the Future of Atmospheric Studies

Understanding hydrogen-water immiscibility is becoming essential for unraveling the structure and evolution of both our Solar System’s giants and distant exoplanets. As upcoming missions like ARIEL prepare to study these atmospheres, recognizing internal processes will be vital for accurate interpretations of their findings.