Our dream of finding ocean-covered exoplanets isn't drying up, but it has become more complex and nuanced. Recent research suggests that some of the most promising candidates, particularly a class of planets known as sub-Neptunes, may not be the water worlds we once thought they were. This challenges the idea of "Hycean" planets (hydrogen-rich atmospheres with deep global oceans), making the search for extraterrestrial life more difficult, but not impossible.
The New Perspective on Sub-Neptunes
A recent study by researchers at ETH Zurich suggests that sub-Neptunes—planets larger than Earth but smaller than Neptune—are unlikely to be "Hycean worlds." The study modeled the early formation of these planets and found that chemical interactions between the planet's magma ocean and its atmosphere destroy most of the water. Hydrogen and oxygen bond with other compounds, which then sink into the planet's core, leaving very little water (at most a few percent of the planet's mass) near the surface.
This is a significant shift from previous assumptions, which held that these planets, having formed far from their stars in an icy region, would retain vast amounts of water as they migrated inward. The new findings mean that planets like K2-18b, once considered a prime candidate for an ocean world, may have far less surface water than initially speculated.
Where Does This Leave the Search?
While the sub-Neptune revelation is a setback for one specific type of "water world," it doesn't mean the entire search is over. Instead, it directs our focus to other places and methods:
Rocky Planets: The new research suggests that conditions for liquid water are more likely to exist on smaller, rocky planets, more akin to Earth. Finding these planets and analyzing their atmospheres for signs of water and other biosignatures is the next major frontier, with missions like NASA's James Webb Space Telescope (JWST) and upcoming observatories like the Habitable Worlds Observatory leading the charge.
Atmospheric Analysis: We've already detected water vapor in the atmospheres of over 10 exoplanets. While this doesn't guarantee a surface ocean, it confirms that the building blocks of water are present and that planets can retain atmospheres containing water. Future telescopes will continue to analyze the atmospheric composition of exoplanets to better understand their potential for hosting liquid water.
Subsurface Oceans: The search for water worlds isn't limited to exoplanets. Our own solar system is teeming with potential ocean worlds. Moons of Jupiter and Saturn, like Europa and Enceladus, are believed to harbor vast liquid water oceans beneath their icy shells. Missions like the Europa Clipper, launching in 2024, are designed to explore these promising targets, offering a close-up look at where life might exist in our own cosmic neighborhood. This search for subsurface oceans is an equally, if not more, compelling avenue in the quest for extraterrestrial life.