Tides and the Hidden Boundaries of Hycean Habitability

In the quest to find habitable worlds beyond Earth, one particularly intriguing class of exoplanets has emerged: hycean planets. These are mini-Neptune-sized planets with thick hydrogen-rich atmospheres, vast water oceans, and potentially icy interiors. Previously, scientists believed these worlds could support life across a wider range of distances from their stars than Earth-like planets. However, in a new study, Joseph Livesey and collaborators challenge that assumption by exploring how an often-overlooked factor—tidal heating—might narrow this so-called Hycean Habitable Zone (HHZ), especially around small, cool stars called M dwarfs.

Revisiting the Habitable Zone with Tidal Heating in Mind

The paper opens with a review of what defines a habitable zone (HZ), the region around a star where temperatures could allow liquid water to exist. Hycean planets, due to their unique atmospheric and surface properties, were expected to have a much broader habitable zone than rocky Earth-like planets. Previous models even suggested they could remain habitable at extremely close or far distances from their stars. However, this optimistic view didn’t fully account for tidal forces—gravitational interactions between the planet and its star that can heat a planet from the inside, potentially pushing it out of the habitable range.

Modeling the Inner Edge: When Tides Make a Planet Too Hot

To test how tidal heating might affect habitability, Livesey et al. model a hycean planet orbiting very close to an M dwarf star. Using a well-established model for tidal forces, they show that even moderate orbital eccentricity (i.e., how non-circular the orbit is) can generate enough heat inside the planet to raise its surface temperature significantly. This additional heat effectively pushes the inner edge of the HHZ farther out from the star than previously thought. They also show that current estimates of the HHZ do not include this effect, meaning many planets previously thought to be habitable might, in fact, be too hot.

The Role of Companion Planets: Keeping the Heat On

But how does a planet maintain a non-circular orbit long enough for tides to matter? In Section 3, the authors investigate how gravitational nudges from a second, more distant planet in the same system can keep the inner planet’s orbit slightly eccentric over billions of years. This process, called “forced eccentricity,” can continuously fuel internal tidal heating. They even provide formulas and diagrams showing how different planet arrangements impact this effect. Remarkably, even a modest outer planet can cause long-term tidal heating, altering the potential for life on the inner world.

Quantifying the Effect: How Much Warmer Do Tides Make Planets?

Next, the authors combine their tidal heating and orbital dynamics calculations to create a simple formula for how much hotter a hycean planet gets due to tides. This formula depends on various physical properties of the planet and star and can be used to estimate how much previous models have underestimated the temperature of these planets. Importantly, they find that tidal heating could raise temperatures by enough to make otherwise habitable planets uninhabitable.

Implications for Life and the Search for Biosignatures

Finally, in the discussion, the authors highlight the broader implications of their results. Not only does tidal heating reshape the boundaries of the HHZ, but it may also play a role in supporting life. For example, tidal energy can power deep oceans and potentially even speed up biological evolution by warming the planet’s surface. The recent (though debated) detection of dimethyl sulfide—possibly a biosignature gas—on the hycean candidate K2-18 b makes this question especially timely. Livesey and colleagues suggest that tides could both help and hinder life, making them a crucial factor in future habitability studies.

Conclusion: Rethinking Habitability for Watery Worlds

In summary, this paper adds a key layer to our understanding of habitable worlds. While hycean planets remain promising targets in the search for life, their true potential hinges on the invisible but powerful influence of tides.

Source: Livesey