Hycean worlds, also known as ocean worlds, are planets covered in oceans with thick hydrogen atmospheres. While no confirmed Hycean worlds exist, there are numerous candidates that researchers are studying for their habitability. Recent research delves into the role of tidal heating in determining the potential habitable zones of these intriguing planets.

If Hycean planets do exist, they are likely to be common around red dwarfs, the most abundant type of star in the galaxy. The thick hydrogen atmospheres of Hycean worlds could shield them from the intense flaring activity of these small, long-lived stars. While the abundance of water on Hycean worlds may expand their habitable zones, their hydrogen atmospheres could contribute to a runaway greenhouse effect, making their habitability a complex and mysterious subject.

A new study set to be published in The Astrophysical Journal, titled "Tides Tighten the Hycean Habitable Zone," and led by Joseph Livesey from the University of Wisconsin-Madison, explores the impact of tidal heating on the habitable zones of Hycean worlds orbiting low-mass stars. This research sheds light on the unique factors that determine the habitability of these ocean-covered planets.

Unlike rocky planets, Hycean worlds are sub-Neptunes with significant water layers and hydrogen-dominated atmospheres. Determining their habitable zones requires a different approach due to their distinctive characteristics. Tidal heating, which generates heat apart from stellar radiation, plays a crucial role in determining the habitable zones of Hycean worlds with moderately eccentric orbits.

Recent findings suggest that the Hycean Habitable Zone (HHZ) may have smaller semi-major axes compared to the terrestrial habitable zone. Tidal heating from moderately eccentric orbits can shift the HHZ outward, creating a smaller habitable zone than previously estimated based solely on stellar heating. The presence of massive outer planets in solar systems can induce eccentricity in smaller planets, affecting their habitable zones.

While most Hycean candidates are orbiting more massive stars, the impact of tidal heating on the HHZ varies with the star's mass. Tidal flexing on ocean worlds can shift their habitable zones outward, depending on the presence of a more massive companion planet that introduces eccentricity into the Hycean world's orbit.

Although Hycean worlds are still hypothetical, their confirmation may be on the horizon. Exoplanet scientists are intrigued by these planets for their potential habitability and unique atmospheres, making them ideal targets for atmospheric spectroscopy with advanced telescopes like the JWST. The discovery of biosignatures on candidate Hycean worlds, such as K2-18b, further fuels interest in these ocean-covered planets.