New research has strengthened the idea that two planets orbiting a red dwarf in the Kepler-138 system are water worlds, meaning water accounts for a large share of their volume. This finding comes from studies conducted by researchers associated with the University of Montreal and others in the field.
The Kepler-138 system is located roughly 218 light-years away in the constellation Lyra. The planets Kepler-138c and Kepler-138d were analyzed using observations from the Hubble Space Telescope and the Spitzer Space Telescope, enabling scientists to compare their sizes and masses with theoretical models. This comparison revealed that a substantial portion of their volume is likely composed of materials lighter than rock but heavier than hydrogen and helium, with water standing out as the most plausible remaining component. The conclusion marks a significant step in exoplanet science, offering the first concrete examples of planets that can be classified as water worlds with confidence, a concept long discussed in theorizing about planetary composition.
In terms of scale, Kepler-138c and Kepler-138d each exceed Earth’s volume by more than three times and carry more than twice Earth’s mass. Researchers often liken these planets to enlarged icy moons in our own solar system, such as Europa and Enceladus, but with a crucial difference: their measured surface temperatures suggest conditions far too hot for conventional oceans. Thermodynamic calculations indicate their atmospheres are dominated by very dense, superheated vapor. At lower altitudes, this vapor could condense into water in a supercritical state, meaning it would exist as a dense fluid rather than a distinct liquid layer. These characteristics imply a planet-wide environment where water compounds the atmosphere and interior in unusual, high-energy ways. The findings align with predictions from planetary formation models that anticipate water-rich worlds forming around cooler, smaller stars and then evolving along paths that keep water in high-pressure, high-temperature phases. The researchers emphasize that while direct detection of liquid water is not possible with current data, the inferred composition points toward a water-dominated interior and atmosphere. This distinction helps explain why the planets behave differently from larger, primarily rocky super-Earths and supports ongoing efforts to map the diversity of worlds in nearby stellar systems, including those around red dwarfs common in our galaxy, a topic of growing interest in Canadian and American astronomy communities.