Some of the most common planets in our galaxy may be hiding far more water than scientists can currently detect—even with the powerful James Webb Space Telescope.
A new study suggests that water on these distant worlds could be buried deep beneath their atmospheres, making it invisible to today’s telescopes.
The research, led by scientists at the University of Chicago and published in The Astrophysical Journal, focuses on a mysterious group of planets known as mini-Neptunes, or sub-Neptunes.
These planets are slightly smaller than Neptune and are the most common type of planet discovered beyond our solar system.
Yet they remain one of astronomy’s biggest puzzles because no similar planets exist in our own solar system.
Since scientists cannot visit these distant worlds, they rely on telescope observations and computer models to understand what they are like.
One of the most powerful tools for studying exoplanets is NASA’s James Webb Space Telescope, which analyzes starlight passing through a planet’s atmosphere as the planet crosses in front of its star.
This allows researchers to identify gases such as hydrogen, methane, carbon dioxide and water vapor.
The challenge is that an atmosphere may not tell the whole story.
Until recently, many researchers believed that the warm conditions on mini-Neptunes would keep gases and water well mixed, much like ingredients in a thoroughly shaken drink. If that were true, the atmosphere would provide a good picture of the planet’s overall composition.
However, the new study suggests the situation may be much more complicated.
Using detailed computer simulations, the research team investigated how water and hydrogen behave under different temperatures, pressures and chemical conditions.
They found that on some planets, especially those rich in water or with cooler atmospheres, water may separate from hydrogen instead of remaining evenly mixed.
Because water is heavier than hydrogen, it can sink deep into the planet’s interior, leaving very little water in the upper atmosphere where telescopes can detect it.
To explore this idea, the researchers examined TOI-270 d, a planet orbiting a star in the constellation Pictor. Earlier observations by the James Webb Space Telescope detected hydrogen, methane and carbon dioxide in its atmosphere. Based on those gases, scientists expected large amounts of water to be present as well.
The new study suggests that much of that water may have settled far below the atmosphere, hidden beneath layers of lighter gases. If so, current telescopes would have no way of seeing it directly.
Scientists still cannot determine whether TOI-270 d actually falls into this category, but the findings highlight how difficult it is to interpret observations of distant planets. Water itself behaves very differently depending on temperature and pressure. It can exist as ice, liquid, vapor or even as a supercritical fluid—a state that occurs under extremely high pressures and temperatures.
Researchers stress that planets like TOI-270 d are unlikely to be suitable places for life as we know it. Any water they contain would probably lie beneath crushing pressures and intense heat inside thick atmospheres.
Even so, understanding where water exists on these planets is important. Water plays a central role in the formation and evolution of planets and is considered one of the essential ingredients for life.
Learning how water is stored and transported inside these common worlds will help scientists better understand planetary systems across the galaxy and improve the search for truly habitable planets in the future.
