An illustration of a water world. Depositphotos.

Water Worlds More Common in the Universe Than Previously Thought

The results of a new study published in Science suggest that many more planets have large amounts of water than previously thought. Some planets might even possess a half-water, half-rock composition, according to the study.


Our planet’s climate is stable and hospitable because of the cycle of rain to rivers to the ocean to rain. Water is the basis of everything alive on Earth. A planet with water is always at the top of the list when scientists discuss where to look for signs of life throughout the galaxy.

There may be many more water-rich planets than previously thought, some having up to half water and half rock, according to a study published in the journal Science. Rather than flowing as oceans or rivers, all that water is likely embedded in the rock.


Rafael Luque, the first author of the new paper and postdoctoral researcher at the University of Chicago, said it was surprising to see so many water worlds orbiting the most common type of star in the galaxy. “It has enormous consequences for the search for habitable planets.”

The discovery of planets in distant solar systems is becoming easier with the help of better telescope instruments. Researchers can identify demographic patterns more readily if they use a larger sample size, just as they can see trends at an individual level when they look at the population of an entire town.

The research team, led by Luque and co-author Enric Pallé, looked at a group of planets seen around a type of star called an M-dwarf and examined them at the population level. Many planets have been discovered around these stars, which are known to be the most common stars in our galaxy.

Our ability to see planets is limited because the stars are so much brighter than their planets. Scientists, however, detect weak signs of a planet’s effects on its star by measuring its shadow when crossing in front of it or its tug on its motion as it orbits it. The actual appearance of these planets remains a mystery.


Pallé said that different ways of discovering planets give you different information. The diameter of a planet can be determined by catching the shadow created when the planet crosses in front of its star. A planet’s mass can be measured by measuring its gravitational pull on a star.

Scientists can determine the planet’s makeup by combining these two measurements. Maybe it’s a gaseous, giant planet like Jupiter, or maybe it’s a small, dense, rocky planet like Earth.

Individual planets have been analyzed, but the entire known population of such planets within the Milky Way galaxy has been analyzed much less often. Scientists began to see a surprising picture when they analyzed 43 planets.

Many of the planets had densities that suggested they could not be made of pure rock based on their sizes. These planets are probably made up of a combination of rocks and water, or maybe some other type of lighter molecule. You can compare picking up a soccer ball with picking up a bowling ball: they’re roughly the same size, but the one is lighter.

You might imagine these planets as being entirely covered in deep oceans, like those in Kevin Costner’s Waterworld. In spite of this, these planets are so close to their suns that any water on their surfaces would be in a supercritical gaseous state, which would increase their radius. However, the samples do not show that, said Luque. “That suggests the water is not in the form of the surface ocean.”

Alternatively, the water may be mixed with the rock or trapped in pockets beneath the surface. A similar situation is thought to exist beneath Jupiter’s moon Europa, which is thought to contain liquid water.

According to UChicago exoplanet scientist Jacob Bean, whose group Luque has joined to conduct further studies, “I was amazed when I saw this analysis – I assumed they were all dry, rocky planets.”


Scientists found that many exoplanets form farther out in their solar systems and migrate inward, in line with an old theory of exoplanet formation.

In spite of the compelling evidence, Bean said he and the other scientists would like to see “smoking gun proof” that one of these planets has water. This is what NASA’s JWST, the successor to Hubble, hopes to accomplish.

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Written by Ivan Petricevic

I've been writing passionately about ancient civilizations, history, alien life, and various other subjects for more than eight years. You may have seen me appear on Discovery Channel's What On Earth series, History Channel's Ancient Aliens, and Gaia's Ancient Civilizations among others.

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