Surprising findings suggest ‘water worlds’ are more common than we thought

Water is the only thing that all life on Earth needs, and the cycle of rain to river to ocean to rain is an essential part of what keeps our planet’s climate stable and hospitable. When scientists talk about where to look for signs of life throughout the galaxy, planets with water are always at the top of the list.

A new study published in Science suggests that many more planets may have large amounts of water than previously thought – as much as half water and half rock. The catch? All that water is probably embedded in the rock rather than flowing as oceans or rivers on the surface.

“It was a surprise to see evidence that so many water worlds orbit the most common type of star in the galaxy,” said Rafael Luque, first author of the new paper and a postdoctoral researcher at the University of Chicago. “This has huge implications for the hunt for habitable planets.”

Planetary population patterns

Thanks to better telescope instruments, scientists are finding evidence of more and more planets in distant solar systems. A larger sample size helps scientists identify demographic patterns—similar to how you can reveal trends that are hard to see at the individual level if you look at the population of an entire city.

Luque, along with co-author Enric Pallé of the Institute of Astrophysics of the Canary Islands and the University of La Laguna, decided to take a population-level look at a group of planets seen around a type of star called an M dwarf. These stars are the most common stars we see around us in the galaxy, and scientists have so far cataloged dozens of planets around them.

But because stars are so much brighter than their planets, we can’t see the planets themselves. Instead, scientists detect faint signs of the impact of planets on their stars—the shadow created when a planet crosses in front of its star, or the slight jerk in a star’s motion as a planet orbits. This means that there are still many questions about what these planets actually look like.

“The two different ways of discovering planets each give you different information,” Pallé said. By capturing the shadow created when a planet crosses in front of its star, scientists can find the diameter of the planet. By measuring the small gravitational force that a planet exerts on a star, scientists can find its mass.

By combining the two measurements, the researchers can get a sense of the planet’s composition. Maybe it’s a large but airy planet made mostly of gas like Jupiter, or a small, dense, rocky planet like Earth.

These analyzes had been performed for individual planets, but much less frequently for the entire known population of such planets in the Milky Way Galaxy. When the researchers looked at the numbers – 43 planets in total – they saw a surprising picture emerge.

The density of a large percentage of the planets suggested that they were too light for their size to consist of pure rock. Instead, these planets are probably something like half rock and half water, or some other lighter molecule. Imagine the difference between picking up a bowling ball and a football: they are roughly the same size, but one is made of a much lighter material.

Searching for water worlds

It might be tempting to imagine these planets as something out of Kevin Costner’s Waterworld: completely covered by deep oceans. However, these planets are so close to their suns that any water on the surface would exist in a supercritical gas phase, which would enlarge their radius. “But we can’t see it in the tests,” Luque explained. “This suggests that the water is not in the form of a surface ocean.”

Instead, the water could exist mixed in the rock or in pockets below the surface. These conditions would be similar to Jupiter’s moon Europa, which is believed to have liquid water underground.

“I was shocked when I saw this analysis — I and a lot of people in the field assumed these were all dry, rocky planets,” said UChicago exoplanet researcher Jacob Bean, whose group Luque has joined to conduct further analysis.

The finding matches a theory of exoplanet formation that had fallen out of favor in the past few years, which suggested that many planets form further out in their solar systems and migrate inward over time. Imagine that clumps of rock and ice form together in the cold conditions far from a star, and then are slowly pulled inward by the star’s gravity.

Although the evidence is compelling, Bean said he and the other scientists would still like to see “smoking gun evidence” that one of these planets is a water world. That’s something scientists hope to do with JWST, NASA’s newly launched space telescope that is the successor to Hubble.