Water worlds common on red dwarfs? Maybe.

Water Worlds: Planet with ocean orbiting a star.
View larger. | This is an artist’s rendering of the exoplanet TOI-1452 b. It’s slightly larger than Earth and may be completely covered by a deep ocean. Orbiting one of 2 small red dwarf stars, it is located about 100 light-years from Earth in the constellation Draco. Astronomers announced its discovery last month (August 2022). Now a new study says aquatic worlds around such stars are likely common. Image by Benoit Gougeon/ Institute for Research on Exoplanets/ University of Montreal.

The earth is a water world with its vast oceans, lakes and rivers. Elsewhere in our solar system are other bodies with subsurface water oceans, including Jupiter’s moon Europa and Saturn’s moon Enceladus. But are there water worlds? Outside our solar system? Growing evidence suggests it is likely. In fact, a new study by University of Chicago researchers says aquatic worlds — at least one type of them — may be common around red dwarf stars. However, most of this water is probably underground. The researchers published their peer-reviewed findings in the journal Science on September 8, 2022.

Water worlds orbit red dwarfs

The new study focuses on red dwarf stars, the most common type of star in our galaxy. Many of the exoplanets — planets outside of our own solar system — that astronomers have discovered so far orbit red dwarf stars. The researchers wanted to study the populations of planets orbiting these stars. These worlds include rocky, water-rich, and gas-rich planets. Their investigation suggests that aquatic worlds are likely common among these planets. Rafael Luque, lead author of the new study, said in UChicago News:

It was a surprise to see evidence that so many aquatic worlds orbit the most common type of star in the galaxy. It has enormous consequences for the search for habitable planets.

2 different observation techniques

The researchers found clues to the size and mass of the planets by combining two different observational techniques. One is called the transit method, based on a planet passing in front of its star as seen from where we are on Earth. The other is the radial velocity method, in which a very small gravitational pull of a planet on a star is measured by a telescope. Combining the results gives scientists a better idea of ​​how large and massive the planets are. Co-author Enric Pallé from the Institute of Astrophysics of the Canary Islands and the University of La Laguna said:

The two different ways to discover planets each give you different information.

Earth on the right with 6 smaller moons beside it, with white text on a starry background.
View larger. | Earth is just one of many water worlds in our own solar system. This figure shows several of the moons in the outer Solar System, along with Earth, that are now known to have subsurface oceans. Image from the Woods Hole Oceanographic Institution.

Half rock and half ice

Scientists assumed that most of these planets would be rocky like Earth. And they are… sort of. However, the results suggest they are more likely to be half rock and half water. This is because the measured densities of the planets suggest they are too light to be pure rock. As Jacob Bean of the University of Chicago said:

I was shocked when I saw this analysis. I and many people in the field assumed these were all dry, rocky planets.

However, there is a catch. The results suggest that while the planets contain plenty of water, it is most likely mixed with the rock or in subterranean pockets rather than sloshing around on the surface. Why? Because these particular planets orbit their stars very closely. All surface water, if not simply vaporized, would exist in a supercritical gas phase. In this case, this would make the planet’s apparent radius appear larger. But that’s not what astronomers have observed. As Luque noted:

But we don’t see that in the rehearsals. This suggests that the water is not in the form of a surface ocean.

Europe-like water worlds?

If this scenario is true and these planets have most or all water underground, they could become more similar to Europa or other oceanic moons in our solar system. On these moons, the liquid water is under a crust of ice, but the analogy is similar. For Europa specifically, there is not only the subsurface ocean, but also increasing evidence of pockets of water – lakes, if you will – within the ice crust itself.

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Man with glasses, mustache and beard.
Rafael Luque of the University of Chicago led the new study of aquatic worlds around red dwarf stars. Image via the University of Chicago.

Formation of exoplanets

The results also have implications for the formation of planets. They support the theory that many planets actually form farther from their stars and then gradually migrate inward. These once colder planets could then eventually be in the closer region around their star where liquid water would be possible. As noted in the paper:

Formation models that include orbital migrations can explain the observations: Rocky planets form within the snow line, water-rich worlds form outside, which later migrate inwards.

Whitish, planet-like sphere with many dark, curved lines on its surface.
Jupiter’s moon Europa is, next to Earth, the most well-known marine world in our solar system. This image of Europa is a Galileo spacecraft composite of images taken in 1995 and 1998. Do you see the many criss-crossing cracks and ridges that cover Europa’s surface? They give the moon the appearance of a cracked egg. Image via NASA/ JPL-Caltech/ SETI Institute.

Other water worlds

In another July 2022 study, scientists said that super-Earth exoplanets with thick primordial atmospheres could also be potentially habitable aquatic worlds.

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Last month (August 2022), scientists announced the discovery of an exoplanet 100 light-years away on super-Earth that may be completely covered by water. From the studies and discoveries made, it appears that there could be a great variety of aquatic worlds out there in our galaxy. An exciting prospect!

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Bottom Line: University of Chicago scientists say rocky waterscapes around red dwarf stars are likely common. Most of this water is probably underground, but not on the surface as oceans.

Source: Density, not radius, separates rocky and water-rich small planets orbiting M dwarf stars

Source (Preprint): Density, not radius, separates rocky and water-rich small planets orbiting M dwarf stars

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