While Mars is just one other world in our solar system that once had water – and still may have water underground – it’s not the only world with subsurface water. We’ve also found gravitational and magnetic evidence for subsurface water on a host of icy moons and dwarf planets, including Ganymede, Europa, Enceladus, and even Ceres and Pluto. Those discoveries alone have upped the possibility of exoplanetary worlds rich in water as well, and new research published in Science and led by Rafael Luque finds that we might even be looking for evidence of water on those exoplanets using the wrong hypothesis.

The team of researchers analyzed a population of planets found orbiting stars called M dwarfs or red dwarfs; they’re incredibly common stars, and we now have an extensive catalog of exoplanets in those types of systems. Since we cannot see the exoplanets themselves due to the relative brightness of their stars and their close-in orbits, we have to use the transit method to catch the dips in the light and the radial velocity method to calculate the gravitational tugs on the star. Co-author Enric Pallé explains: The two different ways to discover planets each give you different information. By catching the shadow created when a planet crosses in front of its star, scientists can find the diameter of the planet. By measuring the tiny gravitational pull that a planet exerts on a star, scientists can find its mass.

Combine those two measurements and you can get the approximate density of the exoplanets. And when the team analyzed all these measurements from 43 different but similar exoplanets, they saw a pattern: densities that suggested the worlds were too light to be made entirely of rock. Which led to the conclusion that many of these worlds could be half-rock and half-water, or something similarly less dense.

If you think about how close these planets are to their stars, only having water under the surface makes sense. The surface temperatures would still be too hot for water to easily stay liquid, instead becoming supercritical gases that puff up the radius of the planets. And the radius measurements we have don’t match what we expected.

Once again, time for JWST to stare at some exoplanets and get us more detailed data.

More Information

University of Chicago press release

“Density, not radius, separates rocky and water-rich small planets orbiting M dwarf stars,” Rafael Luque and Enric Palle, 2022 September 8, Science