Red dwarf stars are a target for astronomers hunting for potential habitable exoplanets, like the three recently found orbiting within Trappist-1's habitable zone, and Proxima b, discovered in August just four light-years away. Now a team at the University of Bern has run computer simulations to model just how planets may form around these stars, and found that the majority of them might be "water worlds," covered in extremely deep oceans with icy cores – but don't get your hopes up about habitability just yet.

Red dwarf stars are common in our galactic neighborhood, and since they're much smaller and colder than our sun, any planet orbiting within its habitable zone needs to tuck in much closer. Unfortunately, recent studies suggest that even within that zone, the planets would likely have atmospheres too dense to support life.

In order to study the composition and appearance of planets that orbit red dwarfs, the Bern researchers ran computer simulations to see how they would develop. They began with a series of thousands of identical red dwarf stars, surrounded by a disk of dust and gas containing 10 planetary embryos about the mass of Earth's moon, scattered randomly in each instance. Left to run for a few days, the models determined how these embryos would grow and migrate, and what their water content would be.

"Our models succeed in reproducing planets that are similar in terms of mass and period to the ones observed recently," says Yann Alibert, one of the researchers. "Interestingly, we find that planets in close-in orbits around these type of stars are of small sizes. Typically, they range between 0.5 and 1.5 Earth radii with a peak at about 1.0 Earth radius. Future discoveries will tell if we are correct!"

Surprisingly, the models predicted that 90 percent of the planets formed would be "water worlds," meaning more than 10 percent of their mass is liquid water. While Earth is a famously wet planet, our water content is a measly 0.02 percent by comparison. These exoplanets would be home to very deep oceans that extend almost all the way to the core, with a seafloor of high-pressure ice.

But before we start scanning for these planets as potential New Earths, the researchers point out that this much water can tip the scales too far in the wrong direction, creating an unstable surface temperature and climate.

"While liquid water is generally thought to be an essential ingredient, too much of a good thing may be bad," says Willy Benz, another of the project's researchers. "Habitable or not, the study of planets orbiting very low mass stars will likely bring exciting new results, improving our knowledge of planet formation, evolution, and potential habitability."

The research was published in the journal Astronomy and Astrophysics.