In addition to rocky and gaseous planets, there are worlds near red dwarfs where hard rocks are densely mixed with water.
The vast majority of stars in our galaxy are red dwarfs, small and dim. So the question of what planets near such stars look like and whether life is possible on them turns out to be particularly important. To this end, scientists from France and the USA have carried out a "census" of known planets orbiting red dwarfs. After determining their density, astronomers discovered that many such worlds could include huge amounts of moisture - which, however, hardly makes them habitable.
A paper by Rafael Luque and Enric Pallé has been published in Science. In it, astronomers analyze the masses and radii of 43 planets orbiting red dwarfs of the Milky Way. The work reveals that they are divided mainly into three populations that differ sharply in density and other characteristics. First are compact solid worlds like the Earth or Mars; second are gas dwarfs, 'mini-neptunes', rich in hydrogen and helium.
The third group consists of formerly unknown planets, which are small and have a rather low density. This can be explained by their high water content, a substance that is very common in space. Scientists estimate that water could account for up to 50 per cent of the mass of these planets. In comparison, the Earth has thousands of times less: water is only 0.02 percent of the mass of our "blue" planet.
However, we should not think that these water planets are covered with oceans, where life is bubbling and swimming intelligent squids. Most likely, such worlds form relatively far from their stars, where the main masses of water accumulate and then migrate to tighter orbits. Here, large amounts of moisture can no longer remain on the planet's surface without volatilising.
Therefore its main masses are preserved at depth, mingled with hard rocks and, possibly, with molten lava. Such conditions do not lend themselves well to life - at least not in the forms we are familiar with. However, such 'half-water' planets are still too poorly understood and only future research will reveal many of their details.