Researchers say that a new type of exoplanet, one made half of rock and half of water, has been found around the most common stars.

More than 70% of the universe's stars are red dwarfs. The stars are usually one-fifth the size of the sun and up to 50 times dimmer.

The fact that red dwarfs are so common has made scientists wonder if they are the best place to find planets with life. Astronomers who discovered the bright red dwarf Gliese 887 in 2020 may be able to host a planet that is suitable to host liquid water.

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Artist's impression of the view from a half-rock, half-water world.

This is an artist's impression of the view from a half-rock, half-water world. (Image credit: Pilar Montañés (@pilar.monro))

Because of the lack of understanding that researchers have about these worlds' composition, it's not certain if the worlds are potentially habitable. Small exoplanets, those less than four times Earth's diameter, are usually either rocky or gassy and have either a thin or thick atmosphere.

The compositions of exoplanets were examined in the new study. They focused on the red dwarfs that were observed by the NASA's TESS satellite.

Astronomers can't see most exoplanets directly because the stars are so bright. Scientists usually detect exoplanets via the effects these worlds have on their stars, such as the shadow created when a planet crosses in front of its star.

When a planet crosses in front of a star, scientists can see the diameter of the planet. Researchers can find the mass of a planet by measuring its pull on a star.

34 exoplanets were analyzed by astrophysicists who had precise data on mass and diameter. The densities of these worlds were estimated with the help of these details.

According to the study co-author, we can split the worlds into three families. There are at least 21 rocky planets and seven gassy planets, as well as six examples of a new type of exoplanet, watery, which is made of about half-rock and half-water.

Luque said that he was surprised to see evidence for so many water worlds around the most common type of star. It has consequences for the search for planets that are hospitable to humans.

The models suggest the small planets evolved in three different ways. Near their stars, the rocky planets may have formed.

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The demographics of small planets around red dwarf stars.

The demographics of small planets around red dwarf stars. (Image credit: Rafael Luque (University of Chicago), Pilar Montañés (@pilar.monro), Gabriel Pérez (Instituto de Astrofísica de Canarias), and Chris Smith (NASA Goddard Space Flight Center))

The small rocky planets have the same density as Earth. Their compositions must be very similar.

The watery planets are thought to have arisen far away from their stars, past the "ice line" where surface temperatures are freezing. They moved closer in to the spot where the astronomer found them.

Water-rich gassy planets may have formed in the same way as watery planets. They probably initially possessed more mass and were able to gather a hydrogen and helium atmosphere around themselves.

Although the rocky planets are relatively poor in water and the watery planets rich in it, that may not mean the former are arid and the latter are covered in oceans.

Even though three-fourths of the surface is covered in water, the Earth only has 0.05% of its mass in water. Although the watery planets discovered are half-water, that doesn't mean they have huge oceans on their surface. The water and the rock seem to mix.

It is possible that these three kinds of worlds are found around larger stars. Luque said that a new generation of instruments in ground-based telescopes would allow them to make these measurements.

The composition and properties of these watery worlds are something to investigate. Luque said in the interview that they can analyze their atmospheres, if they have any, and see how they store water if they have a space telescope. This will show us a lot about their structure.

Scientists detailed their findings in a journal.

It was originally published on Space.com