Astronomers usually classify the planets of our solar system into two types: rocky planets and gas giants. However, a new study of planets in other star systems in our galaxy finds that there is a third kind of world made up of about 50% water and 50% rock. And such water-rich worlds are an intriguing place for astronomers to test hypotheses about why planets can sustain life.

Rafael Luque, a postdoctoral researcher at the University of Chicago and the first author of the new study, published Thursday, said: “In our lifetimes, we have scientifically proven the habitability of other planets. We may be able to say something for the first time,” he said.in the journal chemistry“And that’s a big big step.”

In recent years, astronomers have rapidly detected new planets orbiting beyond our star, called exoplanets. To date, more than 5,000 exoplanets have been discovered and confirmed. But figuring out exactly what these worlds look like—whether they’re habitable from light-years away—is a daunting task.

Most exoplanets are discovered using what is called the transit method. It indirectly identifies a planet by observing how the star’s light dims slightly as the planet passes in front of it. Astronomers can also infer the radius of an exoplanet by how much it blocks the star’s light. Scientists have used that information to compare these alien worlds to the planets of our own solar system and hypothesize what they might look like. Considered to be quite rocky.

But in orbit around many red dwarfs are the most common stars in the galaxy, the kind of planets that have no relatives in our solar system. Based on their radii, these worlds fit in the size gap between Earth and Neptune.

[Related: Newly discovered exoplanet may be a ‘Super Earth’ covered in water]

Astronomers have long thought that these small planets fall into two categories. Some were thought to be “super-Earths” and some were thought to be “mini-Neptunes”. Planetary scientist Ravi Koparap of NASA’s Goddard Space Flight Center said the idea was supported by the observation that there is a shortage of exoplanets with radii about 1.6 times the size of Earth. new research. The way stellar radiation erodes the planet’s atmosphere has been thought to explain the radial gap, he says.

According to that theory, the “super-Earth” on the smaller side of its radius valley was left with a very thin atmosphere and a mostly exposed rocky surface. On the other hand, “mini-Neptune” retained a thick, puffy atmosphere, so these gas planets had larger radii.

But there may be other ways to build exoplanets with these radii. These worlds may indeed be alien, as they have no analogues in our solar system. In order to elucidate certain substances, we have attempted to determine their densities.

Density is not something that can be directly measured from very far away, but using the planet’s mass and radius makes for a simple calculation (mass divided by volume equals density). The researchers used measurements of the radii and masses of 34 newly detected planets by the Transiting Exoplanet Survey Satellite (TESS), which launched in 2018, to determine the density of these mysterious small exoplanets. samples were collected.

[Related: We may be underestimating how many cold, giant planets are habitable]

Based on their calculations, the radius troughs do not separate the different types of planets in orbit around the red dwarf. Density. And those exoplanets, he speculated, could be one of three types of worlds, rich in rock, gas, or water, a new type.

“We may think of Earth as a water-rich planet, but water on Earth is only 0.02% of its total weight,” says Luque. On the other hand, the densities of these distant water worlds indicate that about half of their mass is water.

But don’t start imagining a space-exposed world with a rocky core and a deep ocean of water splashing over it, says Luque. This water shouldn’t be on the surface,” he says. “Water could be trapped below the surface or mixed with magma, but it’s not going to be in the form of a deep ocean, at least at the surface.”

The closest approximations to such water-rich worlds in our own solar system are the moons of Jupiter and Saturn. For example, one of Jupiter’s moons, Europa, has a deep ocean bobbing beneath a global ice shell of water.

According to Luque, these exoplanets are unlikely to have water ice shells. Because these planets are so close to their stars, surface water evaporates. That is, at least, the sun-facing side of the planet. These worlds do not rotate on their axis to have a day and night cycle like Earth does. Instead, there are permanent bright and dark sides. But Luque says there are regions where light and darkness meet, and like a kind of eternal twilight, the surface temperature may be just right for liquid water to stabilize.

When searching for habitable worlds, astronomers typically use liquid water as a guide. Because it is essential to life as we know it (that is, life on Earth, because it is the only life known so far).

“There is only one template for life in this universe, so we use that as a template to find other life,” Kopparap says. But stable liquid water isn’t the only thing that’s needed to make a place habitable by that definition, and just because a place can sustain life doesn’t mean it’s inhabited. he added.

To investigate the habitability of these distant worlds, astronomers can study the chemistry of exoplanet atmospheres to reveal details about their composition with the newly launched James Webb Space Telescope (JWST). Look to tools like Using telescopes like JWST, astronomers look for water vapor to see the presence of H2O as well as gases such as methane, oxygen, carbon dioxide, and nitrogen found in the Earth’s atmosphere.

“We’re finding more and more evidence that there are many potentially habitable worlds. Our planet is nothing special,” Koppalap says. He uses the following analogy. That’s why he finds so many houses. All you have to do is find someone. “