Observed by the James Webb telescope TOI-270 d, a planet located between two waters – Libération

The universe is full of billions of planets: this is now confirmed thanks to recent developments in telescopes, which continue to discover new objects orbiting stars. It is no longer a question of whether there are other planets outside our solar system. They are everywhere. Astronomers in the twenty-first century are now working to characterize these exoplanets, that is, to determine their characteristics, and we are not at the end of our surprises. These worlds are often very different from our own…. Including when they look like us at first glance.

Two astrophysicists from the University of Cambridge, UK, have discovered an exoplanet rich in water and hydrogen, just like Earth's atmosphere. But it's hot on this planet located 73 light-years from here. too hot. So much so that we don't really know what state all water exists in: as vapor in a super-dense atmosphere? Or on Earth in the form of a liquid ocean but about to boil? The discussion is open.

Infernal temperatures

The data that researchers rely on comes from the James Webb Space Telescope (JWST), which NASA launched at the end of 2021. A few months ago, it was assigned to observe the planetary system TOI-270, because it is interesting, with its three planets orbiting close to the star located in Center. The closest planet is a “super-Earth,” which is rocky and slightly larger than our home planet, and which orbits its star in three days. The other two are “sub-Neptunes” that orbit the star in just six and a half days. The outermost planet, TOI-270 d, showed the presence of water molecules when observed by the Hubble telescope a few years ago.

Space telescopes capture the light emitted by stars, and its different wavelengths can then be analyzed in detail to understand how it is filtered by exoplanetary atmospheres, for example, according to the chemical molecules it is composed of.

And from what we know about the chemistry of TOI-270 d, it is “Hycean-Filter Planet”The two British researchers explained in the introduction to their study, Published this week In the scientific journal Astronomy and astrophysics. The Hessian planet is a plaque coined by the astronomer who led the study, Nikko Madhusudan, to identify the planet with an ocean of liquid water beneath a hydrogen-rich atmosphere. Conditions that seem ideal for potential life forms… well, on paper. Because we must not forget that TOI-270 is only 10.5 million kilometers from its star, or 14 times less than the distance between Earth and the Sun, and therefore the temperatures there are hellish.

If TOI-270 d water is collected in an ocean, this is the ocean “It may rise to 100 degrees Celsius or more.”, Nikko Madhusudan thinks In the guardian. Which is technically possible, because the atmosphere of this planet is very dense and “At high atmospheric pressure, such a warm ocean could still be liquid.” Atmospheric pressure, hundreds of times greater than Earth's, can crush the liquid to the ground and prevent it from evaporating. Only the surface will be covered with a layer of steam.

More precisely, we must imagine an ocean consisting of two parts that are completely different from each other, because the planet has two sides. One is constantly heading towards its star, and is very hot. The other turns his back on the star and remains more moderate. “The ocean will be very warm on the day side. The night side will probably be habitable.”This means that it is conducive to the development of life, judges Madhusudan.

“Like a thick, hot liquid.”

Or there is no ocean. Another international study Conducted by Bjorn Beneke, a Canadian astrophysicist from the University of Montreal, which has just been proposed for publication and has not yet been validated, it relies on the same observations from the James Webb Telescope in favor of a different hypothesis. “Climate modeling [de TOI-270 d] He points out that an ocean of liquid water is highly unlikely, and that if water exists inside the planet, it must be at a supercritical stage.This study presents. liquid Above embarrassment It is neither a liquid nor a gas. It exists under conditions of such extreme pressure that the boundary between gas and liquid disappears, and it has the characteristics of both states. “It's like a hot, thick liquid.”tries to explain Björn Beneke's guardian.

According to him and his colleagues — 31 researchers co-signed on this other study — the planet's surface temperature is about 4,000 degrees Celsius, and there is very little chance the water remains liquid. So it will not be on the planet but around it: “We found that water vapor is abundant in TOI-270 d's atmosphere, which is probably its main component by mass.” Water and hydrogen will mix in a supercritical state, enveloping the planet's rocky core.

TOI-270 d is therefore a bad candidate if we are looking for exoplanets where life could exist, even if they appear to harbor carbon disulfide that is a sign of biological processes on Earth. However, we are still light years away from being able to find or prove the existence of even the slightest extraterrestrial bacteria using a telescope. “We need to be very careful about how we communicate our discoveries about these types of stars. It is easy for the public to quickly conclude that we have found life elsewhere.recalls Madhusudan. All we can say is that we are making giant strides in studying exoplanets from a distance, to the point of knowing how to measure their degree of “habitability.” This is already progress.