discovery of the first rocky exoplanet atmosphere on Cancri 55 e

It is a historic discovery if it is confirmed, that of the first atmosphere of a rocky planet outside the solar system. This was one of the objectives of the James Webb telescope: to capture the spectrum of a small planet, rocky like Earth. It’s done, it’s called Cancri 55 e, even if there’s nothing hospitable about it because it’s an ultra-hot world.



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Cancri 55 e, also called Janssen, is a super-Earth, a rocky planet significantly larger than Earth.  (MARK GARLICK/SCIENCE PHOTO LIBRA / GETTY IMAGES)

Mathilde Fontez, editor-in-chief of the scientific magazine Epsiloon, tells us about this discovery published in the scientific journal Nature: the first detection of the atmosphere of a rocky exoplanet.

franceinfo: Is Cancri 55 e a planet like Earth, but outside our solar system?

Mathilde Fontez: Yes, it’s a very, very distant planet. It is located 41 light years from Earth. So it’s impossible to go there. But we see it with telescopes. It’s called Cancri 55th and it was discovered in 2004.

In 2022 and 2023, it was targeted by the powerful James Webb Space Telescope. The analysis of these results has just been published. For the first time, we seem to detect an atmosphere around a rocky exoplanet, like Earth.

When were exoplanets first discovered?

The first was discovered in 1995. There are more than 5,000 of them. There are exoplanets everywhere, around almost every star. By studying their eclipses, the way they filter light when they pass in front of their star, astrophysicists have already succeeded in analyzing the atmospheres of the largest of them, the gas giants. But until then, we had never been able to see a rocky planet atmosphere.

This has been one of the great objectives of the James Webb Telescope since the beginning. Cancri 55 e is one of these terrestrial planets. Even if it is very different from our Earth, since it is twice as big, and above all, it orbits very close to its star. It is therefore ultra-hot, 2,000 degrees according to the researchers’ thermal studies. Its surface is likely to be a gigantic ocean of molten rock.

What is its atmosphere like?

The researchers conclude that there is a layer of gas measuring a few percent of the planet’s radius, a bit like the Earth (for us it is 7%). It would be an atmosphere of carbon, probably carbon dioxide, or carbon monoxide. All this is conditional because the results have just been published as part of an accelerated scientific publication process – this is specific to the James Webb telescope which triggers a lot of scientific competition.

The community of experts will now discuss and put these data to the test. But the quest advances. That of being able to see, study, other planets of the same type as ours, to finally see their diversity.

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