James Webb rules out the possibility of life on the planet TRAPPIST-1 b

From the moment its discovery was announced on February 22, 2017, the TRAPPIST-1 system became one of astrobiologists' greatest hopes. And with good reason: seven rocky worlds , seven roughly the size of Earth, 'only' about 40 light-years away, all orbiting a tiny, cool red dwarf star. The spectacular discovery, achieved thanks to NASA's now-retired Spitzer Space Telescope, represented a veritable treasure trove of information for scientists. And it gave them the feeling that the big moment, that of finding life outside our world, was near. Suddenly, indeed, we had not one, but seven candidate worlds similar to our own to analyze.

The planet that researchers have now focused their attention on is TRAPPIST-1 d, the third in the family, located right on the edge of the so-called 'habitable zone' , the narrow orbital band around a star where temperatures are suitable for liquid water to exist.

Here in the Solar System, that zone is occupied by Earth, while Mars, which is outside it and farther from the Sun, is an icy planet, and Venus, which is also outside but on the inner edge, closer to our star, is a hell with average temperatures of 400 degrees . Needless to say, the habitable zone isn't always at the same distance, but varies from star to star, depending on their temperature. And also, as we'll see shortly, their 'character'.

In the case of TRAPPIST-1 d, a star much smaller and cooler than our own, the habitable zone is extraordinarily close, just 2% of the distance between Earth and the Sun. Its year, in fact, lasts only four Earth days. But to know if this world has any chance of supporting life, we needed to be able to analyze its atmosphere, something that hasn't been possible until now.

With the launch of the James Webb Space Telescope (JWST) , however, humanity finally gained a tool capable of peering into the atmospheres of these hopeful worlds. And the results have not been long in coming. In a study recently published in The Astrophysical Journal and led by astrophysicist Caroline Piaulet-Ghorayeb of the University of Chicago, the verdict was as clear as it was discouraging: TRAPPIST-1 d does not have a protective atmosphere similar to Earth's. Webb's NIRSpec instrument, an infrared spectrograph designed specifically for this purpose, did not, in fact, find the molecular signatures of gases such as water vapor, methane, or carbon dioxide.

Piaulet-Ghorayeb's conclusion is stark, and marks a milestone in the search for our world's 'twins': "We can rule out TRAPPIST-1 d -he says- from the list of potential twins or 'cousins' of Earth."

But what does this non-detection really mean? Scientists are considering three possible scenarios, each with its own implications. First, it could be that TRAPPIST-1 d actually has such a thin layer of gases, similar to the one surrounding Mars, that Webb simply couldn't detect it. But even if that were the case, and despite the fact that it would technically have an atmosphere, it would still be too thin to retain liquid water on the surface or to protect any life forms.

A second, more intriguing possibility is that the planet is shrouded in a dense layer of high-altitude clouds, similar to those that obscure the surface of Venus. These clouds would act as a veil, blocking the signals from the gases below, which the telescope is so keenly searching for.

Finally, the third scenario, and according to researchers the most likely, is that TRAPPIST-1 d is nothing more than a barren rock, a bare, desolate world without any protection. A not so strange fate for a planet in the vicinity of a red dwarf.

This last possibility leads us to one of the greatest lessons the TRAPPIST-1 system is teaching us. Although red dwarfs are the most common type of star in our galaxy (eight out of ten are of this type), their nature is both a blessing and a curse for life.

On the one hand, they are long-lived and stable stars, burning for many billions of years, giving their planets much more time than Earth had for life to flourish. However, they are also incredibly volatile stars.

TRAPPIST-1, like many of its sister planets, is indeed prone to releasing high-energy flares, massive solar eruptions that bombard their planets with enormous waves of radiation. Imagine if our Sun, without warning, were to hurl a burst of X-rays and particles at Earth with the force of hundreds of atomic bombs. These eruptions, as previous studies have shown, are genuine "swipes" capable of stripping even the most robust planets of their atmospheres .

The lack of an atmosphere on TRAPPIST-1 d, therefore, could be evidence that its host star's "bad mood" has been too much for it. It's proof that a world's habitability depends not only on its distance from the star, but also on its character.

Despite the result, the search for life in the TRAPPIST-1 system is far from over. Scientists are already directing the James Webb probe toward the system's outer planets: TRAPPIST-1 e, f, g, and h. These worlds, being farther from the star, may be more likely to have preserved their atmospheres, far from the brutal onslaught of flares. Therefore, the hope of finding water and other atmospheric components on these planets remains intact, although their greater distance makes them more difficult to observe.

The discovery, therefore, is not a failure but an important step in understanding what exactly makes our planet special. As Ryan MacDonald, co-author of the study, explains, "Thanks to Webb, we now know that TRAPPIST-1 d is far from a hospitable world. We're learning that Earth is even more special than we thought."

While scientists are squeezing every bit of information out of TRAPPIST-1, the search for other habitable worlds continues unabated. James Webb has already offered us clues about other fascinating exoplanets. For example, in 2023, he gave us a glimpse of K2-18 b, a planet classified as a "Hicean," a water world with a hydrogen-rich atmosphere that showed the presence of water vapor and methane. And although it's not a rocky planet like Earth, it opens the door to the existence of a completely different kind of habitable world.

Another recent case is that of LHS 475 b, an Earth-sized exoplanet orbiting the even cooler TRAPPIST-1 red dwarf, the study of whose atmosphere is ongoing. None of these worlds has turned out to be a "twin" of Earth, but there is no doubt that all of them are "natural laboratories," teaching us lesson after lesson about the ingredients and conditions necessary for life to emerge. The search, therefore, continues. And the James Webb, with its infrared eye, remains our best magnifying glass for this fascinating detective work.