JWST Delivers Bad News About Life on TRAPPIST-1 Planet

When world events make us question whether humanity has what it takes to endure, it is only natural to let the mind drift towards other worlds: other life, other intelligent species-and whether they might be better equipped to get through whatever Great Filters lie in their path.

These are imaginative ideas, yet they rest on a practical foundation. The first task is to work out which planets sitting in habitable zones around other stars could genuinely be habitable.

That assessment starts with liquid water-and with a life-supporting atmosphere capable of keeping that water stable.

The TRAPPIST-1 system and the appeal of TRAPPIST-1 d

A few years ago, the discovery of the TRAPPIST-1 system sparked huge excitement. The system contains seven broadly Earth-like worlds, with three-or possibly four-orbiting within the compact habitable zone of the red dwarf.

One of those planets, TRAPPIST-1 d, has been suggested by some studies as a world that could support water at its surface, or at least across parts of it. Yet without the right atmosphere, a planet cannot hold on to surface water. Fresh observations from the James Webb Space Telescope (JWST) now indicate that TRAPPIST-1 d does not possess an Earth-like atmosphere.

In November 2022, JWST watched two back-to-back transits of TRAPPIST-1 d using its NIRSpec/PRISM instrument. A team of researchers from Canada, the United Kingdom, France, and the United States examined the transit data and found that this once-promising exoplanet lacks an Earth-like atmosphere.

Their findings appear in The Astrophysical Journal in a paper entitled "Strict Limits on Potential Secondary Atmospheres on the Temperate Rocky Exo-Earth TRAPPIST-1 d." The lead author is Caroline Piaulet-Ghorayeb of the University of Chicago and the Trottier Institute for Research on Exoplanets (IREx) at Université de Montréal.

"While TRAPPIST-1 d may prove a barren rock illuminated by a cruel red star, the outer planets TRAPPIST-1e, f, g, and h, may yet possess thick atmospheres." - co-author Ryan MacDonald, University of St. Andrews.

JWST, NIRSpec/PRISM, and a flat transmission spectrum for TRAPPIST-1 d

"The nearby TRAPPIST-1 system, with its seven small rocky planets orbiting a late-type M8 star, offers an unprecedented opportunity to search for secondary atmospheres on temperate terrestrial worlds," the authors write in their study.

"Here we present the first 0.6–5.2 μm NIRSpec/PRISM transmission spectrum of TRAPPIST-1 d from two transits with JWST."

Because TRAPPIST-1 d sits right at the inner edge of TRAPPIST-1’s habitable zone, it is an excellent candidate for transmission spectroscopy. These JWST measurements provide the first detailed transmission spectrum for the planet’s atmosphere. The problem is that the spectrum is flat-there are no detectable atmospheric features.

"Ultimately, we want to know if something like the environment we enjoy on Earth can exist elsewhere, and under what conditions. While the James Webb Space Telescope is giving us the ability to explore this question in Earth-sized planets for the first time, at this point we can rule out TRAPPIST-1 d from a list of potential Earth twins or cousins," said lead author Piaulet-Ghorayeb in a press release.

What JWST did (and did not) detect in the atmosphere

JWST did not detect the kinds of molecules found in Earth’s atmosphere, such as methane, carbon dioxide, and water. Even so, that does not automatically mean there is no atmosphere at all; there are still a couple of alternative explanations.

"There are a few potential reasons why we don't detect an atmosphere around TRAPPIST-1 d. It could have an extremely thin atmosphere that is difficult to detect, somewhat like Mars.

"Alternatively, it could have very thick, high-altitude clouds that are blocking our detection of specific atmospheric signatures - something more like Venus. Or, it could be a barren rock, with no atmosphere at all," Piaulet-Ghorayeb said.

Why TRAPPIST-1 d matters beyond habitability: red dwarfs, flares, and atmospheric survival

Investigating TRAPPIST-1 d’s atmosphere is not only about deciding whether it could be habitable. It also feeds into a broader scientific effort.

Red dwarfs-also called M dwarfs-like TRAPPIST-1 are widespread and are likely the most common type of star in the Milky Way. They are known to host plenty of rocky planets, which makes it reasonable to ask whether life might persist on some of them.

However, red dwarfs are also notorious for violent flaring, and TRAPPIST-1 fits that pattern. It flares every couple of days, and it produces between four and six superflares each year. Such intense activity could strip away planetary atmospheres, potentially making the TRAPPIST-1 planets uninhabitable.

Even so, the connection between red dwarf flares and habitability remains uncertain. Some research suggests these planets would be unable to keep atmospheres when faced with coronal mass ejections from the star.

Yet it is still plausible that certain worlds could maintain their atmospheres. Strong planetary magnetic fields, for instance, might act as shields against the star’s outbursts. JWST provides a new way to investigate how red dwarf flaring affects planetary atmospheres.

"Webb's sensitive infrared instruments are allowing us to delve into the atmospheres of these smaller, colder planets for the first time," said Björn Benneke of IREx at Université de Montréal, a co-author of the study.

"We're really just getting started using Webb to look for atmospheres on Earth-sized planets, and to define the line between planets that can hold onto an atmosphere, and those that cannot."

What the spectrum implies: stellar contamination and strict limits on atmospheres

In the JWST spectra, the only apparent features are attributed to stellar contamination rather than absorption in a planetary atmosphere. "Our precise transmission spectrum can be fully explained by stellar contamination alone, and therefore enables us to rule out cloud-free or thick atmosphere scenarios across a wide range of potential atmospheric metallicities," the authors write.

Low molecular weight atmospheres are more difficult for a planet to retain, and these measurements rule out hydrogen-dominated atmospheres of that kind. The same data also exclude thicker atmospheres comparable to Venus’ or Titan’s.

What remains are either extremely thin atmospheres that would be unlikely to support habitability, or atmospheres dominated by high clouds that conceal molecular absorption features from JWST. In practice, the study largely closes off those options as well.

"Therefore, we conclude that (1) thick cloud-free hydrogen-rich atmospheres are ruled out by our transmission spectrum; (2) thin H2-rich alternatives are strongly disfavored when considering TRAPPIST-1 d in the context of its formation and evolution under stellar irradiation; and (3) high-altitude clouds or hazes are not expected to form on TRAPPIST-1 d if it has a low-metallicity atmosphere," the researchers explain.

This work very likely removes TRAPPIST-1 d from the shortlist of potentially habitable, water-supporting exoplanets. Still, as with much in nature, its exclusion is not completely beyond doubt.

"Our observations cannot yet completely exclude other potential atmosphere scenarios for TRAPPIST-1 d which were predicted in the literature," the authors explain, pointing out that other studies using climate models suggest the tidally locked planet might form high-altitude water clouds at its terminator-clouds that could hide atmospheric absorption signals from view.

The rest of the system: outer planets and lingering possibilities

So what does this mean for the other planets orbiting TRAPPIST-1?

"All hope is not lost for atmospheres around the TRAPPIST-1 planets," Piaulet-Ghorayeb said. "While we didn't find a big, bold atmospheric signature at planet d, there is still potential for the outer planets to be holding onto a lot of water and other atmospheric components."

However, the outer planets are not as enticing a target as planet d. Their orbits place them farther from the star, making them colder-and even JWST’s powerful instruments find those conditions challenging. Although the researchers do not yet have detailed spectra for those worlds, they still draw an important inference.

"We find that even complete atmosphere loss for TRAPPIST-1 d would not preclude atmosphere presence for the outer HZ planets TRAPPIST-1 e, f, and g," the authors write in their conclusion. Unlike the inner planets, these outer worlds might have held on to their water "even if they initially accreted only a few Earth oceans of volatiles."

"Our detective work is just beginning. While TRAPPIST-1 d may prove a barren rock illuminated by a cruel red star, the outer planets TRAPPIST-1e, f, g, and h, may yet possess thick atmospheres," added Ryan MacDonald, a co-author of the paper, now at the University of St Andrews in the United Kingdom, and previously at the University of Michigan.

"Thanks to Webb we now know that TRAPPIST-1 d is a far cry from a hospitable world. We're learning that the Earth is even more special in the cosmos."

Why we keep looking

To be human is to witness humanity at its best-its greatest achievements and moments of unity-while also seeing the brutal things we do to one another.

Thoughtful people are inevitably drawn to wonder whether there are other life-bearing worlds out there. Every potentially habitable planet represents a small spark of hope that humans, for all our struggles, are not the only intelligent species in existence.

If we search the skies-and exoplanets-for some sense of relief from humanity’s problems, TRAPPIST-1 d will not provide it. If this study is right, it should be struck from the list of hope-inspiring exoplanets.

On to the next one.

This article was originally published by Universe Today. Read the original article.