When global events set our minds to wondering if humanity has what it takes to persist, it’s natural to wonder about other worlds, other life, other intelligent species, and if those others might be better suited to survive whatever Great Filters they face.
Those are fanciful thoughts, but there’s an underpinning of nuts-and-bolts thinking to them. It starts with identifying which planets in habitable zones around other stars might actually be habitable.
That begins with liquid water and a life-friendly atmosphere that can contain it.
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The discovery of the TRAPPIST-1 system generated a lot of excitement a few years ago. It contains seven roughly Earth-like worlds, and three or perhaps four of them are in the red dwarf’s compact habitable zone.
One of them, TRAPPIST-1 d, could host water on its surface, or at least on parts of its surface, according to some research. But without a suitable atmosphere, a planet can’t retain surface water, and new observations from the JWST show that TRAPPIST-1 d does not have an Earth-like atmosphere.
The JWST observed two consecutive transits of TRAPPIST-1 d with its NIRSpec/PRISM instrument in November, 2022. Researchers from Canada, the UK, France, and the USA analyzed the data from those transits and concluded that the promising exoplanet does not have an Earth-like atmosphere.
Their results are in a paper in The Astrophysical Journal titled “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 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.
“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 research.
“Here we present the first 0.6–5.2 μm NIRSpec/PRISM transmission spectrum of TRAPPIST-1 d from two transits with JWST.”
TRAPPIST-1 d is right at the inner edge of TRAPPIST-1’s habitable zone. It’s a great target for transmission spectroscopy, and these JWST observations provide the first detailed transmission spectrum of the planet’s atmosphere. Unfortunately, the spectrum is flat, meaning 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.
The JWST failed to detect the types of molecules present in Earth’s atmosphere like methane, carbon dioxide, and water. However, that doesn’t completely rule out an atmosphere; there are a couple of other possibilities.
“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.
Studying TRAPPIST-1 d and its atmosphere is about more than just ruling out its habitability. There’s a greater scientific endeavor involved.
Red dwarfs, or M dwarfs, like TRAPPIST-1 are common, and are likely the most plentiful type of star in the Milky Way. They’re known to host their share of rocky worlds where we can reasonably wonder if life persists.
But red dwarfs are also known for their violent flaring, and TRAPPIST-1 is no exception. It flares every couple of days, and each year it emits between four and six superflares. This powerful flaring activity could shred any planetary atmospheres, rendering the TRAPPIST-1 planets inhabitable.
However, there’s considerable uncertainty around red dwarf flaring and habitability. Some research shows that the planets couldn’t retain atmospheres in the face of the coronal mass ejections coming from the star.
But it’s at least possible that some of these planets could retain their atmospheres. For example, powerful planetary magnetic fields could provide a protective barrier from the star’s flaring. The JWST opens a path to understanding red dwarf flaring effects on 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.”
The only features in the JWST’s spectra are attributed to stellar contamination rather than atmospheric absorption. “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.
A low molecular weight atmosphere is harder for a planet to retain, and these observations ruled out those types of hydrogen-dominated atmospheres. The observations also ruled out thicker atmospheres like Venus’ or Titan’s.
The only things left are extremely thin atmospheres unlikely to bolster habitability, or atmospheres dominated by high clouds that mask molecular absorption features from the JWST. But the research effectively rules them out.
“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 almost certainly eliminates TRAPPIST-1 d from a list of potentially habitable, water-supporting exoplanets. This is Nature, so TRAPPIST-1 d’s elimination isn’t absolutely certain.
“Our observations cannot yet completely exclude other potential atmosphere scenarios for TRAPPIST-1 d which were predicted in the literature,” the authors explain, noting that other research involving climate models hints at the possibility that the tidally-locked planet could form high-altitude water clouds at its terminator, blocking atmospheric absorption signals from view.
But what about the other planets in the system?
“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 aren’t the juicy scientific targets that planet d is. They’re further from the star, and colder. Even the JWST’s powerful instruments struggle in those conditions. While detailed spectra aren’t available for those worlds, the researchers still reached some conclusion.
“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. Contrary to the inner planets, it’s possible that these outer planets held onto 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.”
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Being a human being means bearing witness to humanity’s greatest, most triumphant moments of accomplishment and unity, but also to the depraved actions we take against one another.
The minds of thinking people are bound to wonder if there are other worlds out there that host life. Each potentially habitable world is a glimmer of hope that humans, with all their struggles, are not the only intelligent species out there.
If we look to the heavens, and to exoplanets, for some kind of reprieve from humanity’s troubles, TRAPPIST-1 d won’t provide it. If this research is correct, its stricken from the list of hope-inspiring exoplanets.
On to the next one.
This article was originally published by Universe Today. Read the original article.
