Can red dwarf planets support life? Webb investigates TRAPPIST-1

A planet that orbits a star nearby to us once seemed like a good candidate for being Earth-like. It’s rocky, similar in size to Earth, and lies in a zone where surface water could theoretically exist. But new findings suggest this exoplanet doesn’t have an atmosphere anything like ours.

The latest data comes from NASA’s James Webb Space Telescope and focuses on TRAPPIST-1 d, a planet that is 40 light-years away in the TRAPPIST-1 system.

Ruling out a potential Earth twin

Caroline Piaulet-Ghorayeb, lead author of the study, is a scientist at the University of Chicago and the Trottier Institute for Research on Exoplanets (IREx) at Université de Montréal.

“Ultimately, we want to know if something like the environment we enjoy on Earth can exist elsewhere, and under what conditions,” said Piaulet-Ghorayeb.

“While NASA’s 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.”

Why TRAPPIST-1 d looked promising

The TRAPPIST-1 system gained attention in 2017 for hosting seven Earth-sized, rocky planets. It’s the largest group of Earth-sized worlds ever found orbiting a single star. The star at the center is a red dwarf – cooler and smaller than our Sun – which shifts the “habitable zone” much closer in.

TRAPPIST-1 d is the third planet from its star and sits near the edge of this temperate zone. But “near” is relative: it orbits at just 2% of the Earth-Sun distance. A year there lasts only four Earth days.

Despite that tight orbit, its location seemed promising for having liquid water – and maybe an atmosphere.

But when researchers used the Webb telescope’s NIRSpec instrument to scan for signs of key molecules, they found no water vapor, methane, or carbon dioxide.

Lack of a detectable atmosphere

“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,” Piaulet-Ghorayeb said.

She explained that the planet could also have very thick, high-altitude clouds that block our detection of specific atmospheric signatures – similar to Venus. It’s also possible that TRAPPIST-1 d is a barren rock, with no atmosphere at all.

The researchers aren’t ready to draw final conclusions. But one thing is clear: TRAPPIST-1 d doesn’t have an atmosphere that looks like Earth’s.

Life near a red dwarf

TRAPPIST-1, the star at the center of this system, may not be a great neighbor for planets hoping to hold onto their atmospheres. Red dwarfs are known for intense, high-energy flares that can blast nearby planets with radiation.

These flares can strip atmospheres away – especially from planets orbiting close in, like TRAPPIST-1 d.

Scientific interest in these kinds of stars remains high. Red dwarfs are the most common type of star in the galaxy. If a planet can hold onto an atmosphere here, there’s a chance it could do so anywhere.

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

“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.”

Eyes on the outer planets

While planet d didn’t pan out as a strong Earth analog, the story isn’t over. Webb is still collecting data on the outer planets in the system – TRAPPIST-1 e, f, g, and h.

These planets are farther from the radiation blasts of their host star, which might give them a better chance of retaining an atmosphere.

But that distance also makes it harder to detect atmospheric signatures, even with the Webb telescope’s precision.

“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 for planet d, there is still potential for the outer planets to be holding onto a lot of water and other atmospheric components.”

One step closer to the big question

NASA sees this work as laying the foundation for something much bigger: understanding which planets could support life.

“As NASA leads the way in searching for life outside our solar system, one of the most important avenues we can pursue is understanding which planets retain their atmospheres, and why,” said Shawn Domagal-Goldman, acting director of the Astrophysics Division at NASA Headquarters in Washington.

“NASA’s James Webb Space Telescope has pushed our capabilities for studying exoplanet atmospheres further than ever before, beyond extreme worlds to some rocky planets – allowing us to begin confirming theories about the kind of planets that may be potentially habitable.”

“This important groundwork will position our next missions, like NASA’s Habitable Worlds Observatory, to answer a universal question: Are we alone?”

TRAPPIST-1 d may not be an Earth twin. But it’s helping scientists understand what makes a planet capable – or incapable – of sustaining an atmosphere. And in the search for life beyond our solar system, that’s a crucial piece of the puzzle.

Information from a NASA online press release.

Image Credit: NASA, ESA, CSA, Joseph Olmsted (STScI)

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