The finding narrows the search for worlds with conditions similar to Earth, according to lead author Caroline Piaulet-Ghorayeb of the University of Chicago and Universite de Montreal's Trottier Institute for Research on Exoplanets. Webb's NIRSpec instrument found no signs of water, methane, or carbon dioxide in TRAPPIST-1 d's atmosphere.

Possible explanations include an extremely thin atmosphere like Mars, a thick high-altitude cloud cover similar to Venus, or no atmosphere at all. TRAPPIST-1 d orbits its red dwarf star at just 2 percent of Earth's Sun distance, completing a year in four Earth days.

Its host star's intense flares may strip away planetary atmospheres, but red dwarfs are the most common stars in the galaxy, making them important targets. "While we can rule out TRAPPIST-1 d as a potential Earth twin, the outer planets in the system may still hold onto atmospheres," Piaulet-Ghorayeb noted.

Co-author Bjorn Benneke said Webb's infrared capabilities are revealing the limits of atmospheric retention on small, cold worlds for the first time. Ryan MacDonald, also a co-author, added that TRAPPIST-1 d's case underscores Earth's uniqueness.

Ongoing Webb observations will target TRAPPIST-1's outer planets e, f, g, and h, which may have better chances of atmospheric survival despite being harder to study due to their distance and colder conditions.

Research Report:Strict Limits on Potential Secondary Atmospheres on the Temperate Rocky Exo-Earth TRAPPIST-1 d

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