James Webb Telescope Rules Out Earth-Like Atmosphere on TRAPPIST-1 d

Updated on: August 14, 2025 | By: Jameswebb Discovery Editorial Team

The James Webb Space Telescope (JWST) has delivered a decisive update on one of the most closely watched exoplanets in our search for life — TRAPPIST-1 d. Once thought to be a candidate for habitability, this rocky, Earth-sized world shows no signs of an Earth-like atmosphere. This conclusion comes from Webb’s highly sensitive Near-Infrared Spectrograph (NIRSpec) observations, marking another major step in our understanding of rocky planets orbiting red dwarf stars.

Why TRAPPIST-1 d Captured the World’s Attention

The TRAPPIST-1 system, located about 40 light-years away in the constellation Aquarius, holds a record: seven rocky, Earth-sized planets orbiting a single star. First revealed in 2017 through NASA’s retired Spitzer Space Telescope, the system instantly became a prime target for studying potentially habitable worlds.

TRAPPIST-1 d is the third planet from its star and lies on the edge of the so-called habitable zone — the region where conditions could allow for liquid water on the surface. While this makes it intriguing, its orbit is extremely close to its small, cool red dwarf star. At only 2% of Earth’s distance from the Sun, TRAPPIST-1 d completes one orbit — its entire year — in just four Earth days.

Webb’s Breakthrough Observation

Webb’s NIRSpec instrument scanned TRAPPIST-1 d during its transits, when the planet passes directly in front of its star from our viewpoint. This allows scientists to detect changes in starlight caused by atmospheric gases.

The result: no detection of molecules such as water vapor (H₂O), methane (CH₄), or carbon dioxide (CO₂).

Lead researcher Caroline Piaulet-Ghorayeb of the University of Chicago and the Trottier Institute for Research on Exoplanets (IREx) offered three explanations:

1. The planet may have a very thin atmosphere similar to Mars, making it hard to detect.

2. It could have dense clouds at high altitudes like Venus, blocking atmospheric signatures.

3. It might be a bare, atmosphere-less rock, stripped by stellar radiation.
   
   

This means TRAPPIST-1 d can be removed from the list of “Earth twins,” but it remains scientifically important.

Life Around a Red Dwarf Star: A Hostile Environment

Red dwarfs like TRAPPIST-1 are the most common type of star in the Milky Way, but they are far from gentle. They frequently emit powerful flares and streams of charged particles. For planets close to these stars, such radiation can erode atmospheres over millions of years.

Still, the TRAPPIST-1 system is a goldmine for exoplanet studies. If any of its planets retain an atmosphere despite the harsh conditions, it increases the chances of finding similar habitable worlds throughout the galaxy.

The Outer Planets: Hope for Habitability

While TRAPPIST-1 d may be inhospitable, the outer planets — TRAPPIST-1 e, f, g, and h — could fare better. Their greater distance from the star reduces the risk of atmospheric stripping, and they might have stable climates.

However, the farther a planet is from its star, the colder it gets, making it harder to detect atmospheric signatures, even with Webb’s advanced instruments.

“All hope is not lost,” says Piaulet-Ghorayeb. “The outer planets could still be hiding water-rich atmospheres.”

How Webb Studies Exoplanet Atmospheres

The James Webb Space Telescope revolutionizes exoplanet research through transit spectroscopy. When a planet passes in front of its star, starlight filters through its atmosphere. Molecules absorb specific wavelengths of light, leaving unique patterns — a kind of atmospheric fingerprint.

By analyzing these fingerprints, scientists can determine a planet’s composition, temperature, cloud cover, and potential habitability. Webb’s infrared capabilities allow it to detect signals invisible to ground-based telescopes, even for small rocky worlds.

Why This Discovery Matters for the Search for Life

Ruling out TRAPPIST-1 d as an Earth analog is just as valuable as confirming habitability. Every such finding helps refine the criteria for life-supporting planets.

Shawn Domagal-Goldman, acting director of NASA’s Astrophysics Division, emphasizes:

“One of the most important avenues we can pursue is understanding which planets retain their atmospheres, and why. This groundwork will position future missions to answer the universal question: Are we alone?”

The Bigger Picture — TRAPPIST-1 in Context

Since its discovery, TRAPPIST-1 has been a benchmark for exoplanetary science because:

• All seven planets are similar in size to Earth

• They are rocky, not gas giants

• The system’s compact nature allows multiple planets to be studied in detail

For comparison, most exoplanets discovered before TRAPPIST-1 were gas giants like Jupiter. Studying a system full of rocky planets opens new possibilities for finding habitable environments.

Key Facts — TRAPPIST-1 d at a Glance

• Distance: ~40 light-years from Earth

• Star: TRAPPIST-1, a cool red dwarf

• Planet Type: Rocky, Earth-sized

• Orbital Period: 4 Earth days

• Position: Third planet from star, on habitable zone’s inner edge

• Atmosphere: No Earth-like gases detected

• Discovery Tools: NASA’s Webb NIRSpec, Spitzer Space Telescope
  
  

What’s Next for Webb and TRAPPIST-1

Webb will continue monitoring the TRAPPIST-1 system, focusing on the outer planets where the odds for habitability might be higher. By refining methods to separate starlight from atmospheric signals, scientists are laying the foundation for future life-detection missions like the Habitable Worlds Observatory.

TRAPPIST-1 d may not be another Earth, but its study is vital in mapping the boundaries of planetary habitability. With every observation, Webb sharpens our understanding of where life might exist — and where it cannot.

In the grand quest to find another home in the cosmos, even “negative” results bring us closer to the truth.

Sources: NASA, ESA, CSA, STScI