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What is Webb Revealing About the TRAPPIST-1 System?

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Artist’s concept of the TRAPPIST-1 system, which is known to have seven Earth-sized rocky planets orbiting a red dwarf star. The host star, located in the background, is depicted as a bright white circle. It is surrounded by a yellow glow that fades to orange near the edges of the frame. The seven planets appear in the foreground, with their nightsides, the side facing away from the host star, facing the viewer. The planets are depicted as red-brown spheres. Many distant galaxies and stars faintly appear in the background. Text in bottom left corner reads “Artist’s Concept.”
Artist’s concept of the TRAPPIST-1 system, which is known to have seven Earth-sized rocky planets orbiting a red dwarf star.
Image: NASA, ESA, CSA, Joseph Olmsted (STScI)
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After scientists using the ground-based Transiting Planets and Planetesimals Small Telescope (TRAPPIST) spotted what looked like three planets orbiting a red dwarf star in 2015, follow-up observations with space telescopes brought clarity: There are actually at least seven Earth-sized rocky worlds orbiting the star.

Named after the telescope, scientists have continued to observe the TRAPPIST-1 system with various space telescopes across different wavelengths of light, making it one of the most studied planetary systems aside from our own. With NASA’s James Webb Space Telescope, a new chapter of the TRAPPIST-1 tale is underway — exciting and intriguing astronomers and the public alike.

Why is TRAPPIST-1 interesting? How are scientists using Webb to learn about the planets?

The TRAPPIST-1 system offers scientists the opportunity to study a relatively close planetary system that is both similar to and distinct from our own solar system. Based on NASA’s Spitzer Space Telescope data, astronomers measured the planets’ sizes and masses, and calculated their densities. They discovered that all seven exoplanets (b to h) are Earth-sized and probably rocky like Earth. 

However, the TRAPPIST-1 system does have qualities that differ from our own solar system, which make it favorable for study at a relatively close distance of 40-light years.

  • Compactness. All seven planets orbit closer to their star than Mercury (the innermost planet in our solar system) does to the Sun.
  • Orbital period. The planets’ orbital periods range from 1.5 to roughly 19 days. Data from the planets’ multiple transits across the face of their star can be collected over a short period of time.
  • Host star size. The ultra-cool red dwarf star is approximately 12 percent the radius of the Sun (so just slightly larger than Jupiter), making it easier to study the planets as they transit since they cover a larger fraction of the star’s surface than they would if the star was larger.
Graphic titled “TRAPPIST-1 System Versus The Solar System, Orbit Comparison.” The inner solar system is at left and labeled “Inner Solar System.” The Sun is depicted as a bright white circle. 4 planets' orbital paths are labeled: Mercury, Venus, Earth, and Mars. 2 lines extend from between the Sun and Mercury to an overlay of the TRAPPIST-1 system at right. 7 rings, representing the orbital paths of the 7 planets, surround the star. They are labeled starting with b, the closest planet to the star, and ending with h, the planet farthest away. Obital areas of planets b, c, and d are shaded red, signifying they are too close to be in the host star’s habitable zone. Orbital areas of planets e, f, and g are green, signifying they are in the area where conditions are just right for liquid water to exist on a planet’s surface. The green area is labeled “Habitable Zone.” Orbital area of planet h is blue to indicate the planet is too far away to be in the host star’s habitable zone.
All seven planets of the TRAPPIST-1 system orbit closer to their host star than Mercury does to the Sun. This diagram compares the orbits of the seven TRAPPIST-1 planets around their star, which are labeled from b to h, to the orbits of the inner solar system planets around the Sun. The orbital areas of TRAPPIST-1 b, c, and d are red to indicate they are too close to be in the host star’s habitable zone. TRAPPIST-1 e, f, and g are in their host star’s habitable zone (in green), the area where the conditions are just right for liquid water to exist on a planet’s surface. The orbital area of planet h is blue to indicate the planet is too far away to be in the host star’s habitable zone.
Image: NASA, ESA, CSA, Joseph Olmsted (STScI)
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Spitzer data also indicated that three of the TRAPPIST-1 planets (e, f, and g) are in the habitable zone. This area around the host star is where the conditions are just right for liquid water to exist on a planet’s surface.   

Whether these planets could have liquid water depends in part if they can hold an atmosphere. Astronomers used NASA’s Hubble Space Telescope to inspect their atmospheres, which ruled out the presence of puffy hydrogen-rich atmospheres similar to that of Neptune for some of the planets. 

When Webb launched in 2021, it moved past the wavelength and stability limits of earlier observatories, opening up atmospheric study in a way they couldn’t. Webb’s position around L2, infrared sensitivity, and larger mirror make it the only telescope able to continuously lock onto an observational target, collect the optimal wavelengths of light, and observe the faint signals at a precision needed for exoplanet atmosphere characterization.  

Since direct imaging of these planets is not possible because of how close they are to their host star, scientists use different approaches to learn about the system. Each method provides one piece of the puzzle that is necessary to determine whether the TRAPPIST-1 planets have atmospheres. Collectively, the data from Webb is providing scientists the ability to investigate the system’s formation and consider how common these environments are. 

  • Transit. When a planet moves between its star and the telescope, blocking some of the starlight.
Infographic titled “Exoplanet TRAPPIST-1 e Transmission Spectrum” showing data points from the NIRSpec instrument on NASA’s James Webb Space Telescope compared with model spectrums.
During a transit, some of the starlight is absorbed by atoms and molecules in the planet’s atmosphere, if it has one. Webb can use transmission spectroscopy to read and parse the filtered starlight, enabling scientists to note which wavelengths are absorbed and provide evidence for the planet’s atmospheric composition. This transmission spectrum graph compares data collected by Webb (white points) with computer models of exoplanet TRAPPIST-1 e with (blue) and without (orange) an atmosphere. Narrower, darker colored bands show the most likely locations of data points for each model while wider, more transparent bands show areas that are less likely but still permitted by the models. The gray region shows where the two models overlap. Researchers can’t yet confidently rule out an atmosphere for TRAPPIST-1 e since many of the data points fit either scenario. 
Illustration: NASA, ESA, CSA, STScI, Joseph Olmsted (STScI)
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  • Secondary eclipse. When the planet moves behind the star and the light coming from the planet is blocked.
Infographic titled “Rocky Exoplanet TRAPPIST-1 b Secondary Eclipse Light Curve” showing a diagram of a secondary eclipse and a graph of change in brightness over time.
Astronomers can subtract the brightness of the star from the combined brightness of the star and planet to calculate how much infrared light is coming from the planet’s dayside. This is then used to calculate the dayside temperature. If the planet has a thick atmosphere with winds redistributing heat, the dayside will be significantly cooler than it would be without an atmosphere. This light curve shows the change in brightness of the TRAPPIST-1 system as TRAPPIST-1 b moves behind the star. 
Illustration: NASA, ESA, CSA, Joseph Olmsted (STScI); Science: Thomas Greene (NASA Ames), Taylor Bell (BAERI), Elsa Ducrot (CEA), Pierre-Olivier Lagage (CEA)
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  • Phase curve. The star-planet system’s changes in brightness as the planet orbits its star.
Graphic showing how the observed total brightness of a star-planet system changes as the planet orbits the star.
This simplified diagram of an exoplanet phase curve shows the change in total brightness of a star–planet system as the planet orbits the star. The system looks brighter when more of the lit side of the planet is facing the telescope (full phase). It looks dimmer when more of...
Image: NASA, ESA, CSA, Dani Player (STScI), Andi James (STScI), Gregory Bacon (STScI)
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What has Webb revealed about the TRAPPIST-1 system and its planets? And what do we still not know?

Data has been successfully collected for all seven planets. As of December 2025, the science community has reported on their Webb observations for four of the seven TRAPPIST-1 planets: b, c, d, and e. So far, Webb hasn’t seen signs of thick atmospheres on TRAPPIST-1 b and TRAPPIST-1 c. The current data for b suggests it may be a bare rock with no atmosphere. If c does have an atmosphere, it’s very thin. For TRAPPIST-1 d and TRAPPIST-1 e, the data is still under study. For now, scientists have ruled out that these two planets have thick hydrogen atmospheres. 

Artist’s concept titled “TRAPPIST-1 System” showing a section of the red dwarf host star at the bottom and seven circles vertically aligned and labeled b to h from bottom to top against a solid black background. A half circle on the bottom represents the host star, TRAPPIST-1, which has a bright orange mottled surface with flares that extend outward. Artist’s concepts for the planets b, c, d, and e are encased in their respective circles, indicating that scientists have reported on their Webb observations for these planets. Planets b and c have brighter circles around them to indicate that scientists have more confidence in their interpretation, while the circles around d and e are fainter to indicate the data is still under study. Planets f, g, and h are depicted as circles with white dashed lines. A question mark is in each of these three circles to indicate that the atmospheric statuses of these planets are still to be determined as the Webb data continues to be analyzed.
So far, the science community has reported on their Webb observations for four of the seven TRAPPIST-1 planets: b, c, d, and e. Of the four planets, scientists have found little atmospheric evidence for TRAPPIST-1 b and TRAPPIST-1 c (white outlines). For TRAPPIST-1 d and TRAPPIST-1 e, the data is still under study (gray outlines), but scientists have ruled out the existence of thick hydrogen atmospheres. The community is still analyzing the Webb data on TRAPPIST-1 f, g, and h (dashed outlines). 
Image: NASA, ESA, CSA, Joseph Olmsted (STScI)
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While scientists expected the red dwarf host star to be active, its activity is much more intense than originally predicted. TRAPPIST-1’s stellar flares and star spots make it challenging to distinguish between signals from the planets’ atmospheres and “contamination” from the star’s activity. To mitigate such challenging conditions, astronomers will have to take more observations than anticipated.

Advances in our scientific understanding are often gradual, and the study of the TRAPPIST-1 system is no exception. To become more certain in the atmospheric characterization of these planets will require follow-up observations of potentially hundreds of transits with Webb over several years.

Will Webb be able to search for signs of life in the TRAPPIST-1 system?

Webb will help scientists determine which planets do or don’t have atmospheres, therefore helping scientists understand some of the dynamics of atmosphere loss and retention.

If any of the planets in the TRAPPIST-1 system have atmospheres, Webb can begin to study their chemical compositions, which could offer tentative clues about habitability. However, Webb is not likely to detect biosignatures on these planets. 

How does the TRAPPIST-1 system tie into the broader story of exoplanets?

TRAPPIST-1 is an exciting case study, serving as a rich data point that ties into our larger interest in exoplanets. The currently underway Rocky Worlds Director’s Discretionary Time program is a Webb-Hubble collaboration to answer whether planets around red dwarf stars (like TRAPPIST-1’s star) are able to retain atmospheres. 

Astronomers will learn even more about the population of Earth-sized exoplanets with the future Habitable Worlds Observatory, and will continue studying TRAPPIST-1 and other planetary systems with the European Southern Observatory’s upcoming Extremely Large Telescope. 

The scientific tale of TRAPPIST-1 is still being written. Each observation by Webb brings forth new information, gradually building our knowledge and shaping how we perceive  these distant worlds, and driving our shared sense of excitement for what’s yet to come. We can’t help but wonder: What will we discover next about this system? 

By Abigail Major
Space Telescope Science Institute in Baltimore, MD

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