Exoplanet · Deep guide · orbits TRAPPIST-1
TRAPPIST-1e
Astronomy's favorite habitable-zone world: rocky, temperate, and testable.
What is it?
TRAPPIST-1e is the fourth of seven Earth-sized planets circling the tiny red star TRAPPIST-1, 40.7 light-years away — and by many measures the most promising habitable-zone planet we know. It is almost exactly Earth's size, receives close to Earth's amount of starlight, and its density suggests a rocky world that could hold water. Best of all, it transits its star, so telescopes like JWST can actually search its sky for air.
Go deeper
Among the seven, e is the density standout: ~0.69 Earth masses at 0.92 Earth radii implies rock with a possible volatile layer, and equilibrium temperature ~250 K sits squarely temperate for modest greenhouse warming. Resonant-chain transit timing gives exquisite masses; JWST secondary-eclipse work has already shown inner siblings b and c are airless or nearly so, making e the pivotal test of whether M-dwarf habitable zones deserve the name. A thick-atmosphere detection here would be arguably the biggest exoplanet result of the decade.
01 A sky full of sister worlds
From TRAPPIST-1e's surface, the star hangs huge and dull red — and the other six planets parade overhead as visible disks, some appearing twice the size of our full Moon, showing phases as they pass. Sunsets would glow crimson; 'noon' light would resemble a red-tinted early evening on Earth. Its year — one orbit — takes six days.
02 Why 'e' beats its siblings Deeper
b and c roast; d skirts the zone's hot edge; f, g and h drift toward iceball territory. e gets the Goldilocks slot AND a mass–radius combination that rules out a puffy hydrogen envelope. When astronomers modeled which TRAPPIST world most plausibly keeps liquid surface water, e kept winning — which is why it owns a disproportionate share of JWST's calendar.
The deep dive
Researched for the Atlas from Wikipedia — TRAPPIST-1e (10,947 characters read) · updated Sep 20, 2026
03 How astronomers actually found it
TRAPPIST-1e was not found in a single clean discovery moment — it emerged gradually from a puzzle of overlapping shadows. A team led by Michaël Gillon at the University of Liège first used the TRAPPIST telescope at La Silla Observatory in Chile's Atacama Desert, watching the star between September and December 2015. That work, published in Nature in May 2016, identified three planets. But the transit timing patterns refused to add up. As Gillon later described it, 'at some point we could not make sense of all these transits.' What had been logged as a single outer planet turned out to be the blended signals of three separate worlds — TRAPPIST-1e, f, and g — plus a fourth, h, spotted only once. The solution required nearly three weeks of continuous staring by the space-based Spitzer Space Telescope, which confirmed all seven planets and properly sorted each signal to its source. TRAPPIST-1e was essentially hiding in plain sight inside a messy pile of overlapping dimming events.
04 A star barely above the brown-dwarf line
TRAPPIST-1 is so marginal as a star that astronomers describe it as sitting near the boundary between a true hydrogen-fusing star and a brown dwarf. Its mass is only 0.09 times that of the Sun, and its radius just 0.12 solar radii. Its surface temperature is 2,566 K — less than half the Sun's 5,778 K — giving it a deep red glow rather than yellow-white light. With a luminosity of only 0.0566% of the Sun's output, it is extraordinarily faint: its apparent magnitude as seen from Earth is 18.8, thousands of times too dim to detect with the naked eye. One consequence is that TRAPPIST-1e, despite orbiting at barely 3% of the Earth–Sun distance, intercepts only about 60% as much starlight as Earth does. Another consequence involves timescales: stars this small burn their fuel so slowly that TRAPPIST-1 could remain stable for up to 12 trillion years, more than 2,000 times the Sun's remaining lifespan, making it one of the longest-lived stars the universe will ever produce.
05 The view of the sky from TRAPPIST-1e
Standing on TRAPPIST-1e — if solid ground exists there — the host star would look strikingly different from our Sun. Because the planet orbits at only 4.38 million kilometers, compared to Earth's 150 million kilometers, the star would subtend an angular diameter of about 2.17 degrees across the sky. That makes it appear roughly four times wider than the Sun appears from Earth. Despite its imposing angular size, it would glow a deep crimson rather than bright white, and the total illumination at the surface would still be only 60% of what we receive on a clear day. The six other planets in the system would also be far more prominent than any planet in our own sky, given that some complete their orbits in just a few days and travel across a compact arc close to the star. TRAPPIST-1's apparent magnitude of 18.8 from Earth underscores just how faint the whole system is — the entire drama of seven worlds plays out around a star invisible to human eyes from our vantage point 40.7 light-years away.
06 Density, composition, and why it stood out Deeper
When scientists combined the transit-derived radius of 0.920 Earth radii with the mass of 0.692 Earth masses — constrained through transit-timing variations and computer simulations — they could calculate a bulk density of 5.65 g/cm³. That is about 1.024 times Earth's own density of 5.51 g/cm³. For a time, this made TRAPPIST-1e genuinely unusual within its system: it appeared to be the only planet with a density consistent with a pure rock-iron composition and the only one denser than Earth, while most of its siblings seemed to have densities suggesting global liquid oceans, thick steam or CO₂ atmospheres, or ice shells. TRAPPIST-1c also appeared entirely rocky but at a lower density. However, refined later estimates revised this picture considerably: all seven planets in the system appear to have broadly similar, rocky bulk compositions, and TRAPPIST-1e's density, while still Earth-like, is now considered somewhat lower than those early figures suggested. Science rarely stays still, and the story of TRAPPIST-1e's density is a good example of preliminary results being sharpened by improved analysis.
07 Orbital resonance and the clockwork system Deeper
TRAPPIST-1e completes one orbit in just 6.10 days — roughly 146 hours — at a distance of 0.0293 AU from its star. For scale, Mercury, the closest planet in our Solar System, orbits at 0.38 AU and takes 88 days. The entire TRAPPIST-1 system is extraordinarily compact. TRAPPIST-1e is locked into gravitational resonances with its neighbors: it shares a 3:2 resonance with both TRAPPIST-1d and TRAPPIST-1f, meaning it completes three orbits for every two of one neighbor and two orbits for every three of another. These resonances are not coincidental — they reflect the history of the system's formation, likely involving the planets migrating inward and becoming gravitationally synchronized over time. The resonances also make the system unusually stable over long timescales and are what allowed astronomers to use transit-timing variations to measure planetary masses with reasonable confidence, since each planet tugs on the others in predictable, detectable ways.
08 Tidal locking and the terminator world
Orbiting so close to its star, TRAPPIST-1e is very likely tidally locked, meaning one hemisphere permanently faces the star while the opposite hemisphere sits in permanent darkness. This is the same phenomenon that keeps one face of the Moon always turned toward Earth. A tidally locked planet is a profoundly alien environment: the dayside would receive constant stellar heating while the nightside would radiate heat into space continuously. Between them lies a narrow terminator — a band of eternal twilight circling the globe. Scientists initially worried this would devastate habitability, but 2018 modeling suggested the situation is more nuanced. If TRAPPIST-1e possesses a sufficiently dense atmosphere, circulation could transfer warmth from the permanent day side to the dark side, potentially preventing the nightside atmosphere from freezing out entirely. The equilibrium temperature of the planet, calculated at 246.1 K assuming zero albedo, would shift significantly with any greenhouse warming, as Earth's own 33-degree greenhouse effect illustrates.
09 What the JWST found — and didn't find
In 2025, four transit observations of TRAPPIST-1e made with the James Webb Space Telescope's NIRSpec instrument produced a frustratingly ambiguous result. Researchers could not find conclusive evidence either for or against an atmosphere. The data fit two broad scenarios equally well. The first is a flat spectral signal, which could mean TRAPPIST-1e is a bare, airless rock — or equally, that it has an atmosphere of some unknown type completely obscured behind a high, thick cloud deck. The second scenario is a family of nitrogen-rich atmospheres, with nitrogen as the dominant gas, and a tentative preference in the data for trace amounts of methane mixed in. The authors are candid that the biggest obstacle is not instrument sensitivity but the star itself: TRAPPIST-1 is active, and its starspot activity contaminates the atmospheric signal. A new program of 15 additional JWST observations is now underway, using back-to-back transits of TRAPPIST-1e and its neighbor TRAPPIST-1b — believed to be a bare rock — to calibrate out stellar noise.
10 Stellar activity and the threat to any atmosphere Deeper
A 2024 modeling study raised a sobering concern about TRAPPIST-1e's long-term habitability: its host star may be actively stripping the planet's atmosphere away. Ultracool red dwarfs like TRAPPIST-1 can be magnetically active, emitting flares and energetic particles that bombard close-in planets. Because TRAPPIST-1e orbits at only 0.0293 AU and completes a revolution in just 6.10 days, it is constantly immersed in the stellar wind and high-energy radiation environment at very close range. The same erosive process could affect all seven planets in the system. Whether TRAPPIST-1e has managed to retain a significant atmosphere despite 7.6 billion years of this exposure — the star is older than the Sun — is one of the most pressing open questions in exoplanet science. The JWST's inability to definitively confirm or deny an atmosphere in 2025 means the answer remains genuinely open, and the 15-observation follow-up program is specifically designed to push toward a firmer conclusion.
11 Why 2018 was a turning point in the science Deeper
The year 2018 brought a cluster of refined studies that sharpened the picture of TRAPPIST-1e considerably. Mass estimates were tightened using transit-timing variations, yielding 0.692 Earth masses — placing the planet at about 15% less massive than Venus. The density calculation of 5.65 g/cm³ solidified its characterization as a terrestrial world with a solid, rocky surface. Critically, researchers also ruled out a cloud-free hydrogen-dominated atmosphere using transmission spectroscopy. This was a significant constraint: hydrogen-dominated atmospheres are puffy and easily detected, and their absence means that if any atmosphere exists, it is more compact — more like those of Venus, Earth, or Mars — and far harder to observe. A separate November 2018 analysis concluded that among all seven TRAPPIST-1 planets, TRAPPIST-1e had the best prospect of being an Earth-like ocean world and was singled out as the most worthy candidate for deeper habitability research. That assessment helped drive the eventual JWST observation campaign.
12 Gravity, mass, and a planet slightly lighter than us
TRAPPIST-1e has a surface gravity of 82% that of Earth — the third lowest among the seven TRAPPIST-1 planets. In practical terms, a person who weighs 70 kg on Earth would weigh roughly 57 kg on TRAPPIST-1e. Its mass of 0.692 Earth masses is also the third lowest in the system. The radius of 0.920 Earth radii was measured with a small uncertainty of about 83 km — impressively precise given that the planet is 40.7 light-years away. These figures were derived by two complementary methods: the transit depth gave the radius directly, while transit-timing variations — tiny shifts in when each planet crosses the star, caused by gravitational nudges from its neighbors — allowed the mass to be teased out through advanced computer simulations. The combination of both radius and mass with low uncertainty is what makes TRAPPIST-1e one of the most precisely characterized small exoplanets known and underpins all the density and composition conclusions.
13 The metallicity mystery of TRAPPIST-1 Deeper
TRAPPIST-1 carries an unexpected chemical fingerprint. Its metallicity — the abundance of elements heavier than hydrogen and helium — is measured at [Fe/H] = 0.04, meaning it is approximately 109% as metal-rich as the Sun. That would be unremarkable for a Sun-like star, but TRAPPIST-1 sits near the minimum mass threshold for hydrogen fusion, a regime where stars are typically expected to have significantly less metal content than the Sun. The reason for this enrichment is not fully understood, and the article notes that it is 'particularly odd.' Metallicity matters for planetary science because heavier elements — iron, silicon, magnesium, oxygen — are the raw materials from which rocky planets are built. A metal-rich progenitor cloud might favor the formation of multiple rocky worlds, which is consistent with what is observed: seven terrestrial-class planets orbiting a star that, by conventional expectations, should have formed in a metal-poor environment.
14 Open questions that define the next decade
After decades of searching, TRAPPIST-1e sits at the center of some of the most important unsettled questions in exoplanet science. Does it have an atmosphere at all? The 2025 JWST data cannot say for certain. If it does, is it thick enough to sustain liquid water, or thin enough to have been eroded away by 7.6 billion years of stellar activity? If methane is truly present in trace amounts — as the data tentatively hints — is that a sign of chemistry, geology, or something more provocative? Is the planet truly tidally locked, and if so, can its atmosphere circulate heat efficiently enough to keep a dayside ocean from evaporating and a nightside from freezing? The 15-observation follow-up JWST program now underway is specifically designed to confront several of these questions by using TRAPPIST-1b as a stellar activity calibrator. How that program resolves will shape whether TRAPPIST-1e remains the premier target it has been considered since 2018, or whether attention shifts elsewhere in the system.
Could life exist here?
The best-measured temperate rocky planet beyond the Solar System. Everything now depends on whether it kept an atmosphere — an answer being actively pursued.
Our labels: Confirmed · Strong Evidence · Estimated · Hypothesis · Theoretical · Speculative (see Methodology).
How would we get there?
40.7 light-years — ~700 human generations at Voyager speed. This one is for telescopes.
Weird & wonderful
- Its seven-planet system spans less space than Mercury's orbit.
- You could watch other planets' weather (if any) with a small telescope from its surface.
- A 10-year-old there would be about 600 in TRAPPIST-1e years.
