A rocky world that might be an 'eyeball' in the dark

Space · Published 20 October 2026 · Updated 21 October 2026 Swipe more like this →

In short: TRAPPIST-1f is a planet so close to its host star that it likely never rotates. One side faces eternal day, while the other sits in permanent night. This extreme environment creates a unique possibility for life: a narrow sliver of habitable land between two extremes.

Located 40.7 light-years away in the constellation Aquarius, TRAPPIST-1f orbits an ultra-cool red dwarf star. Because it sits so close to its host—at a distance of about 0.0385 AU (roughly ten times closer than Mercury is to our Sun)—it is likely tidally locked. This means one hemisphere is perpetually scorched by the star while the other remains in darkness. Scientists have proposed that such planets could become 'eyeball planets,' where only a specific region, known as the terminator line, maintains temperatures suitable for liquid water.

The planet's composition was once a subject of intense debate. Early 2017 estimates suggested it might be 20% water by mass, potentially making it more like an ice giant with high-pressure water clouds than a rocky world. However, refined density measurements have since shown that TRAPPIST-1f has a mass of 1.039 M and a radius of 1.045 R. These figures indicate it is only slightly less dense than Earth, supporting the theory that it is primarily rocky rather than a volatile-rich water world.

The host star itself presents a cosmic anomaly. It is an ultra-cool dwarf with a mass of just 0.09 M, yet it is unexpectedly metal-rich (109% of solar metallicity). While low-mass stars near the brown dwarf boundary usually have lower metal content, TRAPPIST-1's richness makes it a prime candidate for planet formation. Furthermore, because this star type can live for 4–5 trillion years—roughly 400–500 times longer than our Sun—TRAPPIST-1f may be one of the last remaining habitable environments in the universe as older stars eventually die out.

Current simulations suggest TRAPPIST-1f might possess a thick atmosphere rich in abiotic oxygen, potentially retaining primordial steam from its early evolution. While liquid water could exist on the terminator line at temperatures around 273 K (0 °C), the planet's ultimate habitability depends on whether its atmosphere is robust enough to redistribute heat from the day side to the night side.

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