Imagine a world where the sun never sets—or never rises. A planet locked in eternal embrace with its star, one hemisphere scorched under perpetual daylight, the other plunged into endless frozen night. Between them lies a thin ribbon of twilight, a region of eternal dusk where temperatures might just allow liquid water to exist. These are tidally locked exoplanets, and they may be far more common in the universe than Earth-like worlds. To understand how life could emerge in such alien conditions, we turn to Earth’s most resilient organisms: extremophiles.
Tidal locking occurs when a planet's rotation period matches its orbital period, causing one face to always point toward its star. This phenomenon is common for planets orbiting within the habitable zones of red dwarf stars (M-dwarfs), which make up about 75% of stars in our galaxy. Notable examples include:
The extreme conditions on these worlds—blistering heat on the dayside, cryogenic cold on the nightside—demand a radical rethinking of what life could look like.
The boundary between eternal day and eternal night, known as the terminator zone, may offer the most stable environment for life. Here, temperatures could permit liquid water, and atmospheric circulation might redistribute heat. But what kind of life could thrive in such a place?
Extremophiles are organisms that survive in conditions lethal to most Earth life. By studying them, we can hypothesize how life might adapt to tidally locked exoplanets.
Red dwarfs are prone to violent stellar flares, bombarding nearby planets with UV and X-ray radiation. Earth’s Deinococcus radiodurans can survive doses of radiation thousands of times higher than would kill a human. Could similar organisms evolve shielding mechanisms under perpetual flare activity?
In Earth’s deserts and polar regions, endolithic microbes colonize subsurface rock pores. On a tidally locked world, such organisms might retreat below the surface to escape lethal surface conditions.
The peculiar climate of tidally locked planets could produce unique biosignatures detectable by next-generation telescopes like the James Webb Space Telescope (JWST).
Models suggest that tidally locked planets may develop:
Microbial life could influence atmospheric chemistry, producing gases like methane or dimethyl sulfide in detectable quantities.
On Earth, oxygen is a hallmark of life. But on a tidally locked planet, abiotic processes (e.g., photolysis of CO2) might produce false positives. Alternative biosignatures, like nitrous oxide or methyl chloride, may be more reliable.
Picture a tidally locked world where life exists in three distinct biomes:
Observing such worlds presents difficulties:
The search for life on tidally locked exoplanets requires:
As we peer into the eternal twilight of distant worlds, we carry with us the lessons of Earth’s hardiest survivors. Extremophiles teach us that life is not just possible in the cosmos—it is inevitable, lurking in the cracks between fire and ice.