Planets that always show their star the same face: one side in endless day, the other in endless night
Artist's impression · drag to rotate
What it meansno net change in spin over an orbitusually one turn per orbit, one face to the star[1]
Example at homethe Moonit always shows Earth the same face[1]
TRAPPIST-1 b dayside490 ± 17 Kno detectable heat from its night side (JWST, 2025)[6]
LHS 3844 b dayside1 040 ± 40 Knight side consistent with zero kelvin[7]
LHS 3844 b surfacedark, basalt-like rockJWST spectrum, 2026[8]
55 Cancri eabout 2 700 K (2 430 °C) day, 1 380 K (1 110 °C) night2016 map; a reanalysis differs, and its locking is debated[9]
Proxima Centauri blocked or in a 3:2 spinnot known which[2]
"Eyeball Earth"named in 2010a frozen world with open water under its star (Pierrehumbert)[12]
About the image. Drawn as an "eyeball" planet: a frozen world with one round patch of open ocean facing its red star. That is a climate-model scenario and has not been seen on any planet. The measured planets in this page are dark, bare rock; every surface detail in the picture is imagined.
A tidally locked planet always turns the same face towards its star. It spins exactly once per orbit, so one hemisphere has endless day and the other endless night. The Moon does the same with Earth, which is why we only ever see one side of it. Planets that orbit close to their stars, especially small red dwarfs, are expected to end up this way: over time, the star’s tides slow the planet’s spin until it matches its orbit.
Expected, not seen
None of the studies has measured an exoplanet’s rotation directly. The papers say a planet is “expected” or “likely” to be locked, or that the data are “consistent with” it. All seven planets of TRAPPIST-1 are expected to be locked. Proxima Centauri b, the nearest exoplanet, is either locked or spinning three times for every two orbits, like Mercury, depending on the shape of its orbit and of the planet itself. Some astronomers count that kind of resonance as tidal locking too.
What can be measured is heat. As a planet circles its star, a telescope can follow how much infrared light comes from its day side and its night side in turn.
A hot face and a cold one
In 2019 Kreidberg and colleagues used the Spitzer Space Telescope on LHS 3844 b, a rocky planet 1.3 times Earth’s size that circles its star every 11.1 hours. Its day side was 1 040 K (767 °C), give or take 40, and its night side was consistent with zero kelvin. The hottest spot sat right under the star. That is what a bare, airless rock would look like: with no air to carry heat around, the night side simply goes cold. In 2026 a spectrum from the James Webb Space Telescope showed that the hot face is dark, low-silica rock, like basalt.
Webb has done the same for TRAPPIST-1 b. Its day side reads about 490 K (217 °C), and in 2025 the first phase curves of temperate rocky planets found no detectable heat from its night side. Its neighbour TRAPPIST-1 c came out at about 369 K (96 °C), with no thick carbon dioxide atmosphere.
The lava world 55 Cancri e was mapped in 2016 at about 2 700 K (2 430 °C) on its hottest side and 1 380 K (1 110 °C) at night. It is usually assumed to be locked, but its day side brightness changes from one visit to the next, and in 2024 a Webb programme tested whether it might spin like Mercury instead. The data did not support that, but did not rule out other rotation either.
Could anything live there?
A locked planet is not automatically lifeless. An atmosphere can carry heat from the day side to the night side, so the night does not freeze completely. In 2010 Pierrehumbert described an “Eyeball Earth”: a planet frozen almost everywhere, except for a round sea of open water that never moves, centred where the star stands overhead. A 2023 study by Lobo and colleagues suggested another place to look on drier planets: a temperate ring along the terminator, the line of permanent dusk between day and night.
Gallery
A frozen white planet with cracked ice and one round, jagged-edged patch of deep blue open ocean in the middle of its face, its rim touched with red light, an impression of an eyeball planet locked towards its red star. AI-generated painting.A dim red sun sitting on the horizon at the left, casting a red glow over a flat, bare plain, while the right half of the picture fades into dark blue night, an impression of the boundary between day and night on a tidally locked planet. AI-generated painting.Tidally locked planets, 3D artist's impression, rendered from the interactive model.Tidally locked planets, 3D artist's impression, from another angle, rendered from the interactive model.Tidally locked planets, 3D artist's impression, close-up, rendered from the interactive model.
Timeline
2010Pierrehumbert describes the "Eyeball Earth": frozen except for open water under the star.
2016The first day–night map of a super-Earth, 55 Cancri e: about 2 700 K (2 430 °C) against 1 380 K (1 110 °C).
2019LHS 3844 b's night side is found consistent with zero kelvin: a bare rock.
2023JWST measures TRAPPIST-1 b's day side at about 503 K (230 °C).
2025The first JWST phase curves of temperate rocky planets find no heat from TRAPPIST-1 b's night side.
2026JWST shows LHS 3844 b's hot face is dark, basalt-like rock.
Questions people ask
What does it mean when a planet is tidally locked?
Its spin has settled so that it turns once per orbit and keeps one face towards its star, as the Moon does with Earth. Some astronomers also count resonances like Mercury's three turns per two orbits.
Can a tidally locked planet have life?
It is not ruled out. An atmosphere can carry heat around the planet, and climate models find possible habitable spots: open water under the star on an "eyeball" world, or a temperate ring along the line between day and night.
Is Proxima b tidally locked?
Probably, but in which way is unknown. A 2016 study found its spin is either synchronous or in a 3:2 resonance, like Mercury's, depending on its orbit and shape.