Tidally locked world

Tidally locked world
Artist's impression, not a photograph: the surface of Proxima Centauri b as ESO imagined it · ESO/M. Kornmesser, CC BY 4.0, Wikimedia Commons

Read off: Proxima Centauri b, TRAPPIST-1 e

A world that always shows the same face to its dim red star: endless noon on one side, endless night on the other. If it holds air, models put a warm zone under the fixed sun, ice across the night side, and a twilight band between them where water could stay liquid and a colony might settle. The star sits unmoving in the sky, the shadows never turn, and winds run from the cold side to the hot one without end. Red dwarfs flare, and the surface takes it.

In figures

Gravity
0.82 g (TRAPPIST-1 e)
Day
Likely none: the same face to the star; a year of 11.2 days (Proxima b) (What models say, not a measurement)
Air
Unknown; JWST data for TRAPPIST-1 e fit bare rock or a nitrogen-rich air (A candidate, not settled)
Mean temperature
Equilibrium about −39 °C (Proxima b), not a surface figure
Hottest
up to about 17 °C under the fixed star, with an Earth-like air (What models say, not a measurement)
Coldest
down to about −123 °C in the night side's cold traps (What models say, not a measurement)
Liquids
With enough water, models keep liquid water at least under the star (What models say, not a measurement)
Radiation
10 to 60 times Earth's ultraviolet and X-rays; its star is a flare star
Light
Dim
Going
Slow

Hear it

Hazards

  • Stellar flare. The star throws out a burst of light and particles. Instruments warn minutes to hours ahead; whoever is outside or behind thin walls must get to shelter, and radios crackle for a day. D&D 5e (House rule): House rule: a creature still outside shelter when the flare arrives makes a DC 14 Constitution saving throw, taking 4d10 Necrotic damage on a failure or half as much on a success. Radios and sensors work at Disadvantage for 24 hours. Starfinder 2e: Solar flares (GM Core): far from the star, a flare leaves electronics and radio glitching or out of order for up to 1d6 hours; on a world with no air, add its cosmic radiation. Only on or near the star's surface does it deal massive fire damage. Archives of Nethys
  • Endless wind. Air cooled on the night side flows toward the hot side forever. In the twilight band the wind never drops; structures are built low and facing it. D&D 5e: Strong wind gives Disadvantage on ranged weapon attacks, puts out open flames and scatters fog; a flyer must land at the end of its turn or fall.
  • The night side. Beyond the twilight band the star never rises. The dark is total but for the stars, the cold deepens with every kilometre, and ice caps what may be the world's water. D&D 5e (House rule): House rule: the night side is Darkness, and it plays as Deep cold (a DC 15 Constitution save each minute without heated protection, 2d6 Cold damage and 1 Exhaustion level on a failure). Starfinder 2e: Incredible cold (GM Core, −62 °C / −80 °F or colder): fatigued after 2 hours of travel and moderate cold damage (4d6 to 6d6) every minute. A superior thermal capacitor protects against it. Archives of Nethys
  • Unbreathable air. There is air, but not air a human can live on: too thin, no oxygen, or the wrong gases. A mask and an oxygen supply are enough where the pressure holds; a full suit where it does not. D&D 5e: A creature holds its breath for 1 + its Constitution modifier minutes (at least 30 seconds), then gains 1 Exhaustion level at the end of each of its turns until it can breathe; it then loses all the Exhaustion it gained this way. Starfinder 2e: Thin atmosphere (GM Core): a few hours make a creature that is not acclimated fatigued; in severely thin air it attempts a Fortitude save every hour or becomes drained 1. With no oxygen at all, use holding breath and suffocating (Player Core). Armour environmental protections cover both. Archives of Nethys

What may happen

  1. The red star stands exactly where it stood yesterday, and where it will stand next year.
  2. The star brightens visibly for a few minutes; every alarm on the base goes off at once.
  3. The endless wind drops for an hour, and the silence frightens the colonists more than the gale.
  4. Toward the night, the ground turns white: the edge of ice that covers the dark half of the world.
  5. The shadows all point the same way and never turn; a boulder's shadow has kept the same frost behind it for ages.
  6. Under the dim red light, green leaves would look black: whatever grows here has gone dark to drink every photon.
  7. The colony sits in the twilight band, the red star resting on the horizon like a sun that forgot to set.
  8. Past the terminator the star sinks behind the party for good. The stars come out, and they will stay out.
  9. With no day and no night, the colony sleeps by the clock behind blackout blinds. The newcomers keep forgetting what time it is.
  10. The flare drill: everyone has four minutes to reach the shielded shelter, and the newcomer takes the wrong corridor.
  11. Every rock has a long tail of drifted grit on the same side, laid down by a wind that never changes direction.
  12. The star is a broad red disc, wider in the sky than the sun at home, yet the light it gives is the dim glow of an ember.
  13. Sister planets cross the sky, close enough that one shows a clear disc instead of a point of light.
  14. Where the night ice meets the warmth, meltwater gathers and runs toward the star in a shallow stream.
  15. The colony’s crops grow under extra lamps: the red star gives too little of the light they need.
  16. Nobody carries a compass here. The star marks one way and the cold marks the other, and that is all the map needs.
  17. The ice sheet ends in a cliff of blue-black ice facing the star, and blocks the size of vans break from it slowly.
  18. A letter to Earth from here will take more than four years to arrive. The colonists write to children who will be grown by the reply.
  19. Toward the star a vast cloud tower stands where it always stands, above the point where the star hangs overhead.
  20. The colony’s meteorologist, Ilse, has kept a weather log for nine years. Every line reads the same wind from the night side.
  21. After a flare the ultraviolet reading on the panel goes off the scale, and all outdoor work is cancelled for the day.
  22. At a little over four-fifths of a g, the colonists move lightly and the newcomers keep overreaching for things.
  23. A convoy from the night-side ice mine rolls in with blocks of water ice for sale, its drivers pale from months in the dark.
  24. A new arrival has not slept in two days under the unmoving star. The medic prescribes a dark room and a ticking clock.
  25. The colony’s wind turbines have turned without a pause since the day they were raised. The engineers call it the only honest machine here.

Adventure hooks

  1. A colony in the twilight band wants to move closer to the sun side, and the ice miners on the night side will lose their water.
  2. A flare knocked out the relay to the night side, and a survey team out there has a week of heat left.
  3. Plants that grow black under the red sun are spreading toward the domes, and they eat plastic.
  4. The wind stopped for an hour last week; the meteorologist says it is impossible, and has locked herself in.

Who and what is met

Each is an SRD stat block under another name, as game masters reskin them.

Other worlds

Cold desert world, Ice moon with a hidden ocean, Methane world, Hothouse world, Volcanic world, Airless world, Ocean world, Earthlike world, Acid world, Jungle world, Swamp world

Sources

This work includes material from the System Reference Document 5.2.1 ("SRD 5.2.1") by Wizards of the Coast LLC, available at https://www.dndbeyond.com/srd. The SRD 5.2.1 is licensed under the Creative Commons Attribution 4.0 International License, available at https://creativecommons.org/licenses/by/4.0/legalcode.

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