Games like TRAPPIST-1
Games like TRAPPIST-1 include Mouthwashing, SOMA, Little Nightmares II, Little Nightmares, and Sally Face - Episode One. We matched them by the tags players give TRAPPIST-1 on Steam (Horror, Dark, Psychological Horror, Atmospheric, and Space): closest matches first, and among equally close ones the better-known games.

Mouthwashing
Shared: Horror · Psychological Horror · Atmospheric

SOMA
Shared: Horror · Psychological Horror · Atmospheric

Little Nightmares II
Shared: Horror · Dark · Atmospheric

Little Nightmares
Shared: Horror · Atmospheric · Dark

Sally Face - Episode One
Shared: Psychological Horror · Horror · Dark

No, I'm not a Human
Shared: Horror · Psychological Horror · Atmospheric

Poppy Playtime
Shared: Horror · Dark · Singleplayer

Outer Wilds
Shared: Space · Atmospheric · Exploration

Layers of Fear (2016)
Shared: Horror · Psychological Horror · Atmospheric

Outlast
Shared: Horror · Atmospheric · Psychological Horror

MiSide
Shared: Psychological Horror · Horror · Atmospheric

Fran Bow
Shared: Psychological Horror · Horror · Dark

Detention
Shared: Horror · Psychological Horror · Atmospheric

ROUTINE
Shared: Psychological Horror · Atmospheric · Dark

Amnesia: The Dark Descent
Shared: Horror · Psychological Horror · Atmospheric

Five Nights at Freddy's
Shared: Horror · Psychological Horror · Singleplayer

INSIDE
Shared: Dark · Horror · Atmospheric

Outlast 2
Shared: Horror · Psychological Horror · Dark

Visage
Shared: Horror · Psychological Horror · Dark

Tiny Bunny
Shared: Horror · Dark · Atmospheric

DISTRAINT: Deluxe Edition
Shared: Horror · Psychological Horror · Indie

FAITH: The Unholy Trinity
Shared: Horror · Psychological Horror · Dark

What Remains of Edith Finch
Shared: Atmospheric · Story Rich · Singleplayer

Fears to Fathom - Norwood Hitchhike
Shared: Horror · Psychological Horror · Atmospheric