According to several users on Reddit, No Man’s Sky deletes any discoveries other than names of planets and solar systems after two weeks (via Geek.com).
Further investigation revealed that No Man’s Sky seems to be erasing discoveries in chronological order, from oldest to newest.
Whether a bug or design decision, No Man’s Sky is predicated on exploration, discovery, and putting your mark on those discoveries by naming them—planting your flag in soil untouched by others, so to speak.
“Yesterday I browsed my list of discoveries, and went all the way to my starting planet,” wrote Ultrasilvanus.