Here is the full list of all 27 Protect the Planet achievements.
LetterA-1
LetterB-1
LetterC-1
LetterD-1
LetterE-1
LetterF-1
LetterG-1
LetterH-1
LetterI-1
LetterJ-1
LetterK-1
LetterL-1
LetterM-1
LetterN-1
LetterO-1
LetterP-1
LetterQ-1
LetterR-1
LetterS-1
LetterT-1
LetterU-1
LetterV-1
LetterW-1
LetterX-1
LetterY-1
LetterZ-1
LetterSpace-1