question: What is the smallest period (days) to have a planetary mass of 1, a stellar mass greater than 0.21 and of the type M0?
answer: SELECT MIN(period__days_) FROM table_name_1 WHERE planetary_mass___m⊕__ = 1 AND stellar_mass___m__ > 0.21 AND type = "m0"
context: CREATE TABLE table_name_1 (period__days_ INTEGER, type VARCHAR, planetary_mass___m⊕__ VARCHAR, stellar_mass___m__ VARCHAR)