CREATE TABLE table_name_1 (period__days_ INTEGER, type VARCHAR(255), planetary_mass___m⊕__ VARCHAR(255), stellar_mass___m__ VARCHAR(255))
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?
    
SQL: SELECT MIN(period__days_) FROM table_name_1 WHERE planetary_mass___m⊕__ = 1 AND stellar_mass___m__ > 0.21 AND type = "m0"