The rule for moons of minor planets , such as the formal designation ( 87 ) Sylvia I Romulus for the asteroid moon Romulus , is an extension of the Roman numeral convention that had been used , on and off , for the moons of the planets since Galileo 's time . The convention for satellites of minor planets , such as the formal designation ( 87 ) Sylvia I Romulus for the asteroid moon Romulus , is an extension of the Roman numeral convention that had been used , on and off , for the moons of the planets since Galileo 's time .