How Jupiter’s moons get their names

From Roman numerals to Greek myth: the IAU rules that gave Iocaste its name and its final “e”.

Published 2026-10-23

A newly found moon of Jupiter gets up to three labels: a provisional designation, a Roman numeral, and — usually — a name.^[4][1]

Provisional designation

Iocaste started as S/2000 J 3.^[1] “S” marks a satellite, “2000” the year and “J” Jupiter.

Numerals

For decades many of Jupiter’s outer moons were known only by numerals like Jupiter IX. In 1975 the IAU named satellites V–XIII and set up a formal process for future names.^[4] Iocaste is Jupiter XXIV.^[1]

Who qualifies

Names come from lovers and favourites of Jupiter (Zeus) and, since 2004, also from his descendants.^[4]

The ending code

Names ending in “a” or “o” go to prograde irregular moons (“o” for highly inclined ones); names ending in “e” go to retrograde irregulars.^[4] That’s why the moon is Iocaste rather than Jocasta.^[1]

Not every moon gets one

The IAU now limits naming of very small moons, and some recently confirmed moons have no names yet.^[4]

Numbered citations link to the sources list on the Iocaste overview page.