No planet has exactly 15 moons. As of 2014, according to NASA, Neptune has 13 confirmed moons, Uranus has 27, Saturn has 53, Jupiter has 50, Mars has two and Earth has one. Jupiter has 17 provisional moons, while Saturn has nine and Neptune has one.
The first moons orbiting a planet other than Earth were four of the moons of Jupiter, discovered by Galileo Galilei and Simon Marius in 1610. In 1877, astronomer Asaph Hall discovered the two moons of Mars, Phobos and Deimos. The dwarf planet Pluto has five known moons. The dwarf planet Eris has at least one moon, while another dwarf planet, Haumea, has at least two moons.