LOS ANGELES- Pluto may not be big enough to be called a full-fledged planet, but it has been hiding a big secret.
Astronomers say the Hubble Telescope has discovered a fifth moon—the tiniest moon yet—lurking around the distant and icy dwarf planet.
Astronomers say the mini-moon is 6 to 15 miles across, smaller than the moon scientists spotted last year, which is 8 to 21 miles wide. Pluto's largest moon, Charon, is about 650 miles across.
Until the newly found moon gets a name, it will be known as P5.
Meanwhile, the NASA spacecraft New Horizons is speeding toward Pluto where it will arrive in 2015. When New Horizons launched in 2006, Pluto was a full-fledged planet, but astronomers have since demoted it to dwarf planet.