For something that’s not a planet, Pluto sure acts like one.

Astronomers announced Wednesday that they discovered the tiniest moon around the frozen orb, bringing Pluto’s total to five moons.

“We’re not finished searching yet,” said Hal Weaver of Johns Hopkins University, told the Associated Press.

The researchers used the Hubble Space Telescope to scout the distant former planet.

NASA’s New Horizons spacecraft is due to arrive in 2015, making it the first Earth probe to visit Pluto. When it launched in 2006, Pluto was still considered a planet by the International Astronomical Union.

Pluto’s biggest moon, the 650-mile-wide Charon, was discovered in 1978. Nix and Hydra were discovered in 2005. A fourth moon was spotted last year, and believed to be 8-21 miles wide. Neither it nor the tiny 6-15 mile moon discovered most recently have been named yet.

“The moons form a series of neatly nested orbits, a bit like Russian dolls,” said team lead Mark Showalter of the SETI Institute in Mountain View, Calif., in a statement.

The discovery of the moons will help scientists give plot a good course for New Horizon, allowing it to avoid other pieces of debris or objects in orbit around Pluto.