Pluto may have been demoted to dwarf planet status, but it’s queen bee to a clutch of moons that now numbers four.

Astronomers using the Hubble Space Telescope announced today that Pluto has another moon in orbit, joining Charon, Pluto’s largest moon, Nix and Hydra.

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“P4″ — the new moon’s temporary designation — is the smallest of Pluto’s moons, measuring between 8 and 21 miles in diameter. (Charon, by comparison, is 648 miles wide, and Nix and Hydra each span somewhere between 20 and 70 miles.) The sizes are hard to pin down because Pluto is around 3 billion miles away.


Tiny P4 turned up in a Hubble study to see if Pluto has rings. (That study remains under way.) P4 orbits between sister moons Nix and Hydra, both of which were also discovered by Hubble.

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More details should come from NASA’s New Horizons probe, a robotic spacecraft en route to Pluto and beyond.

“Now that we know there’s another moon in the Pluto system, we can plan closeup observations of it during our flyby,” New Horizons lead researcher Alan Stern, with the Southwest Research Institute, said in a statement.

Scientists believe Pluto’s family of moons is the result of an impact in the early days of the solar system, which formed about 4.5 billion years ago. The breakup of a planet-sized body likely provided the raw materials for the moons to form around Pluto. A similar crash in our neck of the solar system is believed to be responsible for the formation of Earth’s moon.

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NASA says P4 was first seen in a photo taken with Hubble’s Wide Field Camera 3 on June 28. It was confirmed in subsequent Hubble pictures taken on July 3 and July 18. The moon was not seen in earlier Hubble images because the exposure times were shorter, though it may have appeared as a faint smudge in a 2006 image.

Image: Pluto’s growing family. Credit: NASA/ESA/SETI Institute