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Astronomers looking for rings around Pluto have instead made an unexpected find: a fourth moon circling the dwarf planet.

The object, temporarily designated P4, is probably the most dwarvish of Pluto's moons: It's estimated to be just 8 to 21 miles (13 to 34 kilometers) in diameter. In comparison, Pluto's diameter is about 1,400 miles, and its other three moons range in diameter from 648 miles (for Charon) to between 20 and 70 miles (for Nix and Hydra, discovered in 2005). The newfound moon orbits in a region between Nix and Hydra, and makes a complete circuit roughly every 31 Earth days.

P4 was detected in June, during a round of Hubble Space Telescope observations aimed at looking for rings or other potential hazards for NASA's New Horizons probe, which is due to zoom through the Pluto system in 2015. Alan Stern, a planetary scientist at the Southwest Regional Institute who heads the $700 million New Horizons mission, told me in an email that the discovery was a testament to the dwarf planet's continuing ability to surprise.

"Pluto's satellite system is truly knocking our socks off with surprises — it's magnificently complex, and getting more crowded all the time. I can't wait till we get there to see what other surprises this planet and its moons have in store for us!" he said.

The find is also a testament to Hubble's amazing vision. The object was spotted on June 28 using the space telescope's Wide Field Camera 3, and its existence was confirmed through follow-up observations this month as well as a search through archived imagery. The moon was not spotted in earlier imagery because the exposure times were shorter.

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