Astronomers using NASA's powerful Hubble Space Telescope have spotted a tiny fourth moon around Pluto, the smallest ever glimpsed around the icy dwarf planet, the US space agency said Wednesday.

The small moon, named P4 for now until a better name is decided upon, is only about eight to 21 miles (13-24 kilometers) in diameter.

"I find it remarkable that Hubble's cameras enabled us to see such a tiny object so clearly from a distance of more than three billion miles (five billion kilometers)," said lead observer Mark Showalter of the SETI Institute in Mountain View, California.

Pluto's largest moon, Charon, is 648 miles (1,043 km) across. The other two, Nix and Hydra, are between 20 and 70 miles in diameter (32 to 113 km), NASA said.

Hubble discovered Nix and Hydra in 2005. Astronomers at the US Naval Observatory glimpsed Charon in 1978.

The first photo of P4 was taken with Hubble's Wide Field Camera 3 on June 28, and was confirmed by more Hubble snapshots on July 3 and July 18.

"The moon was not seen in earlier Hubble images because the exposure times were shorter. There is a chance it appeared as a very faint smudge in 2006 images, but was overlooked because it was obscured," NASA said.

Hubble is a potent space telescope that has transformed the field of astronomy since it was first launched in 1990.

Pluto, once known as the ninth planet from the Sun, was declassified as a full-fledged planet in August 2006 and joined the new category, dwarf planet.

At about 1,430 miles (2,300 kilometers) wide, it is about two-thirds the size of the moon and has a mass less than one percent of the Earth's.

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