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Dwarfs in Good Company as Fifth Pluto Moon Discovered

The news of a fifth moon discovered orbiting Pluto is exciting for astronomers, as there may be more to the dwarf's system than we previously thought.

The dwarf planet Pluto has another companion, it has recently been discovered, with the announcement of a fifth moon being sighted by the Hubble Space Telescope.

Alan Stern of the Southwest Research Institute in Colorado took to Twitter to proclaim the good news.

“Just announced: Pluto has some company – We’ve discovered a 5th moon using the Hubble Space Telescope!” the tweet read. Stern continued with “we fully expect to discover still more moons”, hinting at an exciting time ahead for astronomers.

“Every time we look harder, we find another.”

The moon was spotted by Mark Showalter of the SETI Institute in Mountain View, California, while pouring over images captured by Hubble.

The new addition to Pluto’s family has not yet been officially named and is being referred to simply as P5 for the time being. The plan at this stage is to wait and see how many more moons Pluto has before working on names.  “If we’re naming them as a group, we’ll just handle it a little differently,” says Stern.

Size-wise, P5 is a tiny dot in space, measuring somewhere between 10 and 25 kilometres in diameter. It joins Charon, Nix, Hydra and the recently discovered P4 in Pluto’s current list of companions. While the team are confident of finding more moons in the future, they concede it is no easy task.

“We’re looking right next to Pluto, so there’s this bright searchlight that we have to deal with.”

This discovery is of interest to Stern as he is the principal investigator of NASA’s New Horizons mission. This will fly by Pluto in 2015, and, as exciting as the discovery of a moon is, it does also represent a potential hazard for the spacecraft heading in that direction at 48,000km/hr. Of this Stern said “the inventory of the Pluto system we’re taking now with Hubble will help the New Horizons team design a safer trajectory for the spacecraft.”

There also seems to be hints of some puzzle in the relationship between Pluto and her moons. Charon, the largest, orbits Pluto in 6.5 days. P5 has an orbit about three times as long as Charon’s. Nix’s “Pluto-year” is four times as long as Charon’s, P4’s five times and Hydra’s six. In the words of Showalter, “the moons form a series of neatly nested orbits, a bit like Russian dolls.”

“The way these things are spaced in a uniform way – there’s a story in that, but we don’t know what it is yet.”

Before it’s demotion to dwarf planet in 2006, Pluto had been known as the ninth planet in our solar system since its discovery in 1930. It was thought to be alone until Charon was discovered in 1977. Nix and Hydra were discovered in 2006 and P4 followed in 2011.


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