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Two newly-discovered moons of Pluto get names

Pluto and its five moons are seen in this graphic
Courtesy Wikimedia

Two newly-discovered moons of Pluto now have names.

The International Astronomical Union announced Tuesday that the fourth and fifth satellites of the solar system’s former ninth planet will be known as Kerberos and Styx.

Like moons of most of Earth’s other planetary neighbors, the names relate to the gods of Greek and Roman mythology.

Kerberos is the name of the dog that guards the underworld in the ancient Greek tales, while Styx is the name of the river that separated Earth and the underworld in those tales.

Both moons are recent discoveries.

“They were both discovered in images from the Hubble Space Telescope,” Mark Showalter, an astronomer associated with the Carl Sagan Center for Study of Life in the Universe at California’s SETI Institute and the leader of the research team that discovered them, said. “P4, which is now Kerberos, was found in 2011, late June or July, and P5 was found about this time in 2012. The discovery of P4 was actually in a short Hubble program to search for dust rings around Pluto. We were doing that by taking longer exposures of Pluto than anyone had attempted before.”

Researchers subsequently decided to look for more evidence of additional satellites of Pluto.

The two moons are very small, on the order of about 100 times smaller than Charon, Pluto’s largest satellite.

“They’re just dots in the sky,” Showalter explained. “The estimates we have are on the order of 10-15 miles in diameter. Three billion miles away, and that’s how big they are. Just to illustrate, P5 is about 100,000 times fainter than Pluto itself. They’re separated on the sky by, basically, a hair’s breadth.”

Charon has a diameter of about 1,200 kilometers.

Kerberos and Styx almost certainly move in synchronous rotation with Pluto, just as Earth’s moon does in relation to our planet.

“Every satellite in the solar system, except Saturn’s moon Hyperion, has been shown to be tidally locked,” Showalter said.

The Hubble Space Telescope was used to find Pluto’s two other known satellites, Nix and Hydra, in 2005. Those moons are named, respectively, for the Greek goddess of darkness and night and the nine-headed serpent that fought Hercules in the ancient myths.

The system of naming moons for characters of Greek and Roman mythology is commonly employed by astronomers in naming the moons of the solar system’s planets.

“Moons of Neptune are named for creatures associated with the oceans in Greek and Roman mythology because Neptune was the god of the ocean,” Showalter said.

Similarly, eight moons of Saturn are named for titans of Greek mythology, while many of Jupiter’s moons – notably excluding Callisto, Europa, Ganymede, and Io – are named for lovers and descendants of Zeus. Mars’ two moons are named for children of Ares, the Greek god of war.

The moons of Uranus are named for characters from English literature, particularly Shakespeare’s plays.

Kerberos and Styx may not be the last moons of Pluto to be discovered. The New Horizons spacecraft is scheduled to arrive in the vicinity of Pluto in 2015, allowing scientists a close-up look at the dwarf planet and its satellites.

“I would be frankly surprised if we don’t find a P6 and a P7 and a P8,” Showalter said. “Whatever is going on in the Pluto system that created this family of moons probably didn’t stop at five.”

He explained that it’s not likely the IAU will have any problem finding names for any newly-discovered Plutonian moons that fit the existing theme.

“I can assure you that is not a concern,” Showalter said. “We had 30,000 write-in candidates put in. There’s a long list. I still have that list. I’d like to think we’ll be returning to that list come 2015 when we find other rocks out there.”

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