Hubble spots fifth Pluto moon

Dwarf planet’s retinue grows

Pluto, the popular dwarf formerly known as a planet, has another little friend: a fifth moon, first reported early in the morning of July 11 on Twitter.

Images from the Hubble Space Telescope captured a fifth moon circling Pluto, dubbed P5. NASA, ESA, M. Showalter

“Just announced: Pluto has some company — We’ve discovered a 5th moon using the Hubble Space Telescope!” tweeted Alan Stern of the Southwest Research Institute in Boulder, Colo.

And it appears that more detections might be on the way. “We fully expect to discover still more moons,” says Stern, the principal investigator of NASA’s New Horizons mission, which will fly by the dwarf planet in 2015. “Every time we look harder, we find another.”

On July 7, ace moon-finder Mark Showalter of the SETI Institute in Mountain View, Calif., spotted the fifth moon, referred to as P5 for now, in images captured by Hubble. 

“It didn’t really jump out at us until Saturday,” Showalter says. “From the time I got the data out of the Hubble archive, until the time I was staring at this thing on my computer screen, was maybe an hour at the most. I stared at it for a while and thought, ‘OK, am I ready to embarrass myself if I’ve got it wrong?’”

P5 revealed itself in 14 sets of images, each containing around a dozen three-minute exposures, according to a dispatch from the International Astronomical Union’s Central Bureau for Astronomical Telegrams. Hubble has been peering at the Pluto system since June, tasked with helping astronomers detect any potential hazards to the New Horizons spacecraft. The team is hoping to have enough time to plan an alternate route through the system if the probe’s current trajectory points toward trouble.

Just a tiny little thing, P5 measures between 10 and 25 kilometers in diameter. It joins a moony menagerie that includes (comparatively) enormous Charon, the smaller satellites Nix and Hydra, and the moon-still-known-as-P4, discovered last year by Showalter. P5 is about 42,000 kilometers from Pluto and lies between the orbits of Nix and Charon, making it the most interior of the smaller moons. That location, plus its small size, explains why P5 has been hard to see. “We’re looking right next to Pluto, so there’s this bright searchlight that we have to deal with,” Stern says.

It’s not just finding another Plutonian moon that’s exciting the team. The system’s architecture is configured so that the smaller moons have orbital periods that are neatly related to Charon’s.

Charon orbits Pluto in about 6.5 days. P5 takes roughly three times that long. Nix? Four times. P4’s Pluto-year is five times as long as Charon’s, and Hydra’s is six. “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,” Showalter says.

As for the name? The team is undecided — and will be for a while, as they’re planning on waiting until they know just how many more moons Pluto might have before suggesting names. “If we’re naming them as a group, we’ll just handle it a little differently,” Stern says.

Showalter says he’s been mulling over some possibilities for P4 and P5 based on Pluto’s namesake, the Greek god of the underworld. “The Greeks were such great story tellers,” he says. “There is one I’d sort of looked at — the story of Orpheus, the only human to ever go into Hades and come back. He went in to rescue his wife Eurydice, so one possibility would be to name these two moons Orpheus and Eurydice.”

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