Should Pluto’s Planet Status Be Reinstated? Not Yet

Discovery of a fifth moon circling Pluto has some questioning where the “dwarf planet” category still applies to this faraway member of the Solar System

Astronomers identified a fifth moon orbiting Pluto (Illustration Credit: NASA, ESA, and L. Frattare (STScI); Science Credit: NASA, ESA, and M. Showalter (SETI Institute))

Last week, astronomers identified a fifth moon–named P5 for now–orbiting Pluto in images taken by the Hubble Space Telescope. The moon is a mere 6 to 15 miles in diameter and orbits in a 58,000-mile-diameter circular orbit around the dwarf planet. “The moons form a series of neatly nested orbits, a bit like Russian dolls,” said team lead Mark Showalter of the SETI Institute.

The finding of P5 has some again questioning Pluto’s demotion to dwarf planet status. New Scientist reports:

The discovery provides some ammunition for those upset at Pluto’s demotion from the planetary ranks. “If you are important enough to have acquired five satellites, you are a planet!” says Kevin Baines, a planetary scientist at NASA’s Jet Propulsion Laboratory.

But having or not having moons is not part of the qualifications for planet status. In 2006, the International Astronomical Union defined a planet as having three characteristics:

1. It orbits the Sun.
2. It has has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.
3. It has cleared the neighborhood around its orbit.

Unfortunately for Pluto fans, Pluto fails on count three, and the IAU does not plan to revisit the issue anytime soon. And so it seems that Pluto is doomed to stay a dwarf planet for the time being.

The discovery of P5, however, does have important implications for the New Horizons spacecraft headed towards Pluto and scheduled to rendezvous in July 2015. “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,” said New Horizons’ principal investigator Alan Stern of the Southwest Research Institute. There is real worry that New Horizons could be destroyed if it runs into even a small piece of debris as it zooms past Pluto at 30,000 miles per hour.

As for what P5 (and P4, discovered last year) will eventually be named, that’s still up in the air, although Showalter told New Scientist that after he had finished his search of the Hubble data and found all of Pluto’s moons he would suggest names in the Hades/underworld theme that gave us Charon, Hydra and Nix. I came up with a few options for moon names last year on this blog (Erberus, Styx and Hypnos–in our poll, our readers liked Styx best), but I think Showalter might be running out of options in his preferred theme and will have to do some real digging into classical history once he gets to P7 and beyond.

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About Sarah Zielinski
Sarah Zielinski

Sarah Zielinski is an award-winning science writer and editor. She is a contributing writer in science for and blogs at Wild Things, which appears on Science News.

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