Pluto: Not a Planet; Still Very Interesting


With the discovery of a fifth moon orbiting Pluto came the inevitable protests about the little world’s planetary status: Can it be called a planet yet?

Sorry Pluto fans, this latest revelation can’t supersize Pluto’s standing in the Planetary Rotary Club, but it does provide a fascinating glimpse at the dwarf planet’s history.

ANALYSIS: Pluto Now Has Five (Yes, Five) Moons

Caltech planetary astronomy professor Mike Brown, discoverer of over 100 minor planetary bodies in the Kuiper Belt and often accredited with being responsible for Pluto’s “demotion,” said that although having a fifth moon may not affect its planetary status, “it does mean something.”


“It’s a really good reminder that you don’t have to be a planet to be interesting,” Brown told Discovery News.

Brown led the Palomar Observatory team that discovered the distant world Eris in 2005. At the time, Eris was believed to be the tenth planet of the solar system, orbiting further out than Pluto.

But in an effort to define what a planet actually is — spurred-on by the fact that more small worlds would likely to be found in the Kuiper Belt — the International Astronomical Union (IAU) set out the controversial criteria for planetary status in 2006.

Sadly, as Pluto crosses the orbit of Neptune, it cannot “clear its own orbit” and is therefore a minor body along with Eris and other small worlds in the Kuiper Belt that are now classified by the IAU as “dwarf planets.”

ANALYSIS: Pluto May Live in a Rough Neighborhood

But now, with the continuing discoveries of small moons orbiting Pluto, there have been calls to turn over the IAU’s ruling.

“All the people clamoring about whether it means Pluto might be a planet are essentially saying: ‘See? Pluto is interesting and complex thus shouldn’t it be a planet?’” Brown added, “and the answer is: ‘No; the solar system is full of interesting and complex things that are not planets.’

“Titan is bizarre with methane lakes; Europa has huge below ground oceans; Uranian satellites once had ice volcanoes. But they’re not planets, they are just a subset of the cool things that the universe does in our backyard.”

One of the “cool” things to come from the discovery of the fifth moon (nicknamed “P5″) is the question of how did it form? Was Pluto hit by a large object long ago in the solar system’s history, generating the debris we see as a system of moons?

“That 5th moon really hammers home the idea that Pluto was, well, hammered home at some point,” Brown said.

HOWSTUFFWORKS: Why is Pluto not a planet?

Brown often refers to the Kuiper Belt as the “blood spatter” from the solar system’s evolution — like a forensic scientist analyzes the blood spatter of a crime scene to reveal the perpetrator of a violent crime, planetary scientists analyze the objects in the Kuiper belt to reveal the processes that shaped our star system.

And with the discovery of P5, and the high likelihood of more Plutonian satellites being discovered, Brown reckons Pluto is revealing its own crime scene of sorts.

“If the whole Kuiper belt is the blood spatter with which we try to reconstruct the whole solar system, the Pluto system is what happens when even the blood itself is ripped apart. Which, really, sounds pretty creepy,” he concluded.

Image credit: NASA