July 16, 2012
Should Pluto’s Planet Status Be Reinstated? Not Yet
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 [five] 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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I’m certainly no astronomer or astrophysiscist, but I have read that if the Earth were placed in the Kuiper Belt, it would not clear its orbit, either.
It then appears to me (and again I’m no expert) that planet classification has as much to do with the accident of location as it does with anything else.
Further, hark back to those old “Planet X” myths: if an Earth-sized world (in hydrostatic equilibrium, as any world that size would be) DID reside in the Kuiper belt, and it hadn’t cleared its orbit, under the current system it would be called a dwarf planet–despite being larger than “standard” planets Mercury and Mars.
I don’t particularly care one way or the other what Pluto is called, but it does seem to me anyway as if the new definitions lack a sort of internal consistency, maybe it’s just me.
Those so-called qualifications for planet status are themselves arbitrary, and it is disappointing to see them treated as some sort of gospel truth. They were voted on by only four percent of the IAU, most of whom are not planetary scientists and therefore not experts on the subject at all. And they were opposed by hundreds of professional astronomers in a formal petition led by New Horizons Principal Investigator Dr. Alan Stern. That third count of having to “clear its orbit” was imposed deliberately to keep Pluto out and to keep the number of solar system planets small. It should not be viewed as some sort of objective truth. Interestingly, Dr. Stern is the person who first coined the term “dwarf planet,” but his intention was to designate a third class of planets, small planets large enough to be in hydrosatic equilibrium but not large enough to be rounded by their own gravity. He never intended for dwarf planets to not be considered planets at all. The four percent of the IAU who voted distorted his term and made it inconsistent with all other uses of the term “dwarf” in astronomy, where dwarf stars are still stars, and dwarf galaxies are still galaxies. Now the IAU is compounding its bad decision with an adamant refusal to revisit the issue when the reality is, it is better to admit one made a mistake and re-open discussion than to stand by the mistake and refuse any additional debate. For this reason, planetary scientists should form their own organization and separately create their own definition of planet, which has to be better than the IAU definition, which, as rastronomicals accurately notes above, results in the absurdity of the same object being a planet in one location and not a planet in another. Dwarf planets are planets too, and those who support this position will continue to advocate for it, as we have done the past six yeas.
Tell you what, I don’t know what more a planet has to do to gain some respect. Yes Pluto is very far away, and we here on Earth cannot see it when we look up. But these latest discoveries of Pluto owning moons and more importantly, having an atmosphere, shows that Pluto is ready to fight to regain ownership of being a very special part of our solar system. If people still want to bring up a slightly eccentric orbit… You try hauling 5 or moons around the sun. That’s a big backpack for a little guy. Our planet only has one moon.
It’s OK to admit a mistake. Don’t let ego’s get in the way. Put Pluto back, it’s been long enough.