Let no one expect anything of certainty from astronomy, lest if anyone take as true that which has been constructed for another use, he go away...bigger fool than when he came to it. ~ Copernicus
Those wild and crazy cats at the International Astronomical Union (IAU) are at it again. As if they hadn't caused enough consternation with their decision to demote Pluto to a "dwarf planet", now they've testing the waters about a possible new classification of objects: Plutoids.
A "plutoid", it seems, is a "bright dwarf planet" that spends most of its time out past Neptune. This "classification" currently covers two objects, Pluto and Eris. Eris, of course, was the object that started all this fuss in the first place (you may have heard it referred to as Xena; fotunately cooler heads prevailed). It's a somewhat torturous definition, given the business about spending most of its time beyond Neptune. This caveat was needed because Pluto periodically cuts across Neptune's orbit, which used to make it the eighth planet from the sun -- when Pluto was a planet.
Frankly, "plutoid" sounds like an alien from a 1950's B sci-fi flick, as in "Attack of the Plutoids!"
It's interesting to note that this is not an official declaration by the IAU, because those only come out after their regular meetings, the next of which won't be until 2009. Evidently, they're tossing a crumb out to see if all the people they ticked off by demoting Pluto will gobble it up. It doesn't appear to be working so far.
What irritates most scientists, I think, is what bothered me about the whole planet-not-a-planet brouhaha: The inability of scientists to come up with a definition in the first place. Even the new plutoid designation will run into trouble if New Horizons arrives at Pluto and finds it to have a significant amont of dust on its surface. There would be a delightful irony in the namesake of plutoids turned out not to be one.
Then there's another fascinating issue I hadn't even considered. Even after the discovery of Pluto, many astronomers thought there still had to be another large planet out there to explain some of the orbital anomalies of Neptune. In recent years, it's been thought that the tenth planet (remember, Pluto was number nine back then) wasn't necessary, because the anomalies might be due to collisions with other objects from the Kuiper Belt.
Now, though, a computer simulation has been run that posits the putative large planet. The simulation attempts to tackle the weird orbits of some Kuiper Belt objects and does it by putting an object a lot bigger than Pluto out there in the realm of the icy dwarfs (or whatever the IAU is calling them this week). Computer simulations should always be viewed with some caution, because they depend heavily on the assumptions used in creating them. But, the idea of another giant planet past Neptune has been around for a long time, and real orbital data from objects like Sedna were used, so we can at least consider the result possible in real terms.
The trouble is that this thing, which anyone would consider a planet, will be a plutoid because it's out past Neptune and shiny. I'm assuming that it will be shiny, because it will most definitely be frozen, but, it could conceivably be covered in dark dust.
I'm sure astronomers hope that it's icy, because, if it exists, a dark planet out in the Kuiper Belt region will be devilishly hard to find. Even an icy object will be difficult. Of course, it could be neither dusty nor icy if the object has internal heating like some of the gas giants have.
I'm rooting for a large object to be found out there, especially a gaseous one, because it might force the IAU and the Pluto-is-a-planet fan club to actually decide what a planet is, once and for all. What would really be a monkey-wrench in the equation is if the object turned out to be something akin to a brown dwarf.
That's unlikely, of course, because something of that mass most likely would have been discovered by now. But, think of the arguments that could start. It would have to be bigger than Jupiter but small enough to have eluded discovery, so it would be a dwarf brown dwarf in or around the Kuper Belt. So it wouldn't be a planet either. Maybe they could all it a trans-Neptunian non-plutoidic ... thingy.
Hey, that's as good as calling it a plutoid.