Wednesday, August 16, 2006

My Very Endearing Mother Certainly Just Shot Up Near Prison Cell 2003

If the IAU has their way, then this title is the menomic that school children will be learning to memorize the planets of the solar system. All twelve of them. Starting with Mercury and ending with 2003 UB313. Further planets, possibly as many as 50, would then be able to apply for planetary status via a committee.

That sucks. Even worse, it strikes me as an anti-Plutoist scheme. A while ago, they suggested that Pluto be demoted, in much the same way as Ceres was in the early 19th century. After that cryophobic definition was met with public outcry, they changed their tune, and this is most likely their fall-back position: use a definition so loose that every quasi-spherical lump of ice this side of Alpha Centauri qualifies.

I suspect they will use the natural resistance to the possibility of 50 planets to come back with, “Well, we could use a tighter definition, but Pluto won’t qualify.”

This, of course, is hogwash.

First, consider the inclusion of Charon, Pluto’s largest moon. Charon is only 8.5 times smaller than Pluto, making it the largest relative moon to its planet. In second place is the Earth-Moon system, with an 81.3 times mass difference. Given that the committee has not suggested removing moon status from the Moon, their double planet cutoff point must lie somewhere between these two numbers.

This is obviously a shameless attempt to further delegitimize Pluto. After all, if Charon is a planet, then everything else with a radius more than 605 km might also claim that title. Then we have 50, 100, or more planets, mostly very dim and very far away. The obvious solution is to leave Charon a moon, and say that only the largest body in an orbital system qualifies as a planet.

2003 UB313 claims planet status on the argument that it is larger than Pluto. Since it is almost twice as far out, nothing is currently known about its orbital interaction with other bodies, its moons, its atmosphere, or even its final name. This just goes to show that the anti-Plutonists are the sorts of guys who believe that size is the only thing that matters, and that activity, process, and detail are irrelevant.

Ceres is a more interesting candidate. It was originally called a planet when discovered in 1801. However, it was later demoted after the discoveries of other asteroids during the mid-ninteenth century. Recently, however, interest has re-emerged, as a result of some geophysical modeling, which suggests that it may be a differentiated body, containing a liquid water ocean between an icy crust and a rocky core.

If this is the case, then Ceres may be tied with Europa as the second most promising body in the solar system on which to find life. And no, the first is not Mars. If you don’t know what the best planet to find life on is, then you really, Really, REALLY need to spend less time in front of your computer, and more interacting with the world around you.

If Ceres does end up having active internal- or even surface- processes, then it may merit a return to the big leagues. After all, imagine if it did host the only non-terrestrial life in the solar system. How would the Ceres microbes feel when getting taunted by our own? “You’re only an asteroid germ.” Fortunately, Ceres is due to be visited by the uncancelled Dawn spacecraft in 2015, the same year that Pluto Express flies by that planetary system.

Unfortunately, the IAU absurdity doesn’t end with the dirty dozen planetary suggestion. The union has additionally suggested that all large trans-Neptunian bodies be called “plutons.” Presumably, this would make the Kupier Belt a “Batholith.” Imagine the confusion that would result if it was determined that Pluto had a granitic core.

But the ignorance of the space cadets who proposed a term already used in planetary science is a side issue to the subject of Pluto, and all the Pluto wannabes in the cold space beyond.

If I had the power of planetary definition, I would suggest that, to be a planet, a body must have a surface younger than the late heavy bombardment, and possibly an atmosphere. Pluto would qualify on both counts, but Mercury would fail this test. Of course, Mercury has been devoid of planetary processes for 3.8 billion years, so even if it is a planet, it is so boring that after the Mariner 10 flyby, it was abandoned for 35 years, and more than half of the surface remains unmapped today. Pluto, Ceres, and Vesta have been mapped with the Hubble Telescope, but Mercury just isn’t worth the risk. In terms of planetary chemical evolution, nothing has happened on Mercury that hasn’t also been observed on the asteroid Vesta, and our own Moon seems to have had a more interesting history.

However, due to historical precedent, I think a more forgiving definition of planets should be devised. In the interest of science, this definition should encourage exploration and expansion of human knowledge, instead of merely provoking committee bickering and cryophobia. It should be possible to craft a definition that allows future planets, if they are sufficiently special, but is not so restrictive that Pluto and Mercury get dumped.

So, my suggestion for a planet is this:
1. It should be round today- so formerly round bodies that cooled, became brittle, and broke up don’t count.
2. It should be chemically differentiated. Thus, in addition to being round, it should have had the unmixable components gravitationally separated.
3. It can’t be orbiting a larger body (because then it is a moon).

The eight large planets obviously qualify. Pluto probably does, based on the density and the presence of an atmosphere and frost (which indicates at least partial differentiation). As for Ceres and 2003 UB313? Well, that’s the beauty of this definition. If a second-grader asks if they are really planets, instead of boring him with committee recommendations and pedantic debate points, we give him a scientific answer: “We don’t know yet; we need to send a spacecraft there in order to find out.” Our planetary system deserves nothing less.


Anonymous said...

I found your link from Phil Platt's blog; I was very dissapointed with his entry, which is easily summed us as "I'm pouting because I don't like the 'arbitrary' rules of the committee.'

I found your entry well-thought-out and informative. Thank you.

Anonymous said...

Good points. Just some additions, since I've been accumulating planet definition elements myself for a while. You do need to delimit the upper end of planethood. That's usually done by limiting planets to objects that can't do nuclear fusion by force of their gravity. Also, in regard to 2003 UB_313, we do know a bit more than you indicate: it does indeed have (at least) one moon, or satellite if you prefer, S/2005 etc. Can't hold not having a name against it. After all we have to wait on the wheels at IAU to grind out one, and it is already informally Xena.

Anonymous said...


C W Magee said...

Is 2003 UB313's companion a satellite, or a co-planet? Anyway, it still (AFAIK) doesn't have orbital resonance with any inner planets, or evidence for differentiation.

On a related not, it seems that Google(tm) is part of the cryophobic conspiracy. They evidently find this blog to be sufficiently offensive to disable paying ads. I'll bet they have no such qualms about the blogs that support an 8 planet system, though. So much for avoiding evil...

Anonymous said...

Put out Mercury, because it is boring? Yeah, very, very scientific. Almost like a children outcry, when astronomers tried dethrone Pluto in past. No wonder that you're proplutonist. Pluto is nothing more, nothing less as "first and one of largest known members of Kuiper belt object". And probably in future even "one of largest" will be dropped, when we discover something really big. New planet. NINEth planet. Thank you.

Chris R said...

Nice post. For those, like me, who'd missed recent proposals that Ceres is differentiated, see here:

The more I think about it, the more I like the differentiation idea. We geologists should take up our hammers, beat some sense into the astronomers, and claim back "pluton" while we're at it ;-)

Steve Challis said...

The demotion of Pluto from planetary status is offensive to the Plutonians.