Plutoids
Published on Jun 11, 2008 at 12:40 pm.
8 Comments.
Filed under planets, solar system.
We have yet another new word to describe some of the things out there beyond Neptune: Plutoids. As you may recall, a couple of years ago, the International Astronomical Union finally defined the word “planet.” It was something of a shock to many people in the public, but astronomers had never actually come up with a definition of “planet” in all these years. Originally, planet meant a “wandering star.” That would be a star that appeared to move from constellation to constellation. Five of them were known. The Sun and Moon also moved around in the sky, so they were often considered planets by the ancients. Eventually, astronomers realized that all of the planets and Earth moved about the Sun. Soon, it became apparent that Earth belonged in the group of bodies known as planets. Then, Uranus was discovered. Then, Ceres, Pallas, Vesta, and Juno were found. Neptune was discovered. All of these were called planets. But, then more and more planets were found between Mars and Jupiter. Eventually, astronomers realized that these bodies were far too small to fit it with the rest of the planets, so they were removed from the list. Then, Pluto was found. At first, astronomers mistakenly thought that it was far bigger than it really is, so it was placed on the list of planets. Eventually, we found that it is really quite small. Then, other bodies similar to Pluto in size and composition were found far beyond Neptune.
For some years, astronomers had expected to find icy things out there. Gerald Kuiper had proposed that there should be a large reservoir of these bodies beyond the farthest planet, much like an icy asteroid belt. These bodies, when their orbits are disrupted into highly elliptical orbits that bring them near the Sun would appear as comets. This is proposed to be a separate reservoir of icy comet nuclei than the Oort cloud. Kuiper’s hypothetical reservoir of comets became known as the Kuiper Belt. Finally, in the 1990s, the first of these Kuiper Belt objects was found. Then, more were found. Then, more. The first few were small, but then astronomers started finding larger ones. Some were nearly the size of Pluto, in similar orbits, and of nearly the same composition. This soon sparked a debate as to whether or not Pluto should be on the list of planets. After all, it was to these other bodies what Ceres was to the objects that became known as asteroids. Astronomers proposed several names for these bodies (including Pluto). Among the ones that I have heard have been: Kuiper Belt Objects (my preferred one), Centaurs (because the first few were named for centaurs), Plutinos (proposed by the team that found the first one past Pluto), Plutons, (a proposed category that never caught on), and Trans Neptunian Objects (the title of most of the AAS sessions on these bodies).
The whole matter came to a head, though, when an object larger than Pluto was discovered out there. That body, now known as Eris, triggered the International Astronomical Union (IAU), the professional organization of astronomers, to finally devise a new definition for the term planet. A committee for the IAU worked on a proposed definition for about a year. Unfortunately, that definition did little to clear things up. Bowing to popular opinion, the new definition was contrived to keep Pluto on the list, but it added Eris to the list and it put Ceres back on the list again. However, it also made Charon a planet. Charon had long been considered Pluto’s moon, but the two actually orbit a center of mass point that is between Pluto and Charon, so the new definition made Pluto and Charon a binary planet. But, the definition also left about a dozen “maybe” planets that needed more work to clear up their status. The IAU general assembly voted down this definition in favor of one that they hurriedly put together. The definition of planet that was accepted leaves Pluto off the list of planets, reducing the number to eight. It creates a new category called “dwarf planets” that includes Eris, Pluto, and Ceres.
Now, according to a press release from the IAU, the IAU’s Committee on Small Body Nomenclature is proposing to call these icy bodies in the outer Solar System Plutoids. The new designation is for bodies orbiting beyond Neptune that have sufficient mass to pull themselves into roughly spherical shapes, but not sufficient mass for them to dominate their portion of the Solar System, clearing their orbits of like bodies. Personally, I am not sure that this definition is really right. I think that rather than keying the definition to the location of the body, it should be keyed to the composition of the body. True, Pluto, Eris, and the handful of bodies that we suspect (but don’t know for sure) might fit the definition are all similar composition. But, what would happen if we found something out there that was more like Ceres in composition? It is not likely, but suppose it did happen. Would it be fair to lump it with Pluto rather than considering it to be an out-of-place body asteroid belt body? As I see things, we are still not coming up with really good definitions here.
And, of course the name itself, Plutoid, is an attempt to mollify those die-hard Pluto supporters. Typically, bodies are named for the largest member of similar objects. The inner, rocky planets, are called Terrestrial Planets because Earth (Terra) is the largest of these. The gas giants are often called Jovian planets because Jupiter is the largest. Of course, I have issues with that, because Jupiter and Saturn clearly go together, but they are quite different from Uranus and Neptune, which are both similar to one another. Eventually, we will need to come up with a better system of classifying these bodies.
-Astroprof
Image Credit: IAU, NASA/ESA Hubble Space Telescope, H. Weaver (JHU/APL), A. Stern (SwRI), the HST Pluto Companion Search Team and M. Brown
andy on June 11, 2008 at 5:08 pm: 1
What’s even better is that Ceres is apparently no longer considered an asteroid (at least, according to the IAU official quoted at the end of this article. It would thus seem that, according to the IAU, the largest members of a population of objects are not actually members of that population at all.
Daniel Fischer on June 11, 2008 at 9:28 pm: 2
The equation is and will always be dwarf planets = plutoids + Ceres, and since Ceres is by far the largest body in the asteroid main belt, there is no way that another dwarf planet will be found there.
Ceres was moved out of the minor planet (aka. asteroid) category because of its size, shape - almost perfect sphere - and mass; it almost weighs as much as all the other main belt asteroids combined. This was all decided in 2006 already; no surprises here.
Suzanne on June 12, 2008 at 2:47 pm: 3
Many professional planetary scientists (myself included) are annoyed/angry about yet another IAU committee decision that confuses rather than clarifies the subject of what is a dwarf planet like Pluto. We do not appreciate having yet another meaningless term hit the news media when we are trying to educate the public about the wonderfully complex system of objects that orbit our sun.
“Plutoid” is silly; “dwarf planet” makes sense, especially since dozens have already been found past Neptune and since many moons of outer planets exist in this size/shape range.
Astroprof on June 12, 2008 at 4:01 pm: 4
I agree, the whole thing is silly. It seems that these decisions and definitions are NOT being made with any scientific rationale. Rather, they are being made based on public opinions and to play on the popular appeal of Pluto. I can see “dwarf planet”. I don’t really see anything wrong with calling all of the things out there Kuiper Belt Objects, with a notation that the largest ones are big enough for gravity to pull them into a roughly spherical shape.
Daniel Fischer on June 13, 2008 at 8:21 pm: 5
Astroprof said that the IAU decisions were “being made based on public opinions and to play on the popular appeal of Pluto” - why then are the plutophiles out there still not happy?
As I explained above, the introduction of the dwarf planets subcategory in 2006 now named plutoids would not have been necessary at all but was made strictly in honor of Pluto for its 62 years reign as the only known Kuiperoid.
Adam on June 22, 2008 at 3:05 pm: 6
Terrestrial planets
Jovian planets
Plutonian planets
KBO’s
As a plant scientist watching this field go through its fits, it was great to have a guy like Linnaeus early in our field, since we have the Linnaean taxonomy system.
Like the Linnaean system, Ceres and Vesta, could be duly dealt with. Personly I hate the movement of the “oid” to “Plutoid”, since I believe Pluto to be really out there as a planet. The classification of “clearing the orbital path” is really a meaningless joke since I think Trojans and the Greeks are clearly exceptions to this rule. There will be many others like this.
There is a high probability that Pluto and Charon are tidally kneaded, and once the Horizons project is out there with Pluto, it will no doubt turn our heads (hopefully) and make us review the situation with fresh eyes.
The Plutoid ruling is plainly confusing. It should be omitted.
Adam on June 23, 2008 at 4:16 pm: 7
KB planets
Laurel Kornfeld on June 24, 2008 at 11:12 am: 8
Supporters of Pluto maintaining its planet status are not happy with this for several reasons. First, the original 2006 vote was done by four percent of the IAU; no absentee voting was allowed. Immediately, 300 planetary scientists led by Dr. Alan Stern, Principal Investigator of New Horizons, signed a petition rejecting the new definition as “sloppy science that would ever pass peer review” and stated they will not use it. Now, we have an even smaller contingent of the IAU making decisions without consulting leading experts in the field of planetary science, who were taken by surprise by this announcement. Clearly, the IAU is using a closed, backroom deal process that exludes a large number of experts in the field. How a group arrives at a decision is just as important as that decision itself.
Second, the concept that a dwarf planet is not a planet at all, which the IAU definition states, makes no linguistic sense. Ceres may have been demoted in the 19th century, but now that we know it is in a state of hydrostatic equilibrium (has enough self gravity to pull itself into a round shape), meaning it has geological activity more akin to the planets, it is clear that asteroid is not the correct categorization for it. That is why the solution that makes the most sense is to establish dwarf planets as a new subcategory of planets that have achieved hydrostatic equilibrium but do not necessarily dominate the neighborhood of their orbits. This distinguishes Ceres from the other asteroids in the asteroid belt and Pluto and Eris (and more round KBOs) from shapeless objects in the Kuiper Belt. The “Plutoid” category is entirely unnecessary, as we could simply describe Ceres as an asteroid belt dwarf planet and the round KBOs as Kuiper Belt dwarf planets.