Poor Pluto - downgraded from planet status to dwarf planet. Now they are saying it may just be a giant comet.
But here’s my question. So what if it was formed by accretion of a bunch of comets? Isn’t that essentially the same way all of the planets formed? By accretion of a whole bunch of matter that didn’t make it into the sun?
In terms of defining the body, isn’t the important question what it is now, rather than what it used to be?
The definition of a planet or minor planet is rather arbitrary, but I am not aware of any current definition that is based on composition. So the popular press saying that Pluto is a comet instead of a planet has no real scientific basis.
The definition of “planet” has repeatedly been tinkered with to keep the number of planets less than 10. The first asteroids were classified as planets until it became apparent that there were a hell of a lot of them. Pluto was classified as a planet until it became apparent that there were other large Kuiper Belt objects out there.
In composition, Pluto is much more like a comet than it is like the Earth. In size, it’s not particularly like either, being much smaller than Earth and much larger than a comet. Intermediate objects exist in both directions: There are many icy bodies known smaller than Pluto but larger than comets, and there are objects like Ganymede which are in between Pluto and Earth in size, and composed of a mix of roughly equal parts rock and ice.
I’ve said it before, but I personally favor ditching the “planet” label entirely, and instead referring to nonstar bodies large enough to collapse into spherical as either gasballs, rockballs, or iceballs, as appropriate. By this standard, Mercury, Venus, Earth, Luna, Mars, and Ceres are all rockballs; Jupiter, Saturn, Uranus, and Neptune are gasballs, and Io, Europa, Callisto, Titan, Triton, Pluto, Eris, Quaoar, Sedna, and too many others to list are all iceballs. I’ll grant that I’m not entirely sure which category to put Ganymede into (any human-made classification system will have some gray areas), but it’s at least a lot cleaner than trying to pretend that Jupiter and Mercury have anything meaningful in common, or that Earth and Luna are particularly different.
For those who want to read the paper, vs the click-baitish like articles here is a link to the paper.
https://arxiv.org/abs/1805.09285
I think a lot of people read these stories in a way that they are implying that Pluto is comprised mostly of Oort cloud material, which is not exactly true.
It is a proposing an explanation that fits one model better but then calls out.
But I agree with the above replies, the title planet is purely convention, and probably way too broad and should be revisited.
Christ, you guys are going to make me re-learn everything! I already embarrassed myself when my 4 year old grandson kept talking about Makemake. I thought he made it up. I looked up maki maki and that doesn’t help. You feel really old when parts of the solar system are discovered way, way after you are out of school.
Dennis
Humans like to categorize things. Solar System bodies run the gamut of all sorts of things, some of which are easy to see similarities between, while others are just sorta convention based on the evidence the ancients had. If we weren’t able to see any planets until we got telescopes, I’d think, like Chronos, we wouldn’t be so easily lumping Mercury and Jupiter into the same general category. The only reason they are, as far as I’m concerned, is that they appeared very similar to the ancients as “wandering” points of light against the star background. Mercury is more like a moon of Jupiter than it is Jupiter itself.
As to this thread, I think in general many bodies might be classified as “comets” at some point in their life, but eventually all the volatile material that gives rise to their tail is spent. I suspect that anything that resembles a comet now has not spent all that much time near the sun; Halley’s comet is a rare treat in this sense, somehow having been knocked into its current orbit in fairly recent times. Most comets are very nearly hyperbolic in orbit, having extremely long periods and most likely getting knocked into slightly different orbits each time they draw near the Sun. There are probably a ton of iceballs way out there that simply are on a more circular orbit that we’ll never get to see unless they somehow have a close encounter with another object in a more extreme orbit that’s able to impart a significant change to its movement. As our telescope techonology improves, we’ll be able to see more and more that might possibly become comets if they ever got knocked into the inner Solar System, but we don’t think of them as comets unless they have the distinct tail even if they would be composed of the exact same material and formed in the same way.
AFAIK, we’ve never seen a truly gigantic iceball on an elliptical orbit that enters the inner solar system; Hale-Bopp with an estimated 40-80 km nucleus was far bigger than most. So it seems to me that “comets” really does denote a certain characteristic set of objects which Pluto is not one of. I seem to recall a similar debate over the Centaurs such as Chiron (not to be confused with Pluto’s moon Charon).
And indeed, by 1986, all or nearly all of Halley’s volatile material was spent.
But the planet idea fits with the current ideal ( meme, fad, collective goal ? ) of finding earth replacements. The idea is that Sol’s planet arrangement creates the goldilocks zone. eg one part of the definition of planet was “clears its part of the orbit of asteroids,etc”
It would seem to be a a convenient way to put solar systems into categories…
eg 12 planets, comprising 3 rock, 4 gas, 5 ice, or some such labelling.
At the moment the labels are Earthlike, gas, kupier ??
The earthlike status is probably not a good way, because there is more to that than composition… The presence of a Lunar-like satellite is probably another part and that is a reason that obviously juniour satellites will be left as moons not given the same status of what we consider planets or minor planets.
Huh? Maybe you’re talking about the surface, which is mostly non-volatile stuff, but there should be plenty still inside.
As for Pluto, if it somehow had its orbit changed so that it plunged deep into the inner Solar System, say a perihelion of no more than Earth’s distance, it would look somewhat like a comet. It has lots of volatiles (substances that sublimate easily like water, carbon dioxide, and carbon monoxide) and not enough gravity to hold onto them. But it does have much more gravity than most comets, so it wouldn’t be as impressive as, say Hale-Bop. It probably would not have a dust tail, just a gas one.
That brings up a question that I’ve though of before (but never tried to do the math): how close would Pluto need to be to the sun before the atmospheric outgassing was significant enough to give Pluto an obvious coma and tail?
From what I see from the demotion of Pluto I think so far* in the solar system there should only be 4 planets - the gas/ice giants, Those with multiple moons and rings are mini systems of their own and quite different then the rocky planets, which perhaps should be demoted to planetoids, or minor planets.
Also I think there should be a distinction of planets and worlds, as NASA indirectly expresses in that they still consider Pluto a planet for exploration purposes, so basically something that may be suitable for habitation and in-situ resources if teraformed. But worlds would include the round moons/astroides as well
NASA considers Pluto a dwarf planet. The Pluto Express team, on the other hand, continues to call it a planet
These sorts of classifications are obviously rather arbitrary, but I would argue that the fundamental quality that the planets all have in common, regardless of composition, is that because they all formed from the protoplanetary disk they all formed at approximately the same time and they all orbit very close to the same orbital plane. Those are pretty important defining characteristics for being members of an astronomical set. The exception to many of the planetary commonalities, including orbital plane, circularity of orbit, and probably age and origin, is Pluto. So it’s a good thing its planetary status was revoked. Just about the only thing it has in common with the other planets is that it orbits the sun.
Pluto and other Kuiper belt objects are a bit of a mystery, all totaled they only add up to ~10% of the Earths mass but it should be 300% more. The main reason for studies like this are to figure out where that other material went if it ever existed. But to put the relative size in scale,
Quaoar 0.000324
Eris 0.0027
Pluto 0.0022
All KBOs 0.01
Moon 0.0123
Not that I care much about naming, but science does need more exact terms, even if it makes a few people upset that there are changes to just have meaningful scientific conversations.
Well, they’ve got a pride thing going, haven’t they?
At a con panel a few years ago there was an astronomer who had been in attendance at the IAU General Assembly in Prague where Pluto was demoted. He said he had no issues with the decision, Pluto being kind of an outlier with its tiny size and weird orbit, but was troubled by the way it was pulled off.
They had been in town for some time busy with the meeting and not doing much touristy stuff. Sunday was the last day and almost all of the American delegation elected to take the day off and tour Prague instead; that was the day the motion was introduced and passed. He commented at least they waited until Tombough was dead before making their move.
Your definitions are probably the best for astrophysicists to use.
We can say that Mercury and Jupiter are points of light to be seen without a telescope. And that Earth and Luna are celestial bodies humans have walked on.
As centuries have passed, the former definition has become no less true, even if its become pointless trivia, and as more centuries pass, the latter will likewise become less true or less significant, but we can always claim it was first.
We just have to stick with what’s been said many times, in this thread and others, arbitrary human definitions are arbitrary.
I remain outraged at the ongoing campaign to denigrate and demean Pluto. Pluto is our ninth planet, PERIOD. It was recognized as our ninth planet for many decades and then cast aside by haters who manipulated their definition of “planet” to arbitrarily exclude that deserving body. Just let be known to all: :mad:
I LOVE Pluto!
That is all.
But aren’t Earth and Luna particularly different, in that Earth rotates around a star, and Luna rotates around a thing that rotates about a star? So maybe Earth is a “First degree rockball” while Luna is a “Second degree rockball”. Or maybe Luna could be a “rockball once removed”.
BTW, is it possible for a rockball (or gassball or iceball) to be the center of a system? Is there a reason that only a star can be at the center of an orbital system?
Er, what now? There are multiple known examples of rockballs and gasballs and iceballs at the center of orbital systems.