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Questions About the Accretion Model of Planet Formation


The most common explanation for the formation of planet Earth is that it formed by gravitational collapse from a cloud of particles (gas, ice, dust) swirling around the Sun.  Specifically, the idea is that small planetesimals form as the various particles clump together (perhaps initially by cohesion, then by gravity), eventually growing into planets.  Known as the “accretion hypothesis,” this is the standard model of planet formation, not just for Earth, but for nearly all planets.*

Artist's Conception of Circumstellar Disk Courtesy, NOVA
Artist’s Conception of Circumstellar Disk
Courtesy, NOVA

Significant debate continues regarding the formation of the Moon, but the most widely-held hypothesis is that the Moon formed in a similar way via accretion of impact material produced by a violent collision between a Mars-sized object and the Earth.

For purposes of the current discussion, I want to set aside the debate about Moon formation for a moment and focus on the formation of a planet from a circumstellar disk.

Evidence for the Accretion Hypothesis

There is a decent amount of circumstantial evidence one can point to in support of the idea of planet formation via accretion from a circumstellar disk.  The evidence is primarily two-fold:

First, a number of stars have been imaged with a circumstellar disk around them.  These are thought to be solar systems in the early stages of formation, prior to the time when the planets would have cleared out the vast majority of the disk particles.

Second, we have witnessed live examples of meteors, comets and asteroids striking an object that is many orders of magnitude larger and “accreting” onto those much larger objects.  For example, numerous meteors strike the Earth’s atmosphere on a daily basis, with the smaller bodies leaving behind dusty remnants and some larger bodies striking the Earth in more solid form as meteorites.  There are also examples of comets and other small bodies colliding with other planets in our solar system – Comet Shoemaker-Levy 9’s impact with Jupiter in 1994 being perhaps the most well-known and heavily-studied example.

Why, then, do I refer to these pieces of evidence, strong as they may be, as “circumstantial”?  Precisely because in these events we do not see planets forming.  It may be true that these kinds of events eventually produce planets, but given the timescales involved, we have not, and indeed simply cannot say that we have ever, seen a planet form.  Planet formation lies in a timeframe that is inaccessible to our instruments: either in the deep past for existing planets or in the far future for planets that might arise from the circumstellar dust disks we have imaged.

This does not mean that the accretion theory is wrong.  Indeed, it has quite a lot going for it, as mentioned.  But it does mean that in trying to reconstruct the origin of planets, including Earth, we are dealing with historical science, rather than lab science.  Meaning that, rather than being able to conduct repeatable experiments in the lab, we are left to examine competing hypotheses to decide which hypothesis of past events best explains what we see in existence today.

In addition to the circumstantial evidence, there are many computer models that have been put together based on the accretion hypothesis.  These can be very useful to the extent they accurately model the physical realities.  Yet it is important to keep in mind that these models are not data, they are not evidence in the sense of actual observations.  True, to the extent we believe the models accurately reflect physical realities these models can help us understand the processes in question.  Perhaps they can even help confirm our suspicions or guide our thinking in a particular way.  But we need to keep in mind that they are at least one step removed from actual hard evidence.  (As an aside, and there are nuances and exceptions to this aside, we can generally acknowledge that to the extent a model is based upon a particular hypothesis, it might be useful in understanding how the hypothesis would play out, but it cannot be relied upon to demonstrate the truth of the hypothesis.)

So the accretion hypothesis of planet formation enjoys (i) a decent amount of circumstantial evidence, and (ii) support from some computer models.

Accreting Questions

I am not here to argue that the accretion hypothesis is wrong.  It may be spot on.  It may be precisely what happened to give rise to Earth and the other planets.  Yet, there are some lingering doubts, some open questions.  Nothing yet rising to the level of a cogent argument against the accretion hypothesis, mind you; just a hint of unease with the usual explanation.

Are the doubts and hints of unease foundational?  Are they enough to warrant skepticism?  Or perhaps it is simply the case that our exploration and understanding of space are tentative, that the overall story is tight and secure, that there are merely a few details to be filled in?  I’m not sure.  But perhaps a quote or two will give us a hint of some open questions.

NASA’s webpage about the Origin of the Earth and Moon indicates:

About 130 scientists met December 1-3, 1998, in Monterey, California, to share ideas about the formation and very early history of the Earth and Moon. Conference organizers constructed the program to allow time for participants to discuss crucial issues, leading to lively and spirited debate.

The existence of “lively and spirited debate” is interesting, particularly given the certainty with which some people cling to particular hypotheses.  Granted, this quote seems to be focused primarily on the Moon’s origin, but not exclusively.

The later statement about stellar and planetary formation jumped out at me:

The cloud from which the Solar System formed was composed of gas and dust. Somehow in that dusty cloud, the Sun formed in the center and the planets formed around it.

Notice the word “somehow” that plays such a prominent role.  The article goes on to propose what that “somehow” may have been, but already we are getting a sense that things are not quite pinned down and that the general explanation might be a bit simplistic.

Again, this is not an argument against the idea.  But a careful student of the hypothesis might be forgiven for pausing and thinking, “Hmmm . . .”

Recent Asteroid Collision

What about more direct evidence for accretion?  NASA’s Spitzer Telescope had the opportunity to witness what researchers believe was a collision of asteroids just a few months ago.

The press release is instructive, as it relates to the accretion hypothesis.  A few quotes of note:

NASA’s Spitzer Space Telescope has spotted an eruption of dust around a young star, possibly the result of a smashup between large asteroids. This type of collision can eventually lead to the formation of planets.

Notice the statement that a collision of two asteroids is the kind of collision that can “lead to the formation of planets.”  Yet notice how one researcher describes what the evidence is actually showing:

We think two big asteroids crashed into each other, creating a huge cloud of grains the size of very fine sand, which are now smashing themselves into smithereens and slowly leaking away from the star . . .

Wait a minute.  What that researcher is describing is most definitely not accretion.  If anything it is dispersion.

Another researcher confirmed:

We not only witnessed what appears to be the wreckage of a huge smashup, but have been able to track how it is changing – the signal is fading as the cloud destroys itself by grinding its grains down so they escape from the star.

In contrast, a third researcher, apparently less interested in the actual evidence and more excited about the possible implications, gushed: “We are watching rocky planet formation happen right in front of us.  This is a unique chance to study this process in near real-time.”

Sorry, no.  What you saw was a collision between bodies that tore them apart, which was then followed by further collisions of the smaller bodies that broke them apart, followed by an ever-further dispersion of the resulting debris.

We might be forgiven for pointing out that the dispersion they witnessed is precisely what we see whenever we witness a collision . . . certainly a collision of bodies that have even close to a similar mass.

Imagine a decent-sized meteor striking an old satellite in orbit around the Earth.  Do we expect accretion to take place?  Of course not.  Rather, we instead now have the risk of multiple smaller pieces of debris that have to be tracked — not clumping together like kids in front of a candy story window, but instead with each piece of debris now on its own independent trajectory.  This is not a hypothetical.  This is what in fact occurs and what we are spending hard-earned dollars to track.

More on point, if we start looking at related scenarios, the questions multiply.  Scientists still debate how Saturn’s rings were formed, but one of the primary proposed scenarios involves the breakup of moons.  NASA’s website for kids explains:

But scientists aren’t sure when or how Saturn’s rings formed. They think the rings might have something to do with Saturn’s many moons.  Earth has only one moon. But Saturn has at least 60 moons orbiting it that we know about. Asteroids and meteoroids sometimes crash into these moons and break them into pieces. The rings could be made from these broken pieces of moons.

I don’t know about you, kids, but to me that doesn’t sound like accretion.  Precisely the opposite.

Other scientists propose that the rings are left over from the early nebular material in the solar system, which better preserves the accretion idea, but presents challenges of its own with little resolution.


Conclusion?  I don’t have one.  Again, I am not presenting any strong argument in the above against the accretion theory of planet formation.  What I am presenting are some questions – questions that immediately arise as soon as we start to look into the idea and thinking through about the details.  Questions which remain unresolved after decades of study and research.

Let me know your thoughts.  Is the accretion hypothesis sound?  Do the processes we actually do see in action (cometary impacts, asteroid collisions, satellite debris, etc.) support the idea of accretion?  Given a primordial circumstellar disk, how would the transition take place from (a) collisions that primarily result in breakup and dispersion, to (b) collisions that result in accretion?



* I add here, to prevent anyone going down the wrong path, that I appreciate and respect the work that is done in the astronomical community, including in the area of exoplanet research.  I find the effort to discover another Earth-like planet to be extremely interesting, exciting and worthwhile, and follow it closely.  I do not share the views, apparently held by some on this site, that the search for Earth-like planets is a fool’s errand, that Earth as a habitable planet is alone in the cosmos, that other intelligent life does not exist, or that some religious-based apple cart will be upset by the discovery of other habitable planets or other intelligent beings.

This thread is not devoted to such issues.  To the extent possible, please focus comments on the actual mechanics and physics of planetary formation as they relate to the accretion hypothesis.

Scientific models are really not much more than educated guesses or hypotheses. Take for example the recent announcement that scientists are going to take a core sample from the Chicxulub Crater to see if their models are accurate or not on Fox News. Here is a brief quote from it:
Scientists THINK that, when a big rock smashes into Earth at high enough velocities, the collision causes the crust temporarily to act sort of like a liquid, first forming a so-called transient crater (like the indentation that forms on a lake surface after a rock is thrown in), and the center rebounds, or splashes, upward and then outward. "We think the peak ring is the record of the material that rebounded and splashed outward," Gulick told Live Science. All of these ideas are based on MODELS and aren't necessarily what happened. "We've never gotten a rock back from a peak ring to know if that's correct," Gulick said.
This is an excellent example of why historical science is no where near as accurate as real experimental science that we can test. They will begin drilling in 2016 we are told. It will be interesting to see if their "model" is anywhere close to reality. My prediction is that it will not be. And what will happen then? Nothing major I'm sure. Just tweaking, fudging, and special pleading with the purpose of saving the theory. Time will tell! PART II: When the data doesn't match the assumptions on which you have built your scientific models, what do you do? Here is another example of science being built on false assumptions. This too illustrates the difficulty of building models on assumptions that is unavoidable when doing historical science.
Type 1a supernovae, vital to estimates of the size and expansion of the universe, are not uniform. This has cosmic implications. A team from the University of Arizona has news of cosmic proportions. For many years, Type 1a supernovae have been considered “standard candles” at all distances. This allows astronomers to calculate cosmic distances, and in fact was used to deduce the accelerated expansion of the universe in the late 1990s that won three astronomers a Nobel Prize. Without a known cause for cosmic acceleration, astronomers have proposed some unknown kind of “dark energy” as the cause. All of this has relied on the assumption of uniformity of Type 1a supernovae. A UA press release now calls that assumption into question. Using data from the Swift satellite, which measures stars in ultraviolet light, the UA astronomers found that Type 1a’s fall into two classes. The nearest ones are redder than the more distant ones. Reporter Daniel Stolte titled his article, “Accelerating universe? Not so fast.” Since nobody realized that before, all these supernovae were thrown in the same barrel. But if you were to look at 10 of them nearby, those 10 are going to be redder on average than a sample of 10 faraway supernovae.” The authors conclude that some of the reported acceleration of the universe can be explained by color differences between the two groups of supernovae, leaving less acceleration than initially reported. This would, in turn, require less dark energy than currently assumed. The team is unable to put a number on how much the figures will need to be adjusted, saying further work is needed. ..... If you win a Nobel Prize for a false conclusion, do you have to give the money back? Note once again how assumptions play crucial roles in models that try to understand observations. Perhaps dark energy is real, though less than expected. There’s an outside chance though, if supernova populations are not as uniform as even the UA astronomers believe, that there is no dark energy at all. This should be a lesson in assumptions and unknowns in science.
Eric Anderson: There is most definitely a distinction between repeatable bench science and historical science. It is a well-known difference. They both still use the scientific method. When scientists study layers of volcanic ash to study the history of volcanic eruptions, they are using the scientific method. Eric Anderson: It just means that it does not enjoy the level of certainty that bench science does, nor the balances and checks and confirmations that repeatable experiments provide. Many repeatable experiments have indeterminate results. Many historical findings are so strongly supported as to be considered scientific fact. Dinosaurs did roam the Earth. Zachriel
With respect, Zachriel, you might be served by looking into the philosophy of science a bit. There is most definitely a distinction between repeatable bench science and historical science. It is a well-known difference. That doesn't mean historical science isn't science. It just means that it does not enjoy the level of certainty that bench science does, nor the balances and checks and confirmations that repeatable experiments provide. Instead, it relies more on clues and stories and inferences to explain the past. This also, unfortunately, means that it is much more subject to abuse and storytelling if proponents of particular explanations are not scrupulously careful to consider alternative explanations for those clues and to be cautious with the inferences they draw. Eric Anderson
tjguy: There is even a difference scientifically speaking between what we can know in Experimental Science vs Historical Science. One uses the scientific method and it’s results are much more trustworthy. Both use the scientific method, or they're not science. tjguy: Cosmology suffers from being part of the historical sciences and that means the word “know” when used by cosmologists is even less sure than when used by the experimental or lab sciences. You're drawing an invalid distinction. We have a great deal of certainty about some aspects of history. Dinosaurs once roamed the Earth. Zachriel
Zachriel @ 37
tjguy: I would take the word “know” with a grain of salt.(unless you redefine it to mean less than it means)
Scientific knowledge is always tentative, no matter how strongly supported. Yes, that’s what it means.
I agree. So what we are saying is that the scientific definition of the word "know" is different than the common usage of the word "know". This needs to be understood by impressionable students who do NOT understand this. There is even a difference scientifically speaking between what we can know in Experimental Science vs Historical Science. One uses the scientific method and it's results are much more trustworthy. Cosmology suffers from being part of the historical sciences and that means the word "know" when used by cosmologists is even less sure than when used by the experimental or lab sciences.
tjguy: If the details are still murky, how do they really know that what they think they know is actually true?
We don’t have to know everything to know some things.
In which sense are you using the word "know" here? Common everyday vernacular sense or the scientific sense of educated guess? It is obvious that the cosmologists, since they don't understand the details, SHOULD be using it in the scientific sense. When they write about their discoveries, theories, etc., they should keep in mind the two different meanings this word has and be more accurate with it's usage. Perhaps even avoid using the word "know" when writing to lay people would be more accurate to avoid giving a false impression. - but then that would expose their secret wouldn't it? - that they really don't know for sure. The planet accretion model is simply the best one they have, even though it doesn't answer all the problems yet. But, they hold on to it because it's the best they've got. If/when they find a new and better model, they will then proceed to publicly say how the new model solves all the problems the old model couldn't. Only few people really knew that the old model didn't solve all the problems. The "old model" serves as temporary useful filler until they can find the real answer - if there is one in their worldview. It would be too embarrassing to have nothing on the drawing board. tjguy
Please accept my apologies. I thought you were referring to the ALMA press release. My bad. Yes, the Nature News Feature does claim that some of the basics of accretion theory are right, and then goes on to give a layman's description of the hypothesis. But that doesn't mean we have to accept that layman's summary at face value. Particularly when there are researchers who are actually focusing on the accretion process who have noted significant problems, as was pointed out. For example, the very formation of planetesimals, which the Nature News Feature author treats as a matter-of-fact given, is acknowledged as a significant open issue by some researchers who have actually looked into what is required. The Nature News Feature is focusing on exoplanets and wondering how, given the accretion model, these exoplanets could come to be where they are. It simply assumes that the accretion model is right and then asks, in effect, "What happens afterwards?" Yet some scientists who are looking at accretion itself are thinking it might need an overhaul. This is not uncommon with consensus type theories. Particularly those based more on historical inference and models than actual real-time observations. Scientists working on issue B say, "Yes, there are currently many problems and open questions with issue B that I happen to be looking at, but overall I'm sure the broader theory is sound; after all, other scientists have claimed that part A of the theory is solid." But then we look at what the scientists whose work focuses on part A have found, and it turns out that A isn't really quite so solid either. Eric Anderson
Eric Anderson: You seem so fixated on the particular ALMA press release you cited that you haven’t considered the points in the OP, nor the many citations provided by tjguy @26. We just quoted (#37) from the Nature article tjguy cited, which indicates wide agreement with core accretion. Zachriel
Well, you're either very easily impressed or your study of the issue hasn't extended much beyond a few press releases that toe the party line. You seem so fixated on the particular ALMA press release you cited that you haven't considered the points in the OP, nor the many citations provided by tjguy @26.
. . . it also indicated a great deal of certainty about the general theory of aggregation due to gravity and electrostatic forces.
Oh, your particular press release may give that impression, but things are not nearly as certain as you pretend they are.
. . . scientists aren’t satisfied with just knowing that planets formed by aggregation . . .
The point is that they don't know this. Look at comment #26 again. Many of the issues raised have to do with the very aggregation process itself. The whole theory seems to be up for question at this point. Now we could play a bit of a game and say that if we take the view that the universe (or the galaxy) used to be a bunch of diffuse dust and gas, then in order to get where we are today some kind of "aggregation" must, by definition, be true. But in that case it isn't very helpful to say that our theory of "aggregation" is correct and now we're just filling in a few details. The details are the theory. Absent the details, the "aggregation" theory just dissolves into a general assertion that "Well, some small stuff stuck together and became big stuff." The devil -- and any value in a particular theory of planet formation -- is in the details. So, no, I am not impressed with a press release that sounds very confident in the idea of aggregation generally. And when there are lots of open questions about the most common models of aggregation then we are perfectly entitled to question how much is really known. And I don't feel at all obligated to genuflect and say "Yup, they've really pinned down that planets formed by aggregation. That's solid science. . . . They just don't know how it happened." Eric Anderson
Eric Anderson: The question is: How strongly is something supported? Sure. That's the exact question. We were provided a citation. It indicated a great deal of uncertainty about planet formation, but it also indicated a great deal of certainty about the general theory of aggregation due to gravity and electrostatic forces. The difference is that scientists aren't satisfied with just knowing that planets formed by aggregation, but they want to know exactly how they formed, why some planets are rocky, others are gaseous, whether nearly circular orbits are ordinary or unusual, a whole host of questions. The discovery of exoplanets confirms some theories and contradicts others. Zachriel
Scientific knowledge is always tentative, no matter how strongly supported.
Of course. The question is: How strongly is something supported? There is a huge difference between a reproducible, lab-based, independently confirmed experiment on the one hand, and a never-witnessed, theoretical, process that we only have hazy glimpses of on the other hand. Particularly when the latter has a number of well-known unresolved issues. Trying to pretend the latter is just as solid as the former because, "Hey, all science is tentative," doesn't cut it. Such an approach makes a category mistake and ignores an important distinction between the two. Such an attitude also tends to make some people (as we have seen in the present thread) think they have to reflexively jump in and defend "science," when they should be open-minded enough to ask the uncomfortable questions. Eric Anderson
tjguy: I would take the word “know” with a grain of salt.(unless you redefine it to mean less than it means) Scientific knowledge is always tentative, no matter how strongly supported. Yes, that's what it means. tjguy: If the details are still murky, how do they really know that what they think they know is actually true? We don't have to know everything to know some things. tjguy: Then why, pray tell, did Nature publish an article saying that they might have to rethink the whole thing?! From your citation:
Nature: Planets in chaos: "In the search for an overarching theory, astronomers do agree that core accretion has some things right: planets are leftovers from the birth of stars, a process in which interstellar clouds of hydrogen and helium gas contract until their cores grow dense and hot enough to ignite... As the disk cools, electrostatic charges stick these grains together to form loose conglomerates that eventually grow into kilometre-scale bodies known as planetesimals. At that point gravity takes over, and the planetesimals collide, fragment, mash together and grow into full-sized planets."
Zachriel pointed out this quote in the Nature article:
“Astronomers know that both stars and planets form from interstellar gas clouds — but the details are still murky.”
I would take the word "know" with a grain of salt.(unless you redefine it to mean less than it means) Yes, that is what they said/claimed and I said: If the details are still murky, how do they really know that what they think they know is actually true? If that doesn't bother you, fine, but it obviously does bother other scientists.
Which is why scientists don’t claim to have all the answers, however, accretion is strongly supported, including by the latest results from ALMA.
Strongly supported? Then why, pray tell, did Nature publish an article saying that they might have to rethink the whole thing?! And why are there some scientists trying to come up with new theories to explain planet formation? - Because there is such strong support for the 'core accretion' model? Hmm... Personally, I doubt it, but it seems nothing will persuade you. At the very least, you have to admit there is a controversy right now.
Eric Anderson: We can take beautiful pictures all we want and label them as “planet formation in process,” but it doesn’t mean that is what they are. The ring formation matches the influence a planet is expected to make. Eric Anderson: But it would be nice to have a theory that isn’t fraught with complications at multiple steps in the process and that has to be patched with ad hoc explanations at every turn. Which is why scientists don't claim to have all the answers, however, accretion is strongly supported, including by the latest results from ALMA. Zachriel
New paper out of MIT highlights/updates magnetism role: http://newsoffice.mit.edu/2014/strong-magnetic-field-early-solar-system-1113 ppolish
Zachriel: With respect, I think you may be missing much of the point. We can take beautiful pictures all we want and label them as "planet formation in process," but it doesn't mean that is what they are. The link you provided certainly did not give any information that would help resolve the open issues highlighted in the OP and in tjguy's comment #26. It isn't just that the timescales are too long and so we have to draw an inference. That part is certainly true, and is the case with any historical science. But it is more than that. The fact that we have never seen planets form, that the models are in conflict, that exoplanets don't seem to follow the "rules," that the few real-time observations we have been made tend to be examples of dispersion not accretion, that special pleading and astronomical odds are required to explain certain features -- all of these facts underscore that the accretion hypothesis is in serious trouble in its current incarnation. No-one is asking for infallible deduction founded on lab-based, repeatable observations. That is not possible with historical science. But it would be nice to have a theory that isn't fraught with complications at multiple steps in the process and that has to be patched with ad hoc explanations at every turn. Eric Anderson
tjguy: But you really do not KNOW this. Science doesn't prove, but marshals evidence. While, as you point out, there are many open questions, that doesn't mean nothing is known. Per your citation, "Astronomers know that both stars and planets form from interstellar gas clouds — but the details are still murky." Exoplanets are still a relatively new discovery, and they are on the observational edge, so it will be some time before this new data is fully understood. Meanwhile, Birth of planets revealed in astonishing detail in ALMA’s ‘best image ever’ http://news.science360.gov/obj/story/c30c1013-cc04-4651-9c42-856e7f0aaa33/birth-planets-revealed-astonishing-detail-almas-best-image Zachriel
tjguy "Zachriel, you don’t know that for sure. See this is an example of the problem with historical science. According to your theory – if it is accurate – then you may very well see planets in many stages of formation. But you really do not KNOW this. This is what you think is true. You should add the word “think” to that statement to be accurate, but i have a feeling that you think this area of science is pretty well figured out." Good points and logical answer. Personally I would know how to interpret what I'd be seeing millions or billions of miles away either, but to many of these researchers in the Cosmos fields, making up stories as they go along and calling it science seems to be the pattern. Here is where real science starts. Real Science was undertaken when Scientists and Research engineers put together the Hubble Telescope. Real Science was undertaken when Scientists and Engineers designed and built the rocket systems which put the Hubble into orbit around the Earth. Real Science was used in the calculations with advanced understanding of mathematics for planning the mission. Where science fails in when a fool looks through the telescope and attempts to story tell what he believes from personal religious bias what he thinks is going on billions of miles away. But never underestimate the power of credentials & consensus of like minded ideologues to insist they speak from authority and all knowledge and understanding come only from them. tjguy "Perhaps you need to rethink that thought." Good luck with that. The historical precedent in conversing with this individual by multiple people over the years reveals the exact opposite will most likely happen. DavidD
Y'all are just simply over-thinking this, and ruling out the simplest solution unnecessarily. I mean there's a REASON why all the new exo-planets being discovered are hot and close to the parent star..... Same exact reason that most star systems are "binary" to begin with. These hot new Jupiters are not planets at all..... In fact, there ARE no planets. There is only stars, all in various phases of their life cycle. The Earth is an old star. That's where it got its hot iron core and its magnetic field. The chemical elements and the water did not arrive here through absurd fine-tuning of convenient super-novas and hundreds of millions of cooperative cometary impact - the Earth synthesized its own self first through fusion and then through plasma convection. Our Moon is an even older star. That's why its rocks are still magnetized and carry traces of water and helium 3. ------------ wayne moss
tjguy @26: Thank you for the detailed and extremely enlightening quotes and the helpful review of the current state of affairs. One of the things I appreciate about some of the commenters here at UD is the fact that others have often spent more time with a topic than I. Comment #26 alone should be enough to disabuse anyone of the notion that the accretion hypothesis is settled science. Previously I said:
Again, I tend to think that there is a perfectly reasonable explanation for how stars and planets form through purely natural processes. I’ll keep looking.
I guess I'd still like to think that is the case. But apparently whatever natural process did it, it isn't the accretion hypothesis, at least not in its current incarnation. Here I was focusing in on a couple of specifics, when it turns out the whole concept is rife with open questions, ad hoc explanations, and lucky coincidences. The OP barely sticks the proverbial camel's nose under the tent. You highlighted some great quotes, but perhaps my favorite sentence was the understated: ". . . it should be obvious that any theory that depends for its acceptance on a long string of lucky accidents falls short of the scientific ideal." Eric Anderson
Eric, Thank you for bringing this up. Food for thought! Is star and planet formation simply against the second law? One thing is for sure: the dispersal of matter and energy is in full accord with it! Box
We see planets in many stages of formation.
Zachriel, you don't know that for sure. See this is an example of the problem with historical science. According to your theory - if it is accurate - then you may very well see planets in many stages of formation. But you really do not KNOW this. This is what you think is true. You should add the word "think" to that statement to be accurate, but i have a feeling that you think this area of science is pretty well figured out. Perhaps you need to rethink that thought. tjguy
Eric, Thank you for bringing this up. Cosmology is in trouble these days. You are absolutely right that there are a lot of things about star formation that scientists do not yet understand. Yes, the accretion model is the current “in vogue” model, but you are exactly right in pointing out the limits we face in trying to piece together what happened in the distant unobservable past. Recently, there was an article published by nature entitled “Astronomy: Planets in Chaos.” http://www.nature.com/news/astronomy-planets-in-chaos-1.15480 Here are some quotes from the article:
“The discovery of thousands of star systems wildly different from our own has demolished ideas about how planets form. Astronomers are searching for a whole new theory.” “Astronomers know that both stars and planets form from interstellar gas clouds — but the details are still murky.” I would humbly suggest that if the details are still murky, maybe claiming to “know” this is a bit premature. “Not so long ago — as recently as the mid-1990s, in fact — there was a theory so beautiful that astronomers thought it simply had to be true. They gave it a rather pedestrian name: the core-accretion theory. …. [that was then, this is now] “The findings have triggered controversy and confusion, as astronomers struggle to work out what the old theory was missing. They are trying ideas, but are still far from sure how the pieces fit together. The field in its current state “doesn't make much sense”, says Norm Murray of the Canadian Institute for Theoretical Astrophysics in Toronto. “It's impossible right now to account for everything,” agrees Kevin Schlaufman, an astrophysicist at the Massachusetts Institute of Technology (MIT) in Cambridge. Until researchers reach a new consensus, they will not be able to understand how our own Solar System fits into the grand scheme of things, let alone predict what else might exist.” Such models are appealing, but the concept of migration, especially of the smaller planets, gives some researchers pause — if only because no one has ever seen it happening. The necessary observations may not be possible: stars young enough to have planets migrating through protoplanetary disks are still surrounded by dust, and their light flickers, making it extremely unlikely that current methods will be able to pick out the dimming caused by a transiting planet. The theory is not settled, either. Modellers have found it hard to explain why migrating planets, big or small, would stop in the orbits that astronomers have observed. In simulations, says Winn, they don't: “the planets plop right down on the star”. Perhaps the biggest question is why our Solar System is so different. …. And from 2017, NASA's planned Transiting Exoplanet Survey Satellite (TESS) will look for planetary transits across all the bright stars in the sky. The wider range of possible exoplanet candidates makes it more likely that astronomers will spot a Solar System like ours — if one exists. Meanwhile, researchers continue to nurture their mess of models, which have grown almost as exotic and plentiful as the planets they seek to explain. And if the current theories are disjointed, ad hoc and no longer beautiful, that is often how science proceeds, notes Murray. “Life,” he says, “is like that.”
Fine, but don’t tell us you know how stars formed. Please be honest with everyone! I wish honest admissions like this were found in textbooks! Here is another one: http://creationsafaris.com/crev200603.htm#20060321a A summary of another article that does not seem to be available right now, but here are some quotes:
Solar system theorists are trying to reverse engineer the planets without the recipe.  Planets exist, but they can’t get from a rotating disk of dust and gas to a solar system from their models.  They are at a loss to explain Jupiter, Saturn, Uranus, Neptune and a host of Jupiter-class planets around other stars.     A press release from Astronomy & Astrophysics explains some of the problems.  Two British astronomers found a show-stopper in their models: any hopeful clumps tend to march in lockstep to their deaths in the center, like lumps of oatmeal washing down the drain before they can solidify.  This is called “Type I migration” – the viscosity of the stellar disk carries material inward like a spiral conveyor belt.  The migration is so rapid (a few thousand years), there is simply not time for a gas giant to form by core accretion.  (If the planet is able to open a gap in the disk, a more benign “Type II” migration still keeps it moving inward, but more slowly.)     Dr. Alan Boss (Carnegie Institute of Washington) shared some of his “heretical” views at a presentation March 21 to scientists and engineers at the Jet Propulsion Laboratory.  He listed many cons outweighing the pros of the core accretion theory.  Core accretion was the leading model dating back to Laplace’s original Nebular Hypothesis, until in the 1990s the problem of migration came to light.  The problem was exacerbated by the discovery of “hot Jupiters” around other stars – gas giants closer to their parent stars than Mercury is to the sun.  Earlier theory prohibited gas giants from forming so close.  Also, many red dwarf stars have been found to have Jupiter-size planets, contrary to predictions.  Gas giants seem to form regardless of the metallicity of the star (i.e., the proportion of elements heavier than hydrogen and helium).  Furthermore, our Saturn appears to have a much larger core than Jupiter, when the reverse should be true.     While core accretion is a bottom-up hypothesis, there is an alternative: a top-down approach.  Dr. Boss presented his newer “disk instability” model (the heretical one), not so much to pit it against core accretion (the conventional one), but to pit both models against the observations.  Both leave many problems unsolved.  …. Footnote: Dr. Boss mentioned several times that core accretion is only a problem with gas giants; he claimed it worked well with rocky terrestrial planets like Earth.  In the Q&A session, however, he did admit that there is a gap in our understanding of how the initial particles begin to accrete.  Bodies need to reach at least 10 meters before gravitation can pull in more material.  He referred to studies performed in space demonstrate that dust grains moving with slow relative velocities in a vacuum will clump into filaments and irregular clumps he called “dust bunnies,” but after they get to a certain size, they begin to impact one another too fast for further accretion to occur.  At that stage, more material is lost than accreted.  So he confessed there is a question mark between the dust-bunny stage and the 10-meter stage. cont.
Or how about this:
May 21, 2009 — Constructing planets is a delicate business.  Trying to get tiny bits of dust to join up into balls has never been found to work.  It has to work fast, though, because unless the whole planet clears its dust lane, it will be dragged into the star in short order.  It seems you can’t get there from the bottom up, and even if you could, you’d be in trouble.  These and other problems with planet-building were discussed this month in two papers in the Annual Review of Earth and Planetary Sciences. Making Planets Let’s start from the dust up.  We know many stars possess spinning disks of dust and gas.  Can we understand how small bodies might form?  There are three problems right off the bat, discussed by Erik Asphaug in “Growth and Evolution of Asteroids” in the Annual Review journal.1  Turbulence, accretion and death spirals render theories of their origins problematic.  More on that shortly.  It must be remembered throughout the discussion that planetologists assume the Earth, and the solar system, is 4.5 billion years old.  That means that any lifetimes numbered in 10s or 100s of millions of years represent tiny fractions of the overall age.     Asphaug [UC Santa Cruz] spent a lot of time discussing asteroid populations and characteristics.  He even tossed in a little Tolstoy and Yeats to liven up the dry math and technical jargon.  But when it came to the subject matter of building asteroids, he was less sanguine.  Asteroids, he said, form from the top down, by destructive processes, in relatively short times:
Asteroid origin is ceaseless, as most asteroids are born in the process of catastrophic disruption. …. 
Later, he added, “Asteroids represent the halfway point between the solar system’s turbulent beginnings and the quiescent 4 Ga [4 giga-annum or billion years] that have supported life on Earth.”  Don’t look to them as planetary building blocks, in other words.  Meteorites, by contrast, are primordial—they are remnants of the birth of our solar system, he said.  So how did those smaller chunks form?  Watch for any confidence in the following description of the environment of the planetary maternity ward.  It’s like trying to get born in a shooting gallery with the heaters and air conditioners going berserk:
The first stage of planetary accretion is among the most complex studies in astrophysics.  As they accumulate, planetesimals are entrained within a disk that undergoes violent shocks and propagates gravitational waves and eddies.  Magneto-rotational instabilities might lead to high turbulent viscosities, which lead to radial transport and vertical mixing and viscous spreading.  Solid particles settle to the mid-plane, increasing the density so that planetesimals might eventually coagulate.  Electric discharge and impact heating take place sporadically.  Outside this mid-plane the young sun blasts the gas and sweeps away small material.  Drag against the gas disk forces meter-sized boulders to spiral in from the planet-forming region.     The chemical environment, too, is grossly out of equilibrium.  The disk experiences a wide range of temperatures and pressures and oxidation states, with sharp gradients in time and space.  Where the disk is optically thick, it can remain hot for thousands of years; where it is thin, it can cool in days or even minutes.  Thermodynamic energy is available from solar heating, shock heating (by impacting globules striking the disk, by disk planetesimals colliding with one another, and by shocks in the gas), compression heating (adiabatic work), and radionuclide decay.  This energy is transported radiatively and convectively.
It’s nice that Asphaug acknowledged these environmental hazards, but he so far has not given any reason for hope that bundles of joy will emerge from the wreckage.  In fact, the next thing he talked about was how revolting the results of the Stardust mission were to theorists: ….. What might have happened in the solar system, when planetesimals were presumably progressing from primary to secondary accretion, to form the mysterious chondrules?  When in doubt, call Lucky Strike: “Although impact origin of chondrules is not currently in favor, we have a lot to learn, and large, late collisions deserve a closer look.”  In logic this is known as special pleading.     One would hope that by late-stage planet formation, when the planetesimals are big enough to pull in their own material, the rest of the story would proceed smoothly.  Sorry to pile on the difficulties:
In almost all simulations of late-stage planet formation, giant impacts are treated as “sticky ping pong balls” undergoing perfectly inelastic collisions when they hit, forming a larger equal-mass sphere that conserves linear momentum.  This was proven untenable by Agnor et al. (1999), who tracked angular momentum during such a calculation and showed that perfectly inelastic collisions lead to planets with impossibly fast rotation.
Well, that was ten years ago.  Surely they have solved it by now?  Better simulations by Agnor and others have indeed been performed.  But still, half the collisions end up as hit-and-run events that don’t grow planets.  Only low-velocity, head-on collisions have any hope of causing more growth than damage.  Most of the time, collisions break things down: “The prevalence of hit-and-run collisions makes it a late-stage pathway for the origin of exotic igneous asteroids, for volatile flux and iron-silicate intermingling, and for the bulk removal of planetary mantles and the stripping of iron cores.”  The lucky leftovers might have been the planets as we know them.  That, at least, is the hope.  Maybe a little sweet odor will ease the pain:
The overall trend in a collisional accretionary environment is the loss of atmosphere, ocean, crust, and mantle, the preferential accretion of dense materials into growing planets, the shedding off of mantles, and the occasional disruption of single planets into multiples.  This leads to a primary physical and chemical bias, a dichotomy among the accreted and the unaccreted.  If finished planets are the loaves of bread, asteroids are the scraps on the floor of the bakery.
…. We may not understand how planets formed, but we should be glad they did, he said in his conclusion.  “Whether Earths are common depends in no small part upon the behavior of accreting planetesimals,” he said, waxing philosophical.  In his view, we stand on lucky dust.  “Nearly all of the original mass of our main belt was swept up in the chaos of planet formation, so we may be fortunate not to have lost everything from our habitable zone.”  Add water (which had to be added later by another lucky strike) and the lucky dust became lucky mud, from which you and I sprang.  That’s the tale. …. Saving Planets Now, let’s turn to the other paper and see what happens after you have a planet.  A whole new class of problems for planet formation theories has come to light in the last decade.  John Chambers [Carnegie Institute of Science] discussed those in another article in Annual Review.3  In short, planets don’t stay where they were made, assuming they were made by processes Asphaug just described.  Get ready for migration – the conveyor belt that sends planets toward the oven or the freezer: … Here is the commentary at the end of the article: Whether planet-building is depressing or fun might depend on your temperament, but it should be obvious that any theory that depends for its acceptance on a long string of lucky accidents falls short of the scientific ideal.  Science is supposed to explain things with reference to natural law.  It’s supposed to make predictions.  It’s supposed to be falsifiable.  It’s supposed to have observational support, not reinvent a theory every time new information throws a monkey wrench into the old theory.  And when getting to the goal line requires a series of miracles or credibility gaps, it’s hard to judge whether this type of mythmaking improves on Aristotelianism (or Babylonianism, for that matter).     Much of this type of writing appears to be sophisticated storytelling masquerading as scientific explanation.  Some of the constraints are well characterized – viscosity, half-lives, the physics of inelastic collisions and incidence angles.  But these are like the boundaries and obstacles in a pinball game.  Would you be impressed if we proposed a weirdly improbable path the ball took, circling one obstacle, jumping over others, and getting struck by lightning at some point?  Would you like it if we assumed the boundaries were flexible and the obstacles movable?  Would you be impressed if we said we don’t understand how the ball got from A to B, so we will just pick it up and move it to B for now and leave that problem to someone else in the future?  Would you accept our explanation that the ball scored by taking a lucky random walk?  How about if we said the game made itself and plays itself?  Should such ideas be graced with the word science?     The only observational fact is that Earth hits the jackpot against an improbable odds that only increase with new discoveries.  That lends credibility to the belief that a Master Designer not only built the game, but operated the controls. cont.
And, one more: http://creationsafaris.com/crev201008.htm#20100831a
Exoplanet Hunters Fail Predictions     08/31/2010     August 31, 2010 — Before the first extrasolar planets were discovered, astronomers had high confidence that other solar systems would resemble ours.  We have rocky planets close to the sun, and gas giants farther out.  Planetary scientists were pretty sure the pattern would hold up around other stars.  Now that we have hundreds of examples to compare, the reality has been far different from expectations.  The number of surprises in real exoplanet systems underscores the potential flaws in building models based on a sample size of one.     In Caltech’s latest Engineering and Science magazine,1 John Johnson was interviewed about the state of extrasolar planet hunting.  Johnson has been involved with leading planet-hunting pioneers.  A recurring theme in the interview is the surprise that planetary systems were found to be radically different from predictions.
What are some of the current big questions that you guys are trying to tackle? We’re interested in how the solar system formed.  We’re interested in our immediate environment and describing its origins.  And beyond that, we’re interested in general in how planetary systems formed.  There are some very specific questions that arise at every turn.  There are so many surprises in this field—almost nothing is turning out as we expected.  There are Jupiter-mass planets in three-day orbits.  There are planets with masses that are between those of the terrestrial planets in our solar system and the gas giants in the outer part of our solar system.  There are Jupiter-mass planets with hugely inflated radii—at densities far lower than what we thought were possible for a gas-giant planet.  There are giant planets with gigantic solid cores that defy models of planet formation, which say there shouldn’t be enough solids available in a protoplanetary disk to form a planet that dense.  There are planets with tilted orbits.  There are planets that orbit the poles of their stars, in so-called circumpolar orbits.  There are planets that orbit retrograde—that is, they orbit in the opposite direction of their star’s rotation.  There are systems of planets that are in configurations that are hard to describe given our understanding of planet formation.  For instance, some planets are much too close to one another. But a lot of those surprises have to do with the fact that we have only one example of a planetary system—our solar system—to base everything on, right? What’s interesting is that we’ve found very little that resembles our example.
Johnson went on to say that the leading theory of planetary migration to explain how the so-called hot Jupiters get so close to their star has “gone into the dustbin” now that so many inclined and retrograde examples have been found.  “We’re scrambling to find a new way of describing how these gas giants can move in that also causes their orbits to be tilted,” he added.     Although Johnson reaffirmed the old Laplace nebular hypothesis with a “2.0” upgrade, the number of “wacky” things his team has discovered belies any attestation of confidence.  “We’re going out into the solar neighborhood, where there are things that we thought were just familiar, things that we thought we understood,” he said.  “But just the wackiest stuff comes up—and it’s sure keeping me busy.”  He compared it to going on safari and discovering a blue lion.  “That might be the level of wackiness I would attach to it.” 1.  Marcus Y. Woo, “Discovering New Worlds,” Engineering & Science, Volume LXXIII, Number 3, 2010, pp. 18-23. He didn’t really find a blue lion.  He found a natural lion, but the funny glasses he was wearing made it look blue.     Johnson went on to describe how Stephen Hawking’s book A Brief History of Time had made a profound influence on him.  He also affirmed at the end that he thought humans would figure out that their place in the universe is insignificant, following the theme of the positivists and Carl Sagan: “We are coming out of the darkness from a couple hundred years ago and we’re rubbing our eyes today, realizing that we are on a really small planet around a really average star in an unspectacular part of the galaxy, and we’re learning our place in this whole universe,” he said.  “Once we find more planets like our own, it’ll further define our place and give us a better universal context for what it means to be human.”     This kind of bluffing means Johnson has been a good apprentice.  His mentor Hawking was similarly prone to wild speculation without evidence, pontificating as he did in his book about how close humans were to finding a “theory of everything” when in fact he could point to little more hard evidence than mathematical speculations whirring about in his nimble imagination.  If anyone in economics or sportscasting had this bad a track record of predictions, though, they would be out of a job.  Cosmology is one of the many evolutionary sciences where you can brag about how wrong you have been, and people will still think you are wonderful because you are busy stamp collecting. Cosmology is a mess(Astronomy: Planets in Chaos) and it's only getting worse! Star formation is far from settled science. It is far from being understood. The core accretion hypothesis has some evidence, for it, but there are problems as well, SO, we know it can't be right at least in it's current form. If it could explain the data, there would be no need for another new top down hypothesis. So anyone who thinks that core accretion is settled science is obviously ignorant of the current state of things in cosmology! No doubt you can find plenty of articles that make such claims, but as shown from these articles and the words of the scientists themselves, it ain't so. tjguy
Learned Hand @23: The comments by REC did not rise to the level of constructive "criticism." Not only did he not have any answers to the questions raised, it became clear that he had not ever even thought about the questions in any depth. While I certainly appreciate the fact that scientists should be cautious about "science by press release," the quotes I provided were primarily direct statements from the researchers themselves. Are you suggesting that the researchers' own statements did not reflect what they actually observed? I have not read their full article, because it seems to be behind a paywall. But the abstract of their article is consistent with what the researchers described on the NASA website. Quoting part of the abstract in the journal Science:
We observed a substantial brightening of the debris disk at a wavelength of 3 to 5 micrometers, followed by a decay over a year, with quasi-periodic modulations of the disk flux. The behavior is consistent with the occurrence of a violent impact that produced vapor out of which a thick cloud of silicate spherules condensed that were then ground into dust by collisions.
In other words, what they witnessed was dispersion, not accretion. As to the formation of Saturn's rings, I gave the citation I did because it is a simple, straight-forward description of one of the most long-standing theories. Anyone can confirm this with a quick search. I could have cited any number of other sources for the same point. Don't get hung up on the website. Think about the substance of the theory. Now it could turn out that the moon/comet/bombardment breakup theory for Saturn's rings is wrong -- and indeed, as I mentioned, some people think it is wrong. That in itself is an interesting issue worth thinking through. And the fact that the formation of Saturn's rings is a strongly-debated open issue underscores that the science is not settled in this area. ----- Aside from the particular question of the accretion hypothesis in this thread, I have found it interesting that whenever legitimate questions are raised about this or that "consensus" theory, there are a few people who jump in to defend the consensus theory (even if they haven't thought about it in depth themselves). Some people dig in their heels and fight with the attitude that any questioning of the consensus threatens the very fabric of science. There was a hint of that, but thankfully I don't anyone quite got to that point in this thread. However, it does come up regularly when discussing cosmology, and much more often and aggressively with consensus views regarding climate and evolution. Eric Anderson
We have published, credible and observational historical evidence that the planets did not form by accretion but were created rapidly by God. This should take precedence over scientific theories based not on actual, historical, observational evidence but on computer models and "somehow" just-so stories. tragic mishap
Again, I tend to think that there is a perfectly reasonable explanation for how stars and planets form through purely natural processes. I’ll keep looking.
Where will you look? I think the original criticism was that your sources--press releases and a website for children--are only going to present a shallow and simplified version of the science. Have you read any textbooks on the subject? Learned Hand
In addition to the interesting question about planets forming by accretion, there seems to be a significant open question about how a nebulous cloud of dust and gas could turn into a disk of spinning material in the first place. Some ideas have been proposed, such as a shock wave from a nearby supernova compressing the gas, but that explanation seems quite strained once you start to think through the details. Again, I tend to think that there is a perfectly reasonable explanation for how stars and planets form through purely natural processes. I'll keep looking. Eric Anderson
I always found the views on cosmology, astronomy in Walt Brown's on-line available book very interesting and plausible from a scientific point of view. See this page about his notes on our planetary system formation and among others the hypothesis of planet formation through accretion. Not less interesting is his original hypothesis called The Hydroplate Theory that fascinates me. The theory is interesting and quite plausible. InVivoVeritas
Stephen, ill have to check that out, thanks. Collin
Collin: Perhaps Electric Universe theory would be a good one to start with since it claims to make a lot of predictions about how comets work and we have just managed to get a lander to a comet for the first time. Interestingly, EU theory about comets predicts them to have a harder composition than in the standard theory. Because the lander harpoons were designed with the standard theory in mind, the EU proponents are worried that the harpoons will prove to be more of a hazard than help. StephenA
Eric, (and moderators, Barry, etc) I would love to see a series on this blog of "minority theory" topics. Or "politically incorrect" or "marginalized" topics. Like this topic, or young earth creationism, cryptozoology, etc. You could even invite some guest bloggers. I don't mean to put this OP in the same category of cryptozoology, but some scientists might scoff at it just as they do many other controversial ideas. Collin
Zachriel @12: You have some good points and I'm not sure we're that far apart. I agree that models can be valuable, particularly if they are built from first principles without incorporating as a premise any particular conclusion. Your example of a child growing, however, gives an impression of much more knowledge than we actually have with the accretion model of planet formation. We know how a child grows and have witnessed it both personally and with other children many times. In contrast, we have never seen a planet form. We see various astronomical phenomena that we think might be related to the process, and we can kind of put together a semi-coherent story of how phenomenon 1 might be related to phenomenon 2 which might lead to phenomenon 3, then stir and add lots of time, and we think we might get to where we are today. But it is by no means a sure thing and there are interesting open questions. The moon formation you mentioned, for example, is still hotly debated in the scientific community, notwithstanding something of a nascent "consensus" around the collision hypothesis. Then we're told that Earth and the rings of Saturn were both formed by collisions of objects orbiting around a larger body. But, ironically, the results are precisely the opposite of each other. That alone should give pause. Then when scientists, employing the latest and greatest state of the art telescopes finally image an asteroid collision, they find a process that is exactly the opposite of what the accretion model requires. On the other hand, we do have excellent evidence for accretion of small bodies onto significantly more massive bodies. The accretion hypothesis enjoys enough currency that I'm guessing there must be a good explanation out there somewhere for the open questions I've raised about other parts of the process. I'm not arguing against the accretion hypothesis so much as seeking to understand it. I'd genuinely like to know the answers. At this point, however, I'm not sure we can say, based on a few snapshots of astronomical phenomena and some models, that "it's quite apparent what is happening." My assessment would be more cautious, along the following lines: "One of the most promising proposed hypotheses for the formation of planets is accretion of material in a circumstellar disk. We have observed various phenomena that appear to represent different stages of the accretion process and some models support the viability of this hypothesis. However, there are other observations that seem inconsistent with this hypothesis and important open questions remain." Eric Anderson
We see planets in many stages of formation.
Reference please. Joe
One would have to imagine "dark matter" and "dark energy" have a role too. They are keeping galaxies from flying apart after all. Plenty of imagination required for a planet formation hypothesis:) But Evolution is True. Bill Nye says so. ppolish
Me_Think @8, 9:
A typical International Space station weighs around 450,000 Kg and let’s keep its width (which will be our radius- for easy calculation) to 108.5 m , so the surface gravity of the space station will be G m /r^2 G (gravitational constant): 6.67*10^-11, so the gravity will be just [1.01986*10^-8 m/sec^2]. How do you expect your satellite attract and retain debris ?
Exactly the point. And for a cloud of much smaller particles of gas, ice and dust, what it the gravitational force of those objects? Gravity doesn't do anything much for us initially -- not at the tiny levels of mass we are talking about. As I understand the theory, the idea is that some particles would clump together through cohesion, eventually getting large enough to have meaningful gravity. But how much cohesion/clumping would be required to generate meaningful gravity? Even with Comet Ch-G (which is multi-kilometer sized), the escape velocity is only around 1 m/s, small enough that you could jump off it. And any collision or impact of even modest magnitude is going to blast it apart, with particles heading off in all directions faster than escape velocity, never to return. . . . exactly like the researchers observed in the recent asteroid collision I highlighted. A perfect example of dispersion, not accretion. Again, I'm not saying the accretion model is wrong. It has some things going for it. But even if it is on the right track, there are some interesting open questions. Eric Anderson
On a related note, Rosetta should be landing on Comet Churyumov-Gerasimenko within a few hours. Fingers crossed for a safe landing. Definitely a special moment for all those who have worked on the project for so many years. Rosetta will do a soft landing on the surface and then will harpoon itself to the surface to help make sure it doesn't go anywhere, due to the low surface gravity. Wouldn't take much of a disturbance (or much of a collision impact) to send objects flying off of Comet Ch-G with escape velocity, never to return. This would certainly be even more true of sub-kilometer-sized objects. Eric Anderson
Eric Anderson: In addition to the circumstantial evidence, there are many computer models that have been put together based on the accretion hypothesis. These can be very useful to the extent they accurately model the physical realities. Yet it is important to keep in mind that these models are not data, they are not evidence in the sense of actual observations. No, they are not data. They are hypotheses. However, like any good hypothesis, they make empirical predictions. For instance, the Moon-impact hypothesis predicts the mineral composition of the Moon, lending strong support to the hypothesis. We see planets in many stages of formation. That doesn't mean we have all the answers, but when you see a series of pictures of a child growing, even if you don't know exactly how the process works, it's quite apparent what is happening. Zachriel
Accretion is totally dependent on the Cosmological Constant which has been described as a form of "anti-gravity". An incredibly incredibly incredibly incredibly fine tune feature of our Universe. I left out a few incrediblys. If Cosmological Constant was ever so slightly slightly slightly larger, Gravitational Accretion could not happen. Respect Accretion. Respect the Constant. Awesome stuff. Except it's not "stuff". Dembski proves it's not "stuff":) ppolish
So wikipedia (one level above the NASA kiddies page): http://en.wikipedia.org/wiki/Nebular_hypothesis Look at the "Problems and criticism" section. Me_Think@8: that's one of the major unsolved problems as pointed out on the wikipedia entry. There is no accepted model for going from golf ball sized particles to 1 km sized planetesimals. snelldl
Correction: Actually, 'r' should be 108.5/2 = 54.25, so the surface gravity will be 1.01986*10^-8 m/sec^2 Me_Think
Imagine a decent-sized meteor striking an old satellite in orbit around the Earth. Do we expect accretion to take place? Of course not. Rather, we instead now have the risk of multiple smaller pieces of debris that have to be tracked — not clumping together like kids in front of a candy story window, but instead with each piece of debris now on its own independent trajectory. This is not a hypothetical. This is what in fact occurs and what we are spending hard-earned dollars to track.
For accretion, the 'mother' object ( let's say Space station) has to capture incoming debris in it's gravitational field. A typical International Space station weighs around 450,000 Kg and let's keep its width (which will be our radius- for easy calculation) to 108.5 m , so the surface gravity of the space station will be G m /r^2 G (gravitational constant): 6.67*10^-11, so the gravity will be just 2.549*10^-9 .How do you expect your satellite attract and retain debris ? Me_Think
Planets are stars that stopped shining. Yeah, take a few minutes to wrap your head around that one. After a while, it starts to make sense. Not only does it answer the ridiculous accretion model you just discussed, it also solves the (even bigger) angular momentum problem: http://lifeng.lamost.org/courses/astrotoday/CHAISSON/AT315/HTML/AT31505.HTM Suggests why all of the planets are ridiculously different : instead of forming out of the same trash heap - they are all the same thing just in different stages of metamorphosis. Instead of relying on 5 or 6 perfectly timed supernovas and beneficial (miraculous) collisions that modern geologists need to explain the exact composition of a hot iron core and the very existence of uranium and thorium and the perfect quantity of water -- you realize that the Earth itself has been producing and upwelling these elements all along..... ...why the gas giants are radiating more heat than they receive ...why moon rocks had magnetism and some ages over 10 billion years ...why every new "exo-planet" is huge and close orbiting ..why every planet has an iron core - without relying on the "iron catastrophe" model or the "rain-out" model. Okay - the point here is - is anyone currently researching this hypothesis? Of course not. It conflicts with the textbooks, therefore, it's not worth exploring, because we only dig for confirmation, not for conflict. Check out Wolynski's Stellar Metamorphosis theory here: http://vixra.org/pdf/1303.0157vC.pdf wayne moss
Still not getting the point are you? If you have some substantive answers, please feel free to provide them. Or . . . I guess you could just dig in your heels and complain about the examples some more. Eric Anderson
"Or go ahead and do some research" I think that was precisely my response. Except maybe "research" outside the kids' section at NASA..... REC
Ivar Nielson disagrees with the Accretion Hypothesis. Who is Ivar? Don't know, but he posted the comment under this this recent discovery of "Inside Out Milky Way": http://www.sci-news.com/astronomy/science-milky-way-galaxy-01704.html Ivar says "The consequences of this inside-out formation of course also must be connected to the general formation of our Solar System which is an integrated and orbiting part of the galactic formation and as such, our Solar System logically also once was created in the galactic center and NOT in "a local cloud of gas and dust which suddenly decided to collapse via gravity". ppolish
Based on your snide and completely unhelpful comment, I presume you have no answers to the reasonable questions that I asked, REC. As I clearly stated, I have not made a particular argument nor taken a stance with respect to the accretion model. I have simply provided a few quotes from particular sources to help lay out some of the questions. It could have been done without any examples and quotes, but I think the examples and quotes help people visualize the issues. If you don't like the samples, fine, ignore them. But address the substantive issues. I am simply asking for people's thoughts on these questions -- questions that should be asked by anyone who has thought about the matter in more than a superficial way. If you are acquainted with the issues in depth, then please let me know the answers -- right now, from memory, without looking things up online and without just parroting someone else's explanation. Or go ahead and do some research and then be intellectually honest enough to say, "Well, those are interesting questions. I hadn't really thought about them before, so thanks for bringing up the topic. Based on my research, it looks like the answers are thus and such . . ." Such is the path to a fruitful discussion. Eric Anderson
https://www.thunderbolts.info/wp/ Go nuts! ronvanwegen
Critiques based on press releases and NASA's website for kids. Do you want to be taken seriously?? REC

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