-1
$\begingroup$

I noticed that this was very common in planet formation theories. I would be interested in counterexamples.

Example of finely tuned models:

There are four stages in the supposed evolution of planets, according to the reference:

‘A successful nebular model must account in some detail for four important stages in the solar system’s evolution: the formation of the nebula out of which the planets and sun originate, the formation of the original planetary bodies, the subsequent evolution of the planets, and the dissipation of leftover gas and dust. Modern nebular models (there are more than one!) give tentative explanations for these stages, but many details are lacking. No one model today is entirely satisfactory.’

For the sake of argument, I will just assume that the dust is leftover from a supernova explosion. This is the first stage. Then according to Laplace’s nebular hypothesis, first presented in 1796, the process of planet formation, the second stage, begins with the simple collapse of the dust cloud. There are three theoretical steps in the collapse of the dust cloud and the growth of a planet: 1) gravitational contraction of the dust into small particles, 2) accretion of particles or small aggregates to form large aggregates, and 3) condensation by the accumulation of atoms and molecules on the growing mass.

The most difficult step is the first, gravitational contraction of dust to form small particles. Dust grains must first accrete to form small particles, which must continue to grow until they are at least 10 m in diameter. This size is the point at which gravity is expected to come into its own, accreting and condensing material at a faster and faster rate. Then supposedly, planetesimals would form that are many kilometres across. The planetesimals are finally envisaged to collide to form planets. There are difficult problems with these later steps, but I will focus on the first step: how does the dust collide, stick together and grow before gravity can assert itself? That is the big question. The tiny dust particles must hit each other head on and stick. The process (which is speculative anyhow) is too slow, especially in cold regions of space, according to astronomers. A number of hypotheses are in vogue, but all seem to have fatal flaws.

Steinn Sigurdsson has given up on all the proposed hypotheses because of the extreme unlikelihood that any of them ever occurred. Since planets have obviously formed and they must hold onto their evolutionary belief, he suggests a desperate alternative:

‘ … there could be an extra dimension of space in which gravity alone acts and which until now has gone unnoticed. If this is so, then gravity—which is weak over large distances—gets stronger at the tiny distances encompassed by the extra dimension … .’

In other words, he suggests that gravity would extend into five space dimensions instead of three and would be very strong at very short distances, causing dust and small particles to attract and stick together by gravitational attraction. This would certainly make planet formation much faster and easier. But there is at least one delicate problem with this imaginative hypothesis—the dust grains cannot hit too hard or the incipient particle would break apart:

‘So the turbulence within the disc [flat dust cloud] can’t be too strong, and the acceleration caused by Sigurdsson’s modified gravity can’t be too extreme.’

The idea is actually testable. So far, Newton’s law of gravity still holds down to 218 μm, but experiments are underway to test it at even closer distances. Sigurdsson hopes that his supergravity mechanism will show up when they test gravity at less than 80 μm. It seems to me that if he is correct, there is still the ‘sticky’ problem of how such a small particle can grow larger than 218 μm, above which his hypothetical mechanism would not apply.

Astrophysics makes the absurd ssumption that gravity dominates at small scales.

Reference: Zeilik, M., Astronomy—The Evolving Universe, 8th Ed., John Wiley and Sons, New York, pp. 260–261, 1997

$\endgroup$

closed as unclear what you're asking by peterh, CuriousOne, Wolpertinger, ACuriousMind, sammy gerbil Aug 15 '16 at 1:36

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • 1
    $\begingroup$ 1) I suggest to formulate your question more politely. The best answer can be given to you from a researcher of the field, and it is up to them if they answer or not. 2) Make some induvidual research, look for it, make it clear you are really trying to understand something, but you hit a wall here, here, and here. People answer this type of question much more happily as the single-sentence ones. $\endgroup$ – peterh Aug 14 '16 at 22:47
4
$\begingroup$

If you read this Wikipedia Planets article, you will see that planetary formation is still open to debate, so inevitably there will need to be assumptions made, although there is still such a lot to learn, I don't think the parameters need to be fine tuned :)

I read last month's Planetary Grand Tack, Scientific American, whose cover article was based around how Jupiter and Saturn may have influenced the present location and history of the inner planets.

I include an excerpt from the above article, to illustrate how data on the history of the solar system is still very much required.

I doubt very much if there are counter examples in planetary formation, given that the relatively simple three body problem is unsolvable analytically, to best of my knowledge, and that we only have the one example to study in detail.

There are hundreds of reasons to suspect that our solar system used to have more and bigger inner planets—the hundreds of multiplanetary systems discovered by planet-hunting projects such as NASA’s Kepler mission. Although our solar system is essentially empty inward of Mercury, equivalent regions around most other stars appear to be packed with close-in, intermediate-mass planets—those between the size of Earth and Neptune. Hopeful astronomers have dubbed these worlds “super-Earths” but most of them seem to be more like hydrogen-rich, gas-shrouded mini-Neptunes—very unearthly indeed. “Now that we can look at our own solar system in the context of all these other planetary systems,” Laughlin says, “the standard-issue planetary system in our galaxy seems to be a set of super-Earths with alarmingly short orbital periods. Our solar system is looking increasingly like an oddball.”

enter image description here  

If so, the obvious question is how it got that way. According to Batygin, there’s no reason to suspect that the actual process of planet formation occurred very differently around our sun than around other stars. Instead, the explanation for our solar system’s outlier status may be found in the details of its subsequent evolution—controlled to a remarkable degree by Jupiter.  

Astronomers used to consider planetary systems reasonably static and stable. Planets would coalesce out of the swirling disks of gas and dust around young stars, a bit like trees springing up from dirt, putting down roots and scarcely budging from where they were born. Small, rocky planets would form in the intense light and heat close to stars, whereas gas-giant planets would form farther out, where cold temperatures preserved more gassy feedstock. Small or large, gassy or rocky, most planets would move about their stars in pristine, near-circular orbits. All this cohered with our understanding of our own solar system. But we may have been wildly mistaken about what is the norm.   Twenty years ago when astronomers found the first planets orbiting other stars, they also began realizing that planetary systems are chaotic places. Some planets did not orbit in near-circles but in oblong “eccentric” paths that took them swinging close and then far from their stars—almost as if they had been thrown off-kilter by the gravitational influence of other worlds. And most of the newfound giant planets were very different than Jupiter—in scorching, star-hugging orbits far inward from the cold outer regions where they must have formed. Planets could migrate, too, propelled by gentle interactions with their formative disks or by close encounters with their planetary siblings.

$\endgroup$
  • 1
    $\begingroup$ Your answer could be summed up as "no". $\endgroup$ – D J Sims Aug 22 '16 at 19:02

Not the answer you're looking for? Browse other questions tagged or ask your own question.