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A long time ago, when I had to help my children with their homeworks, I often had the vexing question « how do we know ? ». It is true that reading a middle-school science textbook is like reading another Bible. Hardly do we have an explanation: « we think we know, and sometimes we do know, because ... ».

Inspired by the question Which planet came first, in which the OP stated « sources suggest that Jupiter might be the first », and by @ProfRob’s answer in which it can be read « The planets all began their formation at more-or-less the same time in the first few hundred thousand years after the protosolar cloud contracted »

  • My question is :

Which are the clues that allow the disqualification of any existing alternative scenario (for the timeline(s) of planets' formation)?

Note: I am not asking which theory is the most accepted. I am just asking about the scientific approach(es) used to support one particular theory.

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    $\begingroup$ Good question!! $\endgroup$
    – ProfRob
    Jan 25 at 14:17
  • $\begingroup$ @ProfRob, that a professor would no doubt answer in a didatic and eye-opening way! $\endgroup$
    – Ng Ph
    Jan 25 at 16:16
  • $\begingroup$ @NgPh the question actually deals with our 8 planet system or planet formation in general ? $\endgroup$ Jan 25 at 16:30
  • $\begingroup$ @Kavin Ishwaran, my focus is the Solar system (because I assume that astronomers have more supporting evidence, although I accept that this assumption can be proven wrong). If there is a general approach, general observed similarities, universal "law", why not extending the question. $\endgroup$
    – Ng Ph
    Jan 25 at 16:53
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    $\begingroup$ @Justin Tackett, may be we can make use of the resolution of the current analysis tool as definition of "same time", or "more or less same time". If we can't separate with certainty two dates, we consider them as same. If we can't separate with a certain level of confidence, we say more or less the same. $\endgroup$
    – Ng Ph
    Jan 25 at 19:22

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It depends exactly what one means by "begin formation at the same time". In my answer to the original question what I really meant is within a timescale that is short compared with the overall duration of planet formation which is of order 10-100 million years (and which does vary significantly from from planet to planet as I pointed out in my answer to the previous question). It also depends on what you think constitutes the beginning of planet formation - the formation of the first solid bodies or the formation of objects above some threshold size. However, I think your definition (in comments) as unresolved using the available temporal measurement tools is also a good way of thinking about it.

The absolute ages of ancient meteorites are readily measured using things like Uranium-Lead and Lead-Lead radiometric dating. These studies identify Calcium-Aluminium-rich inclusions (CAIs) as the oldest solids in the Solar System and that they formed with a relatively small age spread - e.g. $4567.30 \pm 0.16$ milion years ago (Connelly et al. 2012, Science, 338, 651, which is similar to the precision of the measurements. Admittedly, this is based on only a small sample of such objects. Thus if the definition of "beginning formation" is the formation of the first solids - then this appears to have happened co-temporally within a few hundred thousand years using measurement techniques that are only precise to a hundred thousand years or so.

Other ancient meteorites - carbonaceous chondrites and iron-rich meteorites that may have come from already differentiated (and hence larger) bodies then suggest ages that are a little older by no more than a few million years (e.g. Connelly et al. 2017; Pape et al. 2019). i.e. The growth of bodies big enough to be influenced by gravity appears to then happen within a few million years of the formation of CAIs.

The timescales for the growth and differentiation of planetestimals from the initial building blocks represented by the CAIs are more accurately probed using radiometric techniques with short-lived radionuclides (e.g. Hf-W chronology). Recent work has suggested that the parent bodies of non-carbonaceous and carbonaceous chondrites - thought to have formed at different radial locations in the protoplanetary disk by virtue of differing Fe/Ni and isotopic ratios - "accreted about contemporaneously within ~1 Ma after CAI formation, but at different radial locations in the disk" (Spitzer et al. (2021). Thus the formation of large solid bodies appears to have taken place almost at the same time at different locations in the disk.

I suppose the final part of the answer is that it is assumed (supported by models) that both terrestrial planets and gas/ice giants begin from the clumping together of solids in the protoplanetary disk. Whilst the former is certain, there are alternatives to the so-called "core-accretion model" for gas-giants that could result in them forming directly from gas in almost their entirety in about the time it takes for the first solids form (but that is still fairly co-temporal in my view). In the case of Saturn, the presence of a solid core is reasonably well-established; Jupiter is complicated and the cores of the "ice giants", Uranus and Neptune, are also still uncertain, but their bulk compositions are not so H- and He-rich, probably precluding the rapid gas-instability model in their case.

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    $\begingroup$ Great demonstration of the extent of your science. Agreed that there are loose ends in the question, but they are not really critical for the “how do we know”. I am still struggling with one mystery of Cosmogony (perhaps because I just skimmed over the references). When I look at a man, I can guess his age. But when I look at a couple, how do I guess when they got married? $\endgroup$
    – Ng Ph
    Jan 26 at 11:58
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    $\begingroup$ @NgPh Because they melt and differentiate? However this boils down to what does one mean by "begin formation". I took it to mean the formation of the solids from which the planets are made. The end of the formation (final configuration) is what you are now querying and this was discussed in my answer to the original question; is likely to be quite different for each planet; and is more uncertain than the range of times quoted in this answer. $\endgroup$
    – ProfRob
    Jan 26 at 13:01
  • $\begingroup$ I think I understand better. By the determination of the age of CAIs (using basically chemistry), you are able to estimate precisely when the conditions were ripe for the formations to begin (when the men can start dating the women). It doesn't mean at all the time when we can start identifying 10,9 or 8 protoplanets of sufficient sizes to conclude confidently that this will be a star with approximately 8 planets. According to @Ludo's answer below, that time has been estimated to be no more than 10 My (9 planets if we include Theia). $\endgroup$
    – Ng Ph
    Jan 27 at 6:03
  • $\begingroup$ @NgPh Radionuclide dating is not chemistry. The other answer is based on the argument that when we look in other star forming regions, protoplanetary discs last about 10 Myr (that timescale is uncertain by a factor of 2 at least) and planets must start forming in a disk. You are again mixing up "when planet formation begins" with "when planet formation is complete". There is no ambiguity in my answers $\endgroup$
    – ProfRob
    Jan 27 at 8:36
  • $\begingroup$ Thanks, and agreed that my use of terminology is often not accurate. But is it pushing too much if I ask whether somebody can have an alternative definition of "begin formation" (e.g. when planetesimals form) and then, which techniques would be used to date such events? It seems that in your "story line" there isn't any place for extrinsic causes: if we have sufficiently the "first solids", history is written in marble by gravity modelling alone. Also, is it possible to have an alternative story that formation of Jupiter, at a particular time, influences the rest of the process? $\endgroup$
    – Ng Ph
    Jan 29 at 11:22
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Planets form in a protoplanetary disk; the gas and dust surrounding a newly formed star. These disks are dynamic environments, in which many different physical processes are at work. While it is important to consider each of these when building complex physical models, I will try to answer this question by focusing on just one: radial drift.

Radial drift is the phenomenon whereby dust particles orbiting the star feel a 'headwind' due to the gas in the disk. This causes them to lose angular momentum and drift in towards the star. The larger the particle, the greater the effect, so as particles grow larger through collisions they drift in faster. This is just one of the physical processes that puts a time limit on the lifetime of the disk. The disk will eventually dissipate and leave no material left for planetary formation.

We can get an idea for the lifetime of a disk by first measuring its mass. This is harder than it sounds, because most of the disk is composed of molecular hydrogen, which is essentially undetectable. Many studies instead focusing on more detectable species such as isotopes of carbon monoxide. The total disk mass can then be inferred by relating the amount of carbon monoxide the the amount of hydrogen through a known ratio.

Once the mass of the disk is calculated, we then need to estimate the timeframe for disk dissipation, given that mass. We find that, on average, a protoplanetary disk tends to last around 5-10 million years. So when we say that planets form at the 'same time', we mean within this same 5-10 million year period. This is of course a very short time period when compared to the billion year(s) lifetimes of stars and planets.

This is not to say that all planets form 'together' during the disk stage. The timescale of formation for any given planet is highly dependant on its location in the disk and which materials it is formed from.

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  • $\begingroup$ Thanks for this additional element. So, this radial migration process is inferred from gravity modelling, right? (Not by observing physical or chemical properties of meteorits). When you say the time of formation of a given planet (say Mercury) is dependent on its location (because of the physics of the radial drift?) "and which materials it is formed from", does this mean that the primeviral constituing materials are very different for Mercury compared to those of Earth? How do we know that? $\endgroup$
    – Ng Ph
    Jan 27 at 6:12
  • $\begingroup$ Disk lifetimes in other young stars are inferred from model-dependent ages for star clusters. A diagnostic of disk presence is then looked at in clusters of different ages and an average disk lifetime inferred. The age scale of these inferences is uncertain by a factor of two. Whilst the duration of the formation of planetary material might be defined by the disk lifetime, this does not give the time span over which the assembly of solid bodies started, which is what the question is about. Neither is it the timescale on which planetary assembly is completed. @NgPh $\endgroup$
    – ProfRob
    Jan 27 at 8:43

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