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View Full Version : Planetary Migration - Is it still happening in our Solar System



jkmccrann
2007-Sep-20, 05:57 PM
Planetary Migration, whether inward or outward, is one of the more interesting phenomena I have come across over the last 5-7 years or so. More interesting because it does go some way to explaining how various various objects in our Solar System have ended up where they are - and also because it is an explicit support to the fact that we live in a dynamic, rather than a static, planetary system.

That is obvious, but before I heard of planetary migration I had the rather simplistic idea that there was a primordial mass of objects orbiting around the sun and they gradually coalesced into the planets we have today - pretty much at their present locations and had been orbiting there ever since - although there were always of course the smaller objects that were bounced around the whole system by the orbits and gravitational perturbations of the planets - and many of them ended up colliding with the planets.

Anyway, hearing about and learning about planetary migration has obviously shaken this simplistic view up - and particularly re-enforced in me the view that we do indeed live in a dynamic rather than a static planetary system.

Where this particularly interests me is in relation to the planetary migrations within our own system. It seems to be well accepted these days that the gas giants in particular have engaged in various migrations, in both directions, and with regards to Neptune, that its outward migration has caused a kind of shepherding of the Kuiper Belt with many plutinos, twotinos and other resonances between many of the objects in this region and Neptune.

The question that immediately strikes me, and one that I haven't really seen explored in depth is, who is to say that this planetary migration - particularly in relation to Neptune and its outward migration, has stopped?

If it has stopped, then when did it stop? What evidence is there to back up the fact that it has stopped? As I understand it, we have yet to observe Neptune on 1 full orbit of the Sun from the time of its discovery!

As a rough calculation, if Neptune were situated in the orbit that Uranus occupied today - and it was migrating outwards by 1% of its orbit (lets say for argument’s sake say starting from Uranus’ perihelion distance @ 18.37AU) – what would the progression be and how long would it take over 10 orbits? 100 orbits? 1000 orbits?

Uranus orbits in 84.3 years (18.37AU)
Neptune orbits in 165.2 years (30.50AU)

Uranus at perihelion = 18.37AU (84.3yrs)
+1% = 18.55AU (85.5yrs)
+1% = 18.73AU (86.7yrs)
+1% = 18.91AU (87.9yrs)
+1% = 19.09AU (89.1yrs)
+1% = 19.28AU (90.3yrs)
+1% = 19.47AU (91.6yrs)
+1% = 19.66AU (92.9yrs)
+1% = 19.85AU (94.2yrs)
+1% = 20.04AU (95.5yrs)
+1% = 20.24AU (96.8yrs)
+1% = 20.44AU (98.1yrs)
+1% = 20.64AU (99.4yrs)
+1% = 20.84AU (100.7yrs)
+1% = 21.04AU (102.1yrs)
+1% = 21.25AU (103.5yrs)
+1% = 21.46AU (104.9yrs)
+1% = 21.67AU (106.3yrs)
+1% = 21.88AU (107.7yrs)
+1% = 22.09AU (109.1yrs)
+1% = 22.31AU (110.5yrs)
+1% = 22.53AU (112.0yrs)
+1% = 22.75AU (113.5yrs)
+1% = 22.97AU (115.0yrs)
+1% = 23.19AU (116.5yrs)
+1% = 23.42AU (118.0yrs)
+1% = 23.65AU (119.5yrs)
+1% = 23.88AU (121.0yrs)
+1% = 24.11AU (122.6yrs)
+1% = 24.35AU (124.2yrs)
+1% = 24.59AU (125.8yrs)
+1% = 24.83AU (127.4yrs)
+1% = 25.07AU (129.0yrs)
+1% = 25.32AU (130.6yrs)
+1% = 25.57AU (132.3yrs)
+1% = 25.82AU (134.0yrs)
+1% = 26.07AU (135.7yrs)
+1% = 26.33AU (137.4yrs)
+1% = 26.59AU (139.1yrs)
+1% = 26.85AU (140.8yrs)
+1% = 27.11AU (142.6yrs)
+1% = 27.38AU (144.4yrs)
+1% = 27.65AU (146.2yrs)
+1% = 27.92AU (148.0yrs)
+1% = 28.19AU (149.8yrs)
+1% = 28.47AU (151.6yrs)
+1% = 28.75AU (153.5yrs)
+1% = 29.03AU (155.4yrs)
+1% = 29.32AU (157.3yrs)
+1% = 29.61AU (159.2yrs)
+1% = 29.90AU (161.1yrs)
+1% = 30.20AU (163.1yrs)
+1% = 30.50AU (165.1yrs)

Using that simple progression, if Neptune migrates by this 1% each orbit, it takes 6344.5yrs to migrate outwards from the orbit of Uranus to its current orbit.

If it migrates outward by 1% every 10 orbits, that is a time period of 63, 445 years.

If it migrates outward by 1% every 100 orbits, that is a time period of 634, 450 years.

If it migrates outward by 1% every 1000 orbits, that is a time period of 6.34m years.

The point is, when looking at the age of the Solar System, these time periods are still not significant – even up to 1000 orbits, or even for that matter 10,000 orbits!


As to the questions that I raised earlier, and also the study that has been done into planetary migration, as I understand it, many of these studies seem to have been done to explain how things came to be as we see them today – with regards to the location of all the objects within the Solar System – which suggests to me a kind of static reality in the studies as regards to how things came to be where they are located today.

I haven’t seen anything that really explores how things will develop into the future – or at least any study that suggest there will be any change in the future in terms of the locations of the planets. Of course, any explanation for the future direction of major objects within the Solar System is purely theoretical – because its impossible to always know every variable in such a (potentially) open system – but given all the models out there that attempt to explain planetary migration, where are the predictive flipsides to these models?

As I said before, from what I’ve seen, which is by no means comprehensive, these models tend to suggest the evolution of the Solar System has reached a kind of end point and past this point we can’t really expect any further planetary migration – something I find quite ridiculous, what timescale does this assertion apply to? Is it in relation to our lifetimes? The remainder of this century? The remainder of the millennium? For the next 1m years? For the next 5m or 10m or 100m years? Or until the sun starts to undergo major convulsions?

Obviously it can’t be forever that our Solar System will stay in the mode it is in today – but for exactly how long is it predicted to closely resemble what it is today? Surely if these studies are comprehensive and the science utilised in these studies is rigorous – they have some predictive capacity with regards to the future – except I haven’t seen it yet.

When they assert the planetary migration of an object such as Neptune has stopped – what does that really mean? To me, it often seems that kind of assertion is moulding a theory to reflect our current snapshot of how everything is – a static snapshot, but as I mentioned before, and as I strongly believe, we live in a dynamic Solar System – even if that raw dynamism plays out over time periods we can’t really comprehend fully.

So from all the above, I guess what I’m really interested to know is – are there any planetary migration studies out there that do make some prediction about any further planetary migration within our system – before such time as the sun explodes and renders this kind of speculation completely moot, and are there any studies out there that in particular assert that Neptune’s migration may not in fact have yet finished.

Given the timescales I outlined above, can we really assert with confidence that Neptune is not still migrating outwards? At perhaps the 1% per 100 orbits (630,000 years?) I’m a little sceptical that the great dynamism within our system is dead – but can anyone direct me towards anything that supports that or is anyone able to strongly assert that our system is in an extended static phase – by extended I mean at least 100m years, or even if not assert that – argue what evidence there is to really suggest that that sort of long time-scale dynamism is not evident?

I guess the final point of this question is, surely it is possible that scientific models that allow for Neptune’s continued migration could be created and backed up by observed and observable data and in effect moulded to fit the scientific data we have, as some current models of planetary migration are in some way moulded to the same data. Are there examples of this?

frankuitaalst
2007-Sep-20, 06:12 PM
The situation today is that , if we consider the solar system on its own , the planets are in some dynamical equilibium , which means they interchange energy ( call it momentum ) . As a result their eccentricities , inclinations and semi-major axis vary . One can say that if one object is slowing down another object must accelerate . Simulations learn that the outer planets are higly involved in this exchange . Also Saturn and Jupiter exchange energy . The inner planets are more stable . For the Earth it is known that Earths orbit has a changing eccentricity . Wikipedia can provide some interesting links.
A link which provides simulations about this system and data can also be found in the www.orbitsimulator.com message bord .

EDG
2007-Sep-20, 06:24 PM
It's http://www.orbitsimulator.com actually :)

Everything in our solar system is still slowly evolving as gravity influences moons and other planets - just not drastically. For rapid migration you need gas drag, which only happens while planets are forming since the disk is full of gas and dust. I'm sure the planets' orbits are changing very slightly because of their influence on eachother, but they're not evolving so fast that we can really spot a huge difference over a few centuries - we're talking geological timescales here.

frankuitaalst
2007-Sep-20, 06:57 PM
Heres a link to the above site : http://www.orbitsimulator.com/cgi-bin/yabb/YaBB.pl?num=1184354144.
As EDG states the movements are really really small over centuries , but they exist .

mugaliens
2007-Sep-21, 12:53 PM
Actually, all our planetary orbits are fairly locked in position due to the gravitational interaction between the planets. If one planet's orbit changes, the others will, too, to compensate and restabilize. It takes time, but generally speaking, all of us, including Pluto and a few of it's sisters, are locked into our orbits with one another.

rtomes (http://www.bautforum.com/members/rtomes.html)has a great thread on the harmonic relations of things, including planets, here (http://www.bautforum.com/against-mainstream/63081-harmonics-theory.html).

EDG
2007-Sep-21, 08:40 PM
Actually, all our planetary orbits are fairly locked in position due to the gravitational interaction between the planets. If one planet's orbit changes, the others will, too, to compensate and restabilize. It takes time, but generally speaking, all of us, including Pluto and a few of it's sisters, are locked into our orbits with one another.

We're not though. Sure, the semimajor axes of planetary orbits hardly change at all because tidal forces are generally so small and there's no drag anymore... but it'd be false to say that we're locked. An orbit only "locked" if it's in a strong resonance (e.g. 2:1, 3:2), and not all planets are in resonances with eachother.