In most of the models I have read up on, the farther out from the centre of the solar "dust cloud", the longer it will take to form a planet. Recently I came across the claim that Neptune and Uranus are just too far out to have formed according to these models, even over the 4.5-billion-year age given to the solar system.

The following quotes were given:

‘What is clear is that simple banging together of planetesimals to construct planets takes too long in this remote outer part of the solar system. The time needed exceeds the age of the solar system. We see Uranus and Neptune, but the modest requirement that these planets exist has not been met by this model.’

Taylor, S.R., Destiny or Chance: our solar system and its place in the cosmos, Cambridge University Press, Cambridge, p. 73, 1998.


‘There have been many attempts to model the evolution of a swarm of colliding planetesimals … Safronov calculated the characteristic time-scales for planetary growth. In the terrestrial region he found timescales of 10xE7 (10,000,000) years but the time estimates increased rapidly in the outer regions of the solar system and was 10xE10 (10,000,000,000)years for Neptune—which is twice the age of the solar system.

It is clear that, in view of the large timescales found for the formation of the outer planets, a satisfactory theoretical model for the accretion of planets from diffuse material is not available at present.’

Dormand, J.R. and Woolfson, M.M., The Origin of the solar system: the capture theory, Ellis Horwood Ltd, W. Sussex, p. 39, 1989.


I was wondering about the apparent non-existance of these two planets and what others thought?

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They most definitely exist. First of all, you see Uranus in a telescope, a rather low powered one at that. Secondly (and most importantly), these planets are discovered by measuring their gravitational effects on the planet nearby, in this case Saturn. Because of the pull of Uranus, Saturn's orbit "wobbles" slightly due to the perturbation, and thus a model for another planet is made. The same happened with Neptune, though its effects were on Uranus. I wouldn't worry about it, I'm sure they're out there.

Welcome to the BABB, PhantomWolf!

One of the nice things about astronomy, and the other sciences (as opposed to (ahem) other ways of looking at the universe), is that when theories don't correspond with reliable, repeatable observational data, the theories are changed, not the data (AKA the facts).

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I don't think that PhantomWolf is arguing the existence of the planets themselves. After all, they are most demonstrably there - we've even sent spacecraft past them! What is in question is whether their existence can be explained by the standard model of planetary formation in our solar system.

I haven't seen the studies quoted, but I strongly suspect that the early solar system was a most complex affair that is unlikely to lend itself to a simple model. Just off the top of my head: Could the presence of the proto-Jupiter/Saturn have caused a resonance leading to increased density of the nebula at the appropriate radii, allowing quicker planetary formation? Could the high solar wind of the newborn sun have caused a 'snowplough' effect in the nebula 'boonies'? Did the nebular material at that radius particularly tend to generating electrostatic charge, facilitating accretion? Could the sun have had a stellar neighbour during the formative period of the solar system?

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All good questions, AGN Fuel! At this point all we have is our unrepresentative sample of known planetary systems and the even less representative sample of the one that we live in. When our known (or imagined known) mechanisms don't seem to work, it's time to use the imagination and think of new but still scientifically plausible ones.
=D>

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The bar has been lowered for the promotion of ATM ideas; the bar for the acceptance of ATM ideas must remain high.

I was under the impression that a lot of models these days were looking at the possibility of the giant planets forming closer to the sun and then migrating outwards. People are looking at this possibility because of the ~120 exoplanets that have been found where most are very close to their parent star. Although there is obviously a huge sampling bias, and I'm not sure how those models would account for angular momentum conservation or what mechanism they propose for moving the planet outwards, it would get rid of the problem of not having time to form in the present location.

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stu,

Let me take the opportunity to welcome you to the Bad Astronomy Bulletin Board!

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The bar has been lowered for the promotion of ATM ideas; the bar for the acceptance of ATM ideas must remain high.

Seems to me the same tidal effect that is moving the Moon away from the Earth might be invoked to move close giants out from their sun. Two points against this would be that the fluid state of a star/sun might not give the same tidal effects, from what I understand, and that it would probably just take way too long, the planet's tidal force on its sun being rather minimal compared to that of the Moon on the Earth. All in all, I would doubt this being the explanation, but I thought it worth mentioning.

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Thanks, AGN Fuel. Yeah it was a more why are they there if the models predict otherwise. I guess it wasn't as clear as mud.

I wasn't attempting to question their presence, that's a bit hard as I've seen Neptune so I know it's there. (actually I predicted that it should have rings prior to them being found, though I won't skite cause it was pretty obvius after Uranus and Jupiters were found.) So it was more the Huh? It what I've been being taught an't right, what's going on?

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Apollo: The History and the Hoax
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AGN (or anyone else),

Are you aware of any models that have been run on supercomputers that simulate the formation of the solar system, based on the increased data we now have as compared to, say, 10 years ago? I checked the Cray and NCSA sites and didn't find anything indicating such a study.

It would seem that with the additional amount of knowledge we have of the Solar System as well as what we now know about extra-Solar systems, we might be able to do some meaningful and informative planet formation simulations.

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The last 30 years has seen several paradigm shifts in planet formation. Two important concepts deserve attention:

(1) A planet need not have formed at its current location. There is a growing consensus that both inner and outer migration occurs during accretion.

(2) The reservoir of potential building blocks need not be local. A strong case can be made that the Earth formed from planetesimals originating in at least five distinct regions of the solar system: The inner system (out to Mars), the inner belt, the outer belt, the Jupiter-Saturn region and the Uranus-Neptune region.

Just 30 years ago we would have described the outer planets as four ‘Gas Giants’. Today we describe the outer system as two gas giants (Jupiter-Saturn) and two ice giants (Uranus-Neptune).

http://blueox.uoregon.edu/~courses/B...3/FG13_010.jpg

The superficial appearance can be deceptive. For example, compare Jupiter to Uranus in the following pic.

http://zebu.uoregon.edu/~js/ast121/i...ian_worlds.gif

Although it is obvious that there is a difference between their masses, how great is that difference? It’s actually tremendous. Uranus is about 14.6 Earth masses whereas Jupiter is 318 Earth masses.

http://csep10.phys.utk.edu/astr161/l...lanet_mass.gif

IMO, we’ve come a long way. The current models could probably use a little tweaking and refining, but we’re getting there. (Death, taxes and error bars)

This is what I think took place. The cores of Uranus-Neptune formed in the Jupiter-Saturn region, then migrated outward, acquiring mass along the way while simultaneously scattering the objects which they did not absorb.

Note; the following link is 33 pages. Allow a minute or so to load:

http://arxiv.org/PS_cache/astro-ph/pdf/9902/9902370.pdf

Very good. Thank you for all the information and the links.

I like the mechanisms for "gas giant" transfers from inner to outer orbits, the effects on Jupiter due to energy and angular momentum losses, and the impact of microparticles on all the above.

This has some real potential for modifying our interpretation of the extrasolar star systems we're now finding, where, perhaps, the large planets have not yet started or completed their migrations to the outer parts of the system.

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What an interesting paper. Thanks for the link, Heathen!

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