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Mike Brown's Planets, season 4: sabbatical
ngunn
post Nov 10 2013, 10:35 PM
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I flagged up an article by Mike Brown hoping it would kick off some interesting discussion. I have to say I'm dissapointed with the result so far. Is there really nobody here interested in engaging with his question? I had hoped to learn something, and still do.
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Explorer1
post Nov 10 2013, 11:41 PM
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There's loads of good theories on his website as responses to the post, I suspect that's the reason we're being less chatty than usual.

I was hoping to learn something too, something I've wondered about for a while. Since the bigger KBOs have rocky cores as Brown says, could some of the largest be metallic as well, or were all the metals accreted in the inner solar system back in the day? New Horizons doesn't carry a magnetometer, so we won't have direct evidence for a while.
Plus if the answer is a conclusive no, none of them have metal cores, we've got a ready made definition of planet that will satisfy everyone, ending the debate once and for all!
(Obviously I'm idealistic about that last bit wink.gif)
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Astro0
post Nov 11 2013, 06:15 AM
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ADMIN NOTE: A general reminder to all about UMSF Rule 1.9 Other subjects that are banned here include: Pluto's planethood;.... Let's be careful please. wink.gif
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alan
post Nov 12 2013, 07:58 PM
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I was quite pleased when I identified a solution to the problem, I wouldn't want to spoil it for you by giving away the answer.
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stevesliva
post Nov 13 2013, 12:23 AM
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QUOTE (alan @ Nov 12 2013, 02:58 PM) *
I was quite pleased when I identified a solution to the problem, I wouldn't want to spoil it for you by giving away the answer.


Thanks, Fermat. Too large to fit in this space?
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alan
post Nov 13 2013, 05:03 PM
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Adore witchy sleepyhead whiningly pirating in rubbishes.

Fermat
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Gerald
post Nov 13 2013, 09:45 PM
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To me the most plausible solution to form km-size objects from dust/sand/stones/snowflakes is via gravitational pull of a sufficiently large and sufficiently dense cloud of such bodies with very low relative velocities. Part of the cloud will collapse, part of it will be ejected.
At low temperatures near absolute zero ice/snow won't adhere much different from silicates.

In the outermost part of the solar system (up to 1 lightyear resp. 63,000 au from the sun) relative velocities of objects, which remain in the solar system, should be below a range of between 100 m/s and 1 km/s, depending on the distance to the sun (otherwise particles escape). Collisions / friction should reduce relative velocities further until the precondition for gravitational collapsing clouds is fulfilled.

Collisions of sufficiently large and fast bodies may lead to densified fragments, besides fine debris.

The accretion process will take quite a bit longer than Wiles needed to show the Taniyama–Shimura–Weil conjecture. wink.gif

The Sedna orbit may be explainable e.g. by an instable ternary system of Kuiper objects, which eventually split into a binary system ejecting Sedna near Sedna's perihelion. I don't see the necessity for a big planet or star to explain the orbit.
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JRehling
post Dec 12 2013, 05:55 PM
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A hypothesis I floated a long time ago on Sedna's orbit is that it may have been shaped by a planet that was once there, but isn't anymore.

The rules of celestial billiards require that if two bodies have an orbit-altering encounter, they will leave the encounter with orbits that will cross again. But there's no rule stipulating that they will remain in those orbits.

Suppose, for example, an earth-sized planet (or many) once had elliptical orbits that came close to Neptune (or another giant planet) at perihelion and ventured out towards Sedna's neighborhood at aphelion. Such planets could have tweaked Sedna's orbit, then later ceased to exist, either by being absorbed into a giant planet or being ejected from the solar system.

I think there's a rich field of study ahead on how mass swarms of bodies evolve. This can involve massive computation, and has been investigated for many years, but I'm not sure how exhaustive the investigations have been.
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stevesliva
post Dec 12 2013, 06:30 PM
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QUOTE (JRehling @ Dec 12 2013, 12:55 PM) *
I think there's a rich field of study ahead on how mass swarms of bodies evolve. This can involve massive computation, and has been investigated for many years, but I'm not sure how exhaustive the investigations have been.


Definitely, and most certainly not exhausted. For an idea that seems to be getting such acceptance the Nice model isn't even a decade old yet, and it came about when the ability to simulate coincided with theory. The idea of Jupiter Saturn resonance causing relative chaos, maybe that was older, but the ability to show that it could have happened and might have led to what we see today, that's still new.

And if you want to make massive computation even more massive, you could add in stellar neighbors.
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0101Morpheus
post Dec 12 2013, 10:18 PM
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Or you could start off with an extra Gas Giant or two for more fun!
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Gsnorgathon
post Dec 13 2013, 03:13 AM
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Or just an extra ice giant, as in the jumping-Jupiter scenario (which I'd never heard called that before - it's got a nice ring to it).
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Gerald
post Dec 13 2013, 02:37 PM
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Rather simplified simulations I've done a long time ago (unpublished) showed, that randomly started planetary systems behave highly chaotic in the beginning, including planets being ejected from the system or planets falling into the sun. That's because each close encounter leads to an exchange of kinetic energy throwing one planet further out, and the other closer to the sun, including highly elliptical orbits.
Therefore tracking back the solar system becomes more and more uncertain the further you go back to the past. This means two things: Similar initial conditions can lead to very different results, and rather different initial conditions may lead to similar results. The underlying general principle is deterministic chaos, rather abundant among non-linear dynamical systems.
So, if you like extra ice giants in the early solar system, you'll find a way to define initial conditions which make them vanish later.

If you like to try your own implementation, here is an intro.
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gpurcell
post Feb 26 2015, 07:28 PM
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Poking around this old thread led me to Dr. Brown's blog and from there to the information that he is teaching a free Coursera online course March 30th through June 9th called "The Science of the Solar System."
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centsworth_II
post Feb 26 2015, 08:09 PM
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Looks like the course gets rave reviews.
http://www.coursetalk.com/coursera/the-sci...he-solar-system
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gpurcell
post Apr 4 2015, 01:51 PM
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First week of the course and it is excellent; just got done watching a couple of lectures where Dr. Brown goes through how reflected light spectra work and how the formulas used to measure spectra can determine atmospheric composition, measure the proportion of water, and determine surface temperatures. What's really cool about it is how he works through the increasing complexity of functional form of the modelling process: e.g., how the fairly simple formula to determine surface temperature has to be modified for the angle of incidence of the Sun by a cosine term.
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