I’ve seen popular science television programs that display these theories as fact. I’m curious to know how well established or accepted these theories are.
Further, after recently seeing a beautiful graphic of Neptune plowing through the Kuiper belt, spilling comets throughout the system in the Great Bombardment, I was left with a few questions.
How could a system as whacked out as displayed ever regularize into what we see today, unless there was some tendency to regularization. Yet I am hearing both here in posts and in Scientific American that gas giants really “want” to wander about their systems.
What does this mean for exosystems? Are they more likely to be regular or “goofy”.
How could we have ever figured out celestial mechanics if our planetary orbits were goofy? Or might it be easier to get away from the perfect circles and epicycles?
TV shows that show anything with space/asteroids/planets are gross caricatures. Asteroid belts are empty space and the Kuiper belt is empty space with each body separated from others by 9 AUs. Ultimately the original angular momentum of the solar nebulas was conserved by flattening out. So bodies tended to grow in the plane of rotation. Larger bodies accumulate more mass, becoming harder to disturb as time went on
We have ideas but no firm consensus on planet formation - it’s a lot more complicated than we originally thought with migrating planets and then there’s the meter problem.
Gravity is dependant on the square of the distance and so movements around a central mass will be conic sections (circle, ellipse, parabola and hyperbola). Plug that in with angular momentum on a plane and you get orbiting material. More extreme eccentricities in the orbits of planets might have made it harder to imagine a regular planetary system. Circles seem more intuitive than ellipses
here is an interesting, replicable visualisation for gravity. it suggests how numerous objects on different vectors might settle into an orbit and how a moon might orbit a planet while they orbit the sun.
is this a fair representation of what happened in our solar system? did the majority literally force the rest to orbit in the same direction?
TV shows are quite possibly the worst medium to learn about science. Partly it’s because they don’t have time to delve into details, but mostly it’s because TV producers have no incentive to get things right. They’re just interested in their ratings and will show all kinds of garbage as long as it’s spectacular garbage. Shows on PBS are somewhat of an exception, but even they care about ratings.
Of the three models I posted, the Big Splat has been around the longest, something like 25 years or so. I’m not a scientist, but I get the impression it’s reasonably well established. But like all theories, it has people who disagree partially or completely. The Nice model came out about 8 or 10 years ago. Not quite as well established. The Grand Tack is very new. It was published in online preprint about 6 months ago. I have no clue how it’s been accepted.
If there’s a lot of either gas and dust or planetisimals in the system, any planets are going to be subject to migration. Gas and dust was the cause in the Grand Tack and planetisimals the cause for the Nice model. After the trash has been mostly cleared out, things usually stabilize.
The Solar System has very little gas and dust. Asteroids and KBOs are the remains of the planetisimals, but there aren’t enough of them to do anything significant.
It’s possibly there is no such thing as a “regular” system. But we can’t tell right now, because both methods we have for discovering exoplanets favor finding large planets in close orbits around a star. None of the systems we’ve found really look like the Solar System, although a few sort of resemble it, so right now it looks like the Solar System is the oddball. It’s too bad Kepler crapped out when it did, since it needed more time to find a system like ours.
Any set of stable orbits can be approximated with circles and epicycles. The more eccentric an orbit, the more epicycles are needed to approximate it. One of the problems seen with the geocentric model was the numerous epicycles it needed. So a “goofy” system with lots of highly eccentric planets should make it easier to go to a heliocentric system.
One answer is that the planetary orbits are not known to be stable. Of course, it is a reasonable hypothesis after 5 billion years. The work on the 3 body problem (which has never been solved in closed terms) was partly motivated by that question.
I actually was looking this up last week for one of my students. Wikipedia:
About this post, the others which mention stochastic or chaotic methods and simulations, and even those that don’t but speak from understanding:
What gives? I thought, you know, Newton, billiards, it’s been figured out, or can be, to a T. I go back in time, bring a book, predict the eclipses, and get worshipped like a God. Planets, no?
Now I can’t? I finally have through my brains quantum and non quantum limits, but we’re not there here. ( )
Newtonian Mechanics tells you exactly the forces on the different planets under a given configuration, but there’s no general equation that lets you start from that set up and predict exactly the set up at any future time. You can get such an equation for two planets, but any more then that and the problem isn’t exactly solvable. Instead you have to make due with computer simulations or approximate solutions.
Which are pretty good, good enough to print tables of eclipses out for thousands of years. But eventually the errors build up, and your tables will fall out of sync with the real solar system.
The problem is that once you get into multiple objects (N body problems) the complexity and susceptibility to initial conditions skyrockets. So instead of rock steady equations you use numerical approximations and run simulations until you, or the grad student, drop. Turns out that over large numbers of simulated runs Mercury and Jupiter could wind up interacting to the extent Mercury gets punted. But it all depends on the initial setup and all the interactions between all the planets.
Evidently you think that’s the definition of “co-orbit”, but it’s not. According to wiki:
The only difference between Jupiter and the other planets is that the barycenter is outside the sun. Perhaps there’s a name for this, but it’s not particularly significant, just interesting.
OK, a mistake on my part. It turns out that the Grand Tack was actually published in Nature about 3 years ago.
As for how it’s been accepted, it has competition with another hypothesis which doesn’t seem to have a catchy name, but probably should be called the Embryo Hypothesis. Both are discussed in this fascinating blog entry. Well I find it fascinating anyway; YMMV. It covers a lot of other stuff before getting to the Grand Tack and its compeditor, which you may or may not find interesting.
The term embryo as used here means an object that’s bigger than a planetisimal but smaller than a planet. Say about the size of the Moon. This was a new term to me. In fact, this whole Embryo Hypothesis was new to me.
At any rate, the Grand Tack seems to predict the mass of Mars better than the Embryo Hypothesis. The latter tends to produce a Mars about the same size as the Earth in simulations. The Grand Tack is not perfect, either, so there’s more work to be done.
Well, yes, but how stable are the orbits of moons?
I mean, we know the Earth-moon system is ever so slowly drifting apart, but it has been around for billions of years, do it must be *relatively *stable.
How about Jupiter and Saturn? I’m sure they must pick up or lose the odd asteroidal moon every now and then, but are the overall sytems fairly stable?
Yes, but just as in stock investing, past performance is no guarantee of future results.
And the Earth and Sun are slowly moving apart due mainly to the same cause (tidal drag). In fact, all the planets are slowly moving away from the Sun, but at different rates. And the rates won’t be constant over the life of the system, although they’re never going to get all that big. However, the planets are far enough apart that these minor changes are unlikely to destabilize the system. But we can’t be sure.
When the stability of the solar system is discussed, no one cares about the small junk.
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