Some thoughts on the evolution of planetary systems

[WeepingElf]

I wish to discuss some thoughts on the evolution of planetary systems here.

As you know, there are planetary systems that are unlike ours: "hot Jupiters", "super-Earths" and the like. Indeed, there are more of those known than systems like our solar system, because they are easier to detect (the bigger a planet is and the shorter its orbital period, the easier it is to detect; of our planets, the terrestrial ones would be hard to find because they are so small, and the giants because their orbital periods are so long). Why did those "freaks" evolve?

It is now understood that giant planets evolve rather swiftly, and influence the formation of the terrestrial planets. According to the Grand Tack model, Jupiter migrated inward, bringing the accretion of the terrestrial planets (especially Mars) to a halt, before Saturn pulled it back outward, thus preventing Jupiter from becoming a "hot Jupiter" and saving the day for the terrestrial planets.

My idea is that: 1. "Hot Jupiters" form when there is no "Saturn" to pull the "Jupiter" back, and the "Jupiter" thus moves inward into a very close orbit, thereby obliterating any terrestrial planets. 2. "Super-Earths" form when there is no giant migrating to put a check on their growth, perhaps because the giants have thrown each other out of the system.

What do you think about this? Does it make sense to you?

[Lērisama]

I don't think so. At least, that's not the whole story.

---

My understanding is that there are hundreds of exoplanet systems that we can't explain, or we are only at the ‘it's plausible, but we don't really know’ stage of theorisation about. Specifically there are several types of system that don't particularly fit this theory, including ‘hot super Earths’ (both behind and in front of hot Jupiters, or without them entirely). However, there are several caveats:

• This certainly explains some (Solar System-like) systems, and is partly just logic – you're not going to be finding any expert claiming that disrupting the supply of planetary material doesn't make the planets smaller
• Planetary migration (before the planet gets too big and moves to a slower, steadier type) is really sensitive to minor changes in the protoplanetary disk. This (and the make up of the disk) are probably what makes different systems different, and can lead to lots of very strange systems we don't really understand
• My book on the subject is at home, and several years old (but post Grand Tack) so I can't promise this is entirely accurate or up to date

[xxx]

the less we know, the more we can imagine...
and given the size of the universe, of which we only understand a single-digit percentage, we can go far enough to invent everything...
without the need for parallel universes with different rules...

[WeepingElf]

A fiction writer has an advantage over a scientist: he need not find out what is actually the case, it suffices if what he comes up with is plausible.

[keenir]

the less we know, the more we can imagine...
and given the size of the universe, of which we only understand a single-digit percentage, we can go far enough to invent everything...
without the need for parallel universes with different rules...

if you don't want to answer the question, DO NOT ANSWER IT.

[xxx]

if you don't want to answer the question, DO NOT ANSWER IT.

what the point of your inter...

[fusijui]

This is not an answer, but maybe a pointer to some answers -- or perhaps just a pointer to a Pandora's Box of time-sink geekery: https://universesandbox.com/

It simulates solar systems (ours by default, your own with some customization) in pretty thorough detail, including planetary climates. The older versions I goofed around with were pretty easy to get to learn. The drawback is that it's USD 30.00.

[Glass Half Baked]

Seems like an elaborate scenario that creates as many problems as it solves, all because of something (Mars' relatively small mass) that can easily be explained by chance.

[Lērisama]

It's not just that – planetary migration is expected both through calculations and observations of other planetary systems¹, so an explanation is needed for why Jupiter didn't migrate inwards. The Grand Tack theory solves this problem, and explaining the small size of Mars is just a happy by product. You're right that Mars's small size alone would not require an elaborate explanation, but that, plus the relatively small mass of the asteroids, plus Jupiter not migrating inwards suggest that something did happen, and the Grand Tack theory is our best guess

¹ Many planetary systems are such that a protoplanetary disk wouldn't be stable if the planets developed in situ, so migration must occur, and it is expected to happen because of something about fluids (the gas disk) feeling pressure and the planets not. I don't fully understand it

[WeepingElf]

It is not as if I was to write a non-fiction book (let alone a scholarly work) about the formation of planetary systems - I just desire a set of rules for building realistic planetary systems for science-fictional purposes, so it suffices if it is plausible.

[Lērisama]

It is not as if I was to write a non-fiction book (let alone a scholarly work) about the formation of planetary systems - I just desire a set of rules for building realistic planetary systems for science-fictional purposes, so it suffices if it is plausible.

I probably should have been clearer in my original post then. Yes, your theory is plausible, just keep in mind it is not the whole story.

[WeepingElf]

It is not as if I was to write a non-fiction book (let alone a scholarly work) about the formation of planetary systems - I just desire a set of rules for building realistic planetary systems for science-fictional purposes, so it suffices if it is plausible.

I probably should have been clearer in my original post then. Yes, your theory is plausible, just keep in mind it is not the whole story.

Fine.

[Glass Half Baked]

I am unaware of anyone who claims that type I or II migration should be expected as the default unless some convincing alternative is presented. They are simply possibilities among possibilities. Since we don't know the density and temperature of the starting material, the expected gain or loss of angular momentum cannot be calculated precisely.

This is just Nostratic for astronomy; its proponents are at the mercy of their desire for it to be true, because it would be awesome.

[Lērisama]

I was under the impression that the gas being fluid and feeling pressure meant that it moved marginally slower than the solid planet, causing the planet to lose angular momentum. I don't see an easy way to avoid that force (bear in mind that migration was predicted before we discovered exoplanets, ignored for being weird and unnecessary to explain the Solar System, and then revived when exoplanets made it clear migration was possible). I may be wrong in this, and my source is a book from 2017 (I think) so it could be out of date.

[Glass Half Baked]

Given that the theory posits a net outward migration of Jupiter, the plausibility of type I or type II migration seems immaterial. If Jupiter can migrate outward due to unmeasured and unatested resonance with Saturn, then surely it is also plausible that it could simply stay in place.

[WeepingElf]

What is "type I" and "type II migration"?

EDIT: Wikipedia knows the answer. Case closed

[Torco]

I certainly don't know enough to have an opinion on whether the theory is true. it seems plausible to me, and i think it will look plausible to most scifi and fantasy enjoyers, but it also seems plausible that it's not the case.

that being said, I'm curious as to why in these models of planetary formation etcetera you seldom find gravitational effects that come from the relative motions of stars [with their systems alongside them]. sure, stars almost never crash into each other, they're too small and too far apart, but high eccentricity exoplanets being somewhat common suggests that at least from time to time systems would come near enough to another star for grav effects to be noticeable. furthermore, there's all sorts of other things that may affect a star system, such as planemos/rogue planets, or i don't know big clouds of gas or whatever. possibly even tiny clusters of dark matter, if such a thing exists. the point being, just because our own planetary system seems to be very nice and orderly it doesn't mean that that's the norm, maybe the reason we don't see aliens is cause most planetary systems knock into one another and sterilize the planets periodically or something.

[WeepingElf]

Stars are usually born in clusters, and thus can get close to each other in their early years, as is shown by the high frequency of binary star systems (which usually have eccentric orbits, so the companions are usually captured ones).