B A problem with accretion

[revnice]

We've seen sugar granules clumping in a bag on the spacestation. We know stuff acretes and a small start can resilt in a massive planet. If plotted, the acretion curve would be exponential because a bigger body attracts more stuff. So, we start with a few particles of dust clinging together because of an electrostatic charge and X billion years later, it's the size of a piece of gravel. Then along comes a real piece of gravel and it smashes the ball of dust to smithereens. (The History Channel shows two rocks colliding and sticking to each other!)

We start all over again and this time the acreted body reaches the size of a fridge. The same thing is going to happen, it just needs a slightly larger impact. The gravel, rocks and dust are barely clinging together because a fridge-size object doesn't have enough mass to give it any significant gravity. I could probaly kick it to pieces. In other words ther are conditions attached. To become a planet, the stuff must not encounter an impact that would break it apart and yet it must keep growing.

I fail to see how acreting stuff could ever reach a size that's impact resistant and allows it to keep growing. In its infancy, almost any impact would be enough to reset the process, surely?

 

[Ibix]

If impacts are energetic enough to break matter apart it will break apart, sure. But where's the energy coming from? Stuff starts moving because of the gravity of such slightly over-dense regions, and if they're very small they don't have much gravity and the speeds of matter around them are typically low. Furthermore accretion tends to happen in gas clouds where you have multiple collisions that dissipate energy from anything moving too quickly (what we'd call "drag" in the atmosphere) so matter won't escape too easily. It heats up instead.

 

[sophiecentaur]

To become a planet, the stuff must not encounter an impact that would break it apart and yet it must keep growing.

One explanation of the existence of the asteroid belt is the presence of massive Jupiter whose gravitational effect prevented the formation of a planet forming due to accretion of those rocks in that inner orbital spot. Also, I believe the asteroid belt is likely to become more and more depleted of large objects because of a net loss over time and decreasing accretion into big chunks. So the likelihood of a major asteroid colliding with Earth is likely to decrease over time (not relevant to our timescale though).

 

[Dale]

In its infancy, almost any impact would be enough to reset the process, surely?

In its infancy any impact would be enough to break it apart, but that does not reset the process. In the process of breaking apart there is an overall loss of KE and an increase in thermal energy. The system is not reset, it progressively becomes slower and hotter. As it slows, the time between disruptive collisions increases.

 

[revnice]

Ibix:
>where's the energy coming from?
Something that's already moving. My hypothetical blob of dust couldn't attract anything with enough speed to break it, but something, almost anything, already on the move, could. Who knows how many budding planets have been obliterated by Ouamuwamu (sp).

I'm not arguing that acretion doesn't result in planets, I just have a problem with this one annoying detail. There is no size that can't be broken but there is a size that's more likely to survive and go on to become a planet. The only answer I can come up with is that it must take an untold number of resets and squillions of mega years to get past being constantly smashed.

How do planets form?
Acretion, there you go.

That's not me, I can't just accept an umbrella term answer, especially if I have a problem with it. I won;t be fobbed off by a keychain. :)

 

[Ibix]

As we've already pointed out, accretion often happens in gas clouds where repeated collisions sap the kinetic energy of moving objects - drag. So eventually collisions happen at low speed, driven basically by the self gravity of the gas.

 

[Bandersnatch]

This is actually an active field of research. It's called the fragmentation barrier.
The talk below discusses this in the wider context of the emerging theory of planetary formation (starting around the 23 minute mark). It's pretty technical, but potentially enlightening.

 

[Dale]

it must take an untold number of resets

There are no resets. The collisions are inelastic. Every collision results in reduced velocity and increased temperature

 

[revnice]

Bandeesnatch:

Thank you! Until you posted that I've been feeling brow beaten and shouted down. I didn't understand but just knowing that someone is looking into it is enough. It means I'm not crazy or stupid.

 

[Ibix]

I didn't understand

In summary: accretion happens in gas discs in regions where the gas pressure is high, which keeps relative particle velocities low so collisions occur at low speeds. The talk covers details of the processes that cause the conditions for accretion to occur, and to occur repeatedly within a disc, leading to the formation of multiple planets.

I think the two B-level things the talk says that add to what has been said on this thread (aside from the detailed mathematical modelling and observational evidence to back it up) are that the range of conditions where accretion can happen above a certain particle size are narrow even within a gas cloud (I hadn't appreciated the extent to which the italicised bit is true) and ice formation apparently plays an important part in reducing fragmentation of the particles, which is news to me.

 

[sophiecentaur]

, I just have a problem with this one annoying detail. There is no size that can't be broken but there is a size that's more likely to survive and go on to become a planet.

There are no resets. The collisions are inelastic. Every collision results in reduced velocity and increased temperature

There is the huge factor of the life stage of the star. Big stars live fast and small stars last a lot longer (billions of years more) and that affects the formation of planets because of the progress of the local temperature. As we continue getting actual landing missions on asteroids the processes will be understood better.

I recently read that the probability of nasty collisions with Earth is steadily decreasing because the mean size of asteroids will be decreasing over the (millions of) years. I can't remember where I got this from but i have a feeling it was from a speaker visiting my Astronomical Soc.

PS We started getting top class speakers as a result of Lockdown when we went on line after changes in the committee and our contacts have increased in number and quality. To name drop - we had Jocelyn Bell Zooming in a few years ago.