<p>This is a fascinating question and I do not think there is yet an absolutely agreed consensus even on the likely distribution of the Oort cloud objects and it isn't clear to me how the pictures you show have been produced.</p>
<p>TL;DR There are probably lots of objects with semi-major axes from 100 to a few thousand au, but because they are ejected from the inner Solar System but not strongly influenced by the Galactic tidal field, they have high eccentricities (and so spend longer at aphelion a long way from the Sun) and low inclinations (and so are confined to a disc). Objects ejected even further end up being tidally circularised and the inclinations of their orbits also gets randomised by the Galactic tidal field. This roughly agrees with what is shown in your second image.</p>
<p>Note also the the axes on your diagrams are logarithmic. That means that the volume out to 100000 au is a million times bigger than the volume out to 1000 au. This means that even if the <em>density</em> of objects is higher at smaller radii, their absolute numbers may be smaller in such diagrams.</p>
<p><strong>Now read on</strong></p>
<p>The potential gap you have identified appears to coincide with the region has been termed the &quot;parking zone&quot; by <a href="https://arxiv.org/abs/1506.03105" rel="nofollow noreferrer">Jilkova et al. (2015)</a> and is defined by the &quot;Kuiper belt cliff&quot; at its inner edge (about 100 au) and the start of the Oort cloud at its outer edge (a few thousand au). The contention is that objects in the parking zone are difficult to produce if they scatter out from the inner Solar System or scatter in from the outer Solar System. Therefore they might either formed in-situ or they are captured objects from another star system during the initial (clustered) stage of the Sun's life.</p>
<p>The Oort cloud itself is thought to be mainly populated with objects that formed in the inner Solar System, where the protostellar disc was dense. Small asteroids that are close to the giant planets can pick up a series of orbital perturbations that boost their eccentricity and semi-major axis, whilst keeping their perihelion distance roughly constant. If that process continues then the asteroids will eventually escape the Solar System, and a large fraction probably did so. However, when the asteroids achieve a very large semi-major axis, the Galactic tidal field can act to <em>circularise</em> the orbits - reducing the eccentricity but preserving the semi-major axis. The circularisation timescale is quicker the larger the semi-major axis.</p>
<p>The combination of these two processes - the &quot;conveyor belt&quot; of building up the eccentricity and semi-major axis, competing with the circularisation of the orbits induced by the Galactic tidal field - produces an Oort cloud of objects with relatively low eccentricity orbits between the &quot;parking zone&quot; and the outer edge of the Oort cloud (the Hill radius), which is where the Galactic tidal field will strip the objects out of the Solar System altogether. I think there are though still plenty of objects in the parking zone with relatively high eccentricities, though these may be more confined to a disc with low inclination (see edit).</p>
<p>Further reading: <a href="https://iopscience.iop.org/article/10.3847/1538-3881/ab13aa" rel="nofollow noreferrer">Vokrouhlicky et al. (2019)</a> and <a href="https://arxiv.org/abs/2105.12816" rel="nofollow noreferrer">Portegies Zwart et al. (2021)</a>, which is where the summary diagram below is taken from. Their models follow the development of the Oort cloud from a protoplanetary disc in which giant planets have formed, for a billion years. These show that the Oort cloud forms within 100 million years and evolves slowly thereafter.</p>
<p><a href="https://i.sstatic.net/X2FxN.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/X2FxN.jpg" alt="enter image description here" /></a></p>
<p><strong>EDIT!</strong> However, the overall number density as a function of radius in both the Vokrouhlicky et al. and Portegies Zwart models <strong>do not</strong> show the gap that forms the basis of your question. On the contrary, the mass density of objects between 100 and 1000 au is higher than that in subsequent decades of semi major axis (see the orange line in the plot below)!</p>
<p>Note though that the asteroids at 100 to a few thousand au still have low inclinations (according to the simulations) - i.e. are more confined to a disc, so the high densities might mask the fact that they have relatively low densities at high inclinations (as shown in your pictures). It is only when they get beyond the parking zone that the Galactic tidal field starts to mix up the inclinations as well as circularise the orbits.</p>
<p><a href="https://i.sstatic.net/HUWfY.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/HUWfY.jpg" alt="r distribution of Oort cloud" /></a></p>
<p>I think the main reason for the <em>apparent gap</em> is the use of logarithmic axes in your pictures. The volume of space between say 100 au and 1000 au is about a million times smaller than between 10000 au and 100000 au.  Thus even though the density is increasing with smaller semi major axis, the absolute numbers in a shell on a plot with logarithmic axes will decrease.</p>

CC BY-SA 4.0
2023-10-18T17:56:08.917Z
Quite similar to [Kirkwood gap](https://en.wikipedia.org/wiki/Kirkwood_gap) for asteroid belts. Btw, I found this interesting paper: https://iopscience.iop.org/article/10.3847/1538-3881/abfb6f/pdf.
Nilay Ghosh
CC BY-SA 4.0
2023-10-18T18:17:48.493Z
The Kirkwood gaps are caused by the clearing out of resonant orbits between Mars and Jupiter. That isn't what's happening here and there isn't a gap. @NilayGhosh
ProfRob
CC BY-SA 4.0
2023-10-19T09:09:58.373Z
When Voyager reaches the inner edge in 300 years hopefully there will be a signpost. it will take 300k years to get through the region.
bandybabboon
CC BY-SA 4.0
2023-10-19T17:36:15.183Z
@bandybabboon Yes, we need a signpost: https://space.stackexchange.com/questions/54580/if-the-kuiper-belt-is-only-about-50-au-thick-and-voyager-2-is-153-au-away-then-i
Nilay Ghosh
CC BY-SA 4.0
2023-10-18T12:39:27.290Z
2023-10-18T19:40:18.940Z
2023-10-18T19:40:18.940Z
ProfRob
<p>Can someone please tell what caused the hollow space (pointed by yellow arrow) in the Oort Cloud?</p>
<p>I understand that it is called cloud because the cluster of objects does not align with the major rotation axis of our Sun. But, I could not find information on anywhere what caused the hollow space.</p>
<p>Thank you.</p>
<p><a href="https://i.sstatic.net/WpmUN.png" rel="noreferrer"><img src="https://i.sstatic.net/WpmUN.png" alt="enter image description here" /></a></p>
<p><a href="https://i.sstatic.net/CCdGz.png" rel="noreferrer"><img src="https://i.sstatic.net/CCdGz.png" alt="enter image description here" /></a></p>
<p><a href="https://i.sstatic.net/32vyq.png" rel="noreferrer"><img src="https://i.sstatic.net/32vyq.png" alt="enter image description here" /></a></p>

CC BY-SA 4.0
2023-10-17T09:35:36.000Z
Where did those images come from?
PM 2Ring
CC BY-SA 4.0
2023-10-17T11:00:18.510Z
There are empty(ish) areas in our solar systems, but they're usually governed by gravitational bodies.   The Kirkwood gaps for example.  Or the lack of asteroids between Mars and the Sun relative to the far larger amount between Mars and Jupiter.    I know of no reason why there would be an empty area between the Kuiper Belt and the Oort cloud and I suspect there isn't.   I suspect those images are inaccurate representations.    There might be a higher concentration of objects near the plane of the solar system.   https://cdn.spacetelescope.org/archives/images/large/opo0204i.jpg
userLTK
CC BY-SA 4.0
2023-10-17T14:40:39.823Z
Large eccentricity of orbits? The bodies spend most time around aphelion.
Leos Ondra
CC BY-SA 4.0
2023-10-17T19:06:26.553Z
The second image even has voids resembling p-orbitals in chemistry. Really curious. Great question!
ksousa
CC BY-SA 4.0
2023-10-18T02:34:17.920Z
@LeosOndra objects spend more time in their more distant parts of the orbit.   That might be an explanation.   I hadn't thought of that.    I still have doubt that the images are accurate though.  I'm not sure anyone knows the distribution of larger objects in the Oort Cloud.
userLTK
CC BY-SA 4.0
2023-10-18T12:23:35.023Z
You've found the holy grail there: There are absolutel no 3D simulations of the oort cloud formation since the protoplanetary disk to today, but there are 20 simulations of galaxy formation, which is more complex. Time for someone to do some physics using the basic laws to see the different variables and to answer your query.
bandybabboon
CC BY-SA 4.0
2023-10-18T14:18:16.867Z
@bandybabboon "There are absolutel no 3D simulations of the oort cloud formation since the protoplanetary disk to today" Are you sure? [Jack Hills didn't](https://en.wikipedia.org/wiki/Hills_cloud#New_model)?
uhoh
CC BY-SA 4.0
2023-10-18T17:16:41.273Z
The scale's logarithmic,  so the voids as depicted, especially in the  first image, are _far_ smaller relative to the rest of the oort cloud  than they initially appear.
notovny
CC BY-SA 4.0
2023-10-18T17:49:47.093Z
Also see: https://astronomy.stackexchange.com/questions/13474/lack-of-objects-between-heliopause-and-oort-cloud
Nilay Ghosh
CC BY-SA 4.0
2023-10-22T15:22:51.580Z
@uhoh I can't find any videos of 3D simulations of it, I mean there are 20 video animations of galaxy kinematics and none of the oort cloud that i can find to watch and so on.
bandybabboon
CC BY-SA 4.0
2023-10-22T23:37:53.353Z
@bandybabboon a simulation happens inside a computer, and the output of these calculations will be some raw data and some plots. Making a fancy video is a completely separate thing from doing a simulation. All three papers discussed in ProfRob's answer ([1](https://arxiv.org/abs/1506.03105), [2](https://iopscience.iop.org/article/10.3847/1538-3881/ab13aa), [3](https://arxiv.org/abs/2105.12816)) describe simulations and results, but perhaps nobody saw any value in making a YouTube video of them.
uhoh
CC BY-SA 4.0
2023-10-22T23:39:36.477Z
@bandybabboon You could consider posting a follow-up question asking for videos, either in YouTube or as a supplementary information attachment to a published paper. For those kinds of questions be sure to add the [tag:resource-request] tag and mention that you've searched hard and so far have found none, and link back to this question for background.
uhoh
CC BY-SA 4.0
2023-10-23T12:16:30.697Z
@uhoh, I wrote to some scientists who have galaxy animations based on kinematics on YT and said that it would be nice to have some simulations.
bandybabboon
<solar-system><asteroids><oort-cloud><asteroid-belt>
What's the reason for the hollow region in Oort cloud?
Kevv Keka
# What's the reason for the hollow region in Oort cloud?

(Asked by: username_0 on 2023-10-16)

Can someone please tell what caused the hollow space (pointed by yellow arrow) in the Oort Cloud?

I understand that it is called cloud because the cluster of objects does not align with the major rotation axis of our Sun. But, I could not find information on anywhere what caused the hollow space.

Thank you.

[![enter image description here](https://i.sstatic.net/WpmUN.png)](https://i.sstatic.net/WpmUN.png)

[![enter image description here](https://i.sstatic.net/CCdGz.png)](https://i.sstatic.net/CCdGz.png)

[![enter image description here](https://i.sstatic.net/32vyq.png)](https://i.sstatic.net/32vyq.png)

**Tags:** solar-system, asteroids, oort-cloud, asteroid-belt
> **Comments:**
> **username_1**: Where did those images come from?  
> **username_2**: There are empty(ish) areas in our solar systems, but they're usually governed by gravitational bodies.   The Kirkwood gaps for example.  Or the lack of asteroids between Mars and the Sun relative to the far larger amount between Mars and Jupiter.    I know of no reason why there would be an empty area between the Kuiper Belt and the Oort cloud and I suspect there isn't.   I suspect those images are inaccurate representations.    There might be a higher concentration of objects near the plane of the solar system.   https://cdn.spacetelescope.org/archives/images/large/opo0204i.jpg  
> **username_3**: Large eccentricity of orbits? The bodies spend most time around aphelion.  
> **username_4**: The second image even has voids resembling p-orbitals in chemistry. Really curious. Great question!  
> **username_2**: @LeosOndra objects spend more time in their more distant parts of the orbit.   That might be an explanation.   I hadn't thought of that.    I still have doubt that the images are accurate though.  I'm not sure anyone knows the distribution of larger objects in the Oort Cloud.  
> **username_5**: You've found the holy grail there: There are absolutel no 3D simulations of the oort cloud formation since the protoplanetary disk to today, but there are 20 simulations of galaxy formation, which is more complex. Time for someone to do some physics using the basic laws to see the different variables and to answer your query.  
> **username_6**: @bandybabboon "There are absolutel no 3D simulations of the oort cloud formation since the protoplanetary disk to today" Are you sure? [Jack Hills didn't](https://en.wikipedia.org/wiki/Hills_cloud#New_model)?  
> **username_7**: The scale's logarithmic,  so the voids as depicted, especially in the  first image, are _far_ smaller relative to the rest of the oort cloud  than they initially appear.  
> **username_8**: Also see: https://astronomy.stackexchange.com/questions/13474/lack-of-objects-between-heliopause-and-oort-cloud  
> **username_5**: @uhoh I can't find any videos of 3D simulations of it, I mean there are 20 video animations of galaxy kinematics and none of the oort cloud that i can find to watch and so on.  
> **username_6**: @bandybabboon a simulation happens inside a computer, and the output of these calculations will be some raw data and some plots. Making a fancy video is a completely separate thing from doing a simulation. All three papers discussed in ProfRob's answer ([1](https://arxiv.org/abs/1506.03105), [2](https://iopscience.iop.org/article/10.3847/1538-3881/ab13aa), [3](https://arxiv.org/abs/2105.12816)) describe simulations and results, but perhaps nobody saw any value in making a YouTube video of them.  
> **username_6**: @bandybabboon You could consider posting a follow-up question asking for videos, either in YouTube or as a supplementary information attachment to a published paper. For those kinds of questions be sure to add the [tag:resource-request] tag and mention that you've searched hard and so far have found none, and link back to this question for background.  
> **username_5**: @uhoh, I wrote to some scientists who have galaxy animations based on kinematics on YT and said that it would be nice to have some simulations.  

---

## 1 answers

### Answer by username_9 on 2023-10-18. Score: 11 | Accepted answer

This is a fascinating question and I do not think there is yet an absolutely agreed consensus even on the likely distribution of the Oort cloud objects and it isn't clear to me how the pictures you show have been produced.

TL;DR There are probably lots of objects with semi-major axes from 100 to a few thousand au, but because they are ejected from the inner Solar System but not strongly influenced by the Galactic tidal field, they have high eccentricities (and so spend longer at aphelion a long way from the Sun) and low inclinations (and so are confined to a disc). Objects ejected even further end up being tidally circularised and the inclinations of their orbits also gets randomised by the Galactic tidal field. This roughly agrees with what is shown in your second image.

Note also the the axes on your diagrams are logarithmic. That means that the volume out to 100000 au is a million times bigger than the volume out to 1000 au. This means that even if the *density* of objects is higher at smaller radii, their absolute numbers may be smaller in such diagrams.

**Now read on**

The potential gap you have identified appears to coincide with the region has been termed the "parking zone" by [Jilkova et al. (2015)](https://arxiv.org/abs/1506.03105) and is defined by the "Kuiper belt cliff" at its inner edge (about 100 au) and the start of the Oort cloud at its outer edge (a few thousand au). The contention is that objects in the parking zone are difficult to produce if they scatter out from the inner Solar System or scatter in from the outer Solar System. Therefore they might either formed in-situ or they are captured objects from another star system during the initial (clustered) stage of the Sun's life.

The Oort cloud itself is thought to be mainly populated with objects that formed in the inner Solar System, where the protostellar disc was dense. Small asteroids that are close to the giant planets can pick up a series of orbital perturbations that boost their eccentricity and semi-major axis, whilst keeping their perihelion distance roughly constant. If that process continues then the asteroids will eventually escape the Solar System, and a large fraction probably did so. However, when the asteroids achieve a very large semi-major axis, the Galactic tidal field can act to *circularise* the orbits - reducing the eccentricity but preserving the semi-major axis. The circularisation timescale is quicker the larger the semi-major axis.

The combination of these two processes - the "conveyor belt" of building up the eccentricity and semi-major axis, competing with the circularisation of the orbits induced by the Galactic tidal field - produces an Oort cloud of objects with relatively low eccentricity orbits between the "parking zone" and the outer edge of the Oort cloud (the Hill radius), which is where the Galactic tidal field will strip the objects out of the Solar System altogether. I think there are though still plenty of objects in the parking zone with relatively high eccentricities, though these may be more confined to a disc with low inclination (see edit).

Further reading: [Vokrouhlicky et al. (2019)](https://iopscience.iop.org/article/10.3847/1538-3881/ab13aa) and [Portegies Zwart et al. (2021)](https://arxiv.org/abs/2105.12816), which is where the summary diagram below is taken from. Their models follow the development of the Oort cloud from a protoplanetary disc in which giant planets have formed, for a billion years. These show that the Oort cloud forms within 100 million years and evolves slowly thereafter.

[![enter image description here](https://i.sstatic.net/X2FxN.jpg)](https://i.sstatic.net/X2FxN.jpg)

**EDIT!** However, the overall number density as a function of radius in both the Vokrouhlicky et al. and Portegies Zwart models **do not** show the gap that forms the basis of your question. On the contrary, the mass density of objects between 100 and 1000 au is higher than that in subsequent decades of semi major axis (see the orange line in the plot below)!

Note though that the asteroids at 100 to a few thousand au still have low inclinations (according to the simulations) - i.e. are more confined to a disc, so the high densities might mask the fact that they have relatively low densities at high inclinations (as shown in your pictures). It is only when they get beyond the parking zone that the Galactic tidal field starts to mix up the inclinations as well as circularise the orbits.

[![r distribution of Oort cloud](https://i.sstatic.net/HUWfY.jpg)](https://i.sstatic.net/HUWfY.jpg)

I think the main reason for the *apparent gap* is the use of logarithmic axes in your pictures. The volume of space between say 100 au and 1000 au is about a million times smaller than between 10000 au and 100000 au. Thus even though the density is increasing with smaller semi major axis, the absolute numbers in a shell on a plot with logarithmic axes will decrease.

> **Comments:**
> **username_8**: Quite similar to [Kirkwood gap](https://en.wikipedia.org/wiki/Kirkwood_gap) for asteroid belts. Btw, I found this interesting paper: https://iopscience.iop.org/article/10.3847/1538-3881/abfb6f/pdf.  
> **username_9**: The Kirkwood gaps are caused by the clearing out of resonant orbits between Mars and Jupiter. That isn't what's happening here and there isn't a gap. @NilayGhosh  
> **username_5**: When Voyager reaches the inner edge in 300 years hopefully there will be a signpost. it will take 300k years to get through the region.  
> **username_8**: @bandybabboon Yes, we need a signpost: https://space.stackexchange.com/questions/54580/if-the-kuiper-belt-is-only-about-50-au-thick-and-voyager-2-is-153-au-away-then-i  

---

