<p>This is a &quot;space is big&quot; answer.  There have been about 10000 stars passing through the Oort cloud, and they will cause some disruption to the cometary nuclei that are orbiting out there.</p>
<p>But space is big, and so most of the objects that are in the Oort cloud will never come close to another star, and remain in the outer solar system.</p>

CC BY-SA 4.0
2022-08-16T01:48:46.247Z
But the gravity fields are just as big. Wouldn't a 0.25 solar-mass red dwarf disrupt orbits that pass about twice closer to it than the the sun? So 0.5LY at 1LY distant, etc. This is a large region where objects get "sling-shotted"?
Kevin Kostlan
CC BY-SA 4.0
2022-08-16T04:38:14.133Z
@KevinKostlan You have to realize how big is space ;)
Nilay Ghosh
CC BY-SA 4.0
2022-08-16T04:40:52.357Z
@Kevin Have a look: https://space.stackexchange.com/questions/1382/how-frequently-do-asteroids-collide-with-each-other
Nilay Ghosh
CC BY-SA 4.0
2022-08-16T09:22:15.563Z
Yes, it will perturb those objects, but the passing star will pass, and the Oort cloud comets will (for the most part) settle back into a slow elliptical orbit around the sun.  A few get knocked into the inner solar system, and a few get ejected - now I haven't done any particular simulations of this, but the fact that we still see long period comets means  that the Oort cloud must still be there. Rob has done the maths that demonstrates this very clearly.
James K
CC BY-SA 4.0
2022-08-15T14:05:17.873Z
2022-08-15T14:05:17.873Z
James K
<p>Well, for one thing, the Outer Oort cloud remains theoretical, so we're not sure it exists in any reasonable shape or density.  Further, your own link says &quot;The Hills cloud explains the continued existence of the Oort cloud after billions of years&quot;</p>

CC BY-SA 4.0
2022-08-15T13:06:17.443Z
2022-08-15T13:06:17.443Z
Carl Witthoft
<p>The Oort cloud <em>is</em> disturbed. This is thought to be one of the ways we can get long period comets. It is also depleted of objects over time and it is likely that only a fraction of the original objects in the cloud remaining.</p>
<p>I guess what you really mean is why isn't the Oort cloud completely removed on a short timescale? The reason is that not only does an interloper have to get close enough, it has to be there long enough to strip satellites from the Sun.</p>
<p>The basic physics here is that when a star passes close to the Sun it attracts <em>both</em> the Sun and it's satellites towards it. What matters for whether it will strip a satellite from the Sun is the <em>difference</em> in attraction the Sun and the satellite feel, known as a tidal force. The tidal force depends on the mass of the interloper and the inverse <em>cube</em> of its proximity. A back of the envelope equation might be a condition for tidal stripping would be
<span class="math-container">$$G\frac{M m}{r^3}a &gt; G\frac{M_\odot m}{a^2}\ , $$</span>
where <span class="math-container">$M$</span> is the mass of the interloper, <span class="math-container">$a$</span> is the semi-major axis of the satellite and <span class="math-container">$r$</span> is how close the interloper gets to the Sun.
This <em>rough</em> equation tells us that stripping <em>may</em> occur if
<span class="math-container">$$ r &lt; \left(\frac{M}{M_\odot}\right)^{1/3}a\ .$$</span>
This would suggest that all that is required is for another star to enter the Oort cloud for this to happen.</p>
<p>But the treatment above assumes that <span class="math-container">$r$</span> and <span class="math-container">$a$</span> don't change. Whilst this might be a good approximation for <span class="math-container">$a$</span>, it isn't for <span class="math-container">$r$</span>. Interlopers will have a distribution of relative velocities with respect to the Sun, but a typical value might be about 20 km/s. That means the interloper spends a relatively short time in the regime where <span class="math-container">$r \sim a$</span> and is certainly moving too fast to capture a satellite from the Sun. Again, a <em>rough</em> calculation - the escape speed from the Sun at 50,000 au is about 200 m/s. The acceleration that <em>another</em> 1 solar mass star passing 50,000 au away provides is <span class="math-container">$2\times 10^{-12}$</span> m/s<span class="math-container">$^2$</span>. For this acceleration to add 200 m/s to a satellites velocity takes of order <span class="math-container">$10^{14}$</span> s (or 3 million years). But moving at 20 km/s an interloper will travel 50,000 au in just 10,000 years. Thus the interloper may not be in the vicinity of the Solar System for long enough to accelerate the satellite significantly.</p>

CC BY-SA 4.0
2022-08-16T07:39:38.207Z
And of course the Sun can also steal stuff from the interloper.
PM 2Ring
CC BY-SA 4.0
2022-08-16T22:46:50.313Z
Like pulling a tablecloth too fast for the dishes to be yanked away.
Kevin Kostlan
CC BY-SA 4.0
2022-08-16T07:29:44.680Z
2022-08-16T07:29:44.680Z
ProfRob
<p>Stars pass within one lightyear of Sol every <a href="https://astronomy.stackexchange.com/questions/3601/how-often-do-stars-pass-within-1-light-year-of-the-sun?rq=1">half-million years</a> or so on average. These stars enter the <a href="https://en.wikipedia.org/wiki/Oort_cloud" rel="noreferrer">Oort cloud</a>.</p>
<p>Each time a star passes through it's gravity will disrupt some object's orbits. How did the Oort cloud survive thousands of encounters?</p>

CC BY-SA 4.0
2022-08-16T04:47:19.107Z
https://www.sciencedirect.com/science/article/abs/pii/0019103588900590
Nilay Ghosh
<orbit><oort-cloud>
Why didn't passing stars destroy the Oort cloud?
Kevin Kostlan
# Why didn't passing stars destroy the Oort cloud?

(Asked by: username_0 on 2022-08-15)

Stars pass within one lightyear of Sol every [half-million years](https://astronomy.stackexchange.com/questions/3601/how-often-do-stars-pass-within-1-light-year-of-the-sun?rq=1) or so on average. These stars enter the [Oort cloud](https://en.wikipedia.org/wiki/Oort_cloud).

Each time a star passes through it's gravity will disrupt some object's orbits. How did the Oort cloud survive thousands of encounters?

**Tags:** orbit, oort-cloud

> **Comments:**
> **username_1**: https://www.sciencedirect.com/science/article/abs/pii/0019103588900590  

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## 3 answers

### Answer by username_4 on 2022-08-16. Score: 8 | Accepted answer

The Oort cloud *is* disturbed. This is thought to be one of the ways we can get long period comets. It is also depleted of objects over time and it is likely that only a fraction of the original objects in the cloud remaining.

I guess what you really mean is why isn't the Oort cloud completely removed on a short timescale? The reason is that not only does an interloper have to get close enough, it has to be there long enough to strip satellites from the Sun.

The basic physics here is that when a star passes close to the Sun it attracts *both* the Sun and it's satellites towards it. What matters for whether it will strip a satellite from the Sun is the *difference* in attraction the Sun and the satellite feel, known as a tidal force. The tidal force depends on the mass of the interloper and the inverse *cube* of its proximity. A back of the envelope equation might be a condition for tidal stripping would be
$$G\frac{M m}{r^3}a > G\frac{M\_\odot m}{a^2}\ , $$
where $M$ is the mass of the interloper, $a$ is the semi-major axis of the satellite and $r$ is how close the interloper gets to the Sun.
This *rough* equation tells us that stripping *may* occur if
$$ r < \left(\frac{M}{M\_\odot}\right)^{1/3}a\ .$$
This would suggest that all that is required is for another star to enter the Oort cloud for this to happen.

But the treatment above assumes that $r$ and $a$ don't change. Whilst this might be a good approximation for $a$, it isn't for $r$. Interlopers will have a distribution of relative velocities with respect to the Sun, but a typical value might be about 20 km/s. That means the interloper spends a relatively short time in the regime where $r \sim a$ and is certainly moving too fast to capture a satellite from the Sun. Again, a *rough* calculation - the escape speed from the Sun at 50,000 au is about 200 m/s. The acceleration that *another* 1 solar mass star passing 50,000 au away provides is $2\times 10^{-12}$ m/s$^2$. For this acceleration to add 200 m/s to a satellites velocity takes of order $10^{14}$ s (or 3 million years). But moving at 20 km/s an interloper will travel 50,000 au in just 10,000 years. Thus the interloper may not be in the vicinity of the Solar System for long enough to accelerate the satellite significantly.

> **Comments:**
> **username_5**: And of course the Sun can also steal stuff from the interloper.  
> **username_0**: Like pulling a tablecloth too fast for the dishes to be yanked away.  

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### Answer by username_2 on 2022-08-15. Score: 1

This is a "space is big" answer. There have been about 10000 stars passing through the Oort cloud, and they will cause some disruption to the cometary nuclei that are orbiting out there.

But space is big, and so most of the objects that are in the Oort cloud will never come close to another star, and remain in the outer solar system.

> **Comments:**
> **username_0**: But the gravity fields are just as big. Wouldn't a 0.25 solar-mass red dwarf disrupt orbits that pass about twice closer to it than the the sun? So 0.5LY at 1LY distant, etc. This is a large region where objects get "sling-shotted"?  
> **username_1**: @KevinKostlan You have to realize how big is space ;)  
> **username_1**: @Kevin Have a look: https://space.stackexchange.com/questions/1382/how-frequently-do-asteroids-collide-with-each-other  
> **username_2**: Yes, it will perturb those objects, but the passing star will pass, and the Oort cloud comets will (for the most part) settle back into a slow elliptical orbit around the sun.  A few get knocked into the inner solar system, and a few get ejected - now I haven't done any particular simulations of this, but the fact that we still see long period comets means  that the Oort cloud must still be there. Rob has done the maths that demonstrates this very clearly.  

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### Answer by username_3 on 2022-08-15. Score: 0

Well, for one thing, the Outer Oort cloud remains theoretical, so we're not sure it exists in any reasonable shape or density. Further, your own link says "The Hills cloud explains the continued existence of the Oort cloud after billions of years"


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