<p>I was curious about the same things. I believe it was in the astronomy stack exchange I was referred to an online data base that gives position and velocity vectors for neighboring stars. From those I put together a spreadsheet. Here's a screen capture:</p>

<p><img src="https://i.sstatic.net/rYyUq.png" alt="enter image description here"></p>

<p>I only entered 48 of the closest stars so it's by no mean an exhaustive list.</p>

<p>It looks like your graphic matches my estimates which is reassuring. I don't know why the Ross Stars aren't in my list, possibly the omission is an error on my part when I was entering data to the spreadsheet.</p>

<p>Looks like the closest approaches are around 3 lightyears.</p>

<p>If each star has an Oort cloud, I believe the comets' velocity with regard to our sun would be pretty close to the star's relative velocity. Slowest star wrt our solar system seems to be Gliese 729 which is moving ~14 km/s wrt the sun.</p>

<p>If, for example, some of Van Maanen's Oort cloud came within a light year of our sun, they would have been moving 270 km/s. Those snowballs would have zoomed in and out of our neighborhood.</p>

<p>With these distances and relative velocities I don't see much opportunity for swapping comets.</p>

<p>It's speculated that our sun swapped comets with neighboring stars when our solar system was being formed. From <a href="http://en.wikipedia.org/wiki/Oort_cloud#Origin" rel="noreferrer">Wikipedia</a>:</p>

<blockquote>
  <p>Recent research has been cited by NASA hypothesizing that a large
  number of Oort cloud objects are the product of an exchange of
  materials between the Sun and its sibling stars as they formed and
  drifted apart, and it is suggested that many—possibly the majority—of
  Oort cloud objects were not formed in close proximity to the Sun.</p>
</blockquote>

CC BY-SA 3.0
2014-06-02T15:03:53.713Z
On the scale of 0.0002 billion years, movements of stars near the Sun is available. And interaction with siblings since the beginnings of earthly time is only one part of the equation. The other part is how the Sun might have interacted with comets (or spaceships?) from unrelated much older or younger stars, during the 16 or so times the Sun has orbited the Milky Way like once every 0.25 billion years. How frequently _should_ mass exchanging encounters have occured, is really my question. (Wiki's lack of dates makes terms like "recently" mean little in quickly developing fields of astronomy).
LocalFluff
CC BY-SA 3.0
2014-06-02T15:07:00.657Z
But I think you argue that stars and their comets are most likely to have higher than capture velocity relative to the solarsystem? And for that reason they are unlikely to stick around.
LocalFluff
CC BY-SA 3.0
2014-06-02T15:12:57.630Z
That's my thinking. Even if perihleion were 1/2 a light year, 14 km/s Vinf would be a hyperbola of very high eccentricity -- the path would look like a straight line. That's how I modeled the paths -- as straight lines.
HopDavid
CC BY-SA 3.0
2014-06-02T15:18:21.610Z
It's my understanding they get the stars' distance by Parallax -- which is much more error prone as the stars go further. Radial velocity is measured by blue shift. And then distance plus angular velocity are used to get the other velocity components. So estimates are more prone to error with more distant stars.
HopDavid
CC BY-SA 3.0
2014-06-02T15:20:16.607Z
So distances don't say everything, it's the delta-v that matters for exchanging stuff like comets. It might as well be easier for a more distant star to happen to launch a comet our way that our Sun is more likely to trap.
LocalFluff
CC BY-SA 3.0
2014-06-02T15:21:49.977Z
Our vision is murky but I imagine you're right. Over 4 billion years we might have swapped stuff with other stars doing slow fly bys.
HopDavid
CC BY-SA 3.0
2014-06-02T15:30:34.500Z
Hyperbolas are symmetric. The incoming Vinfinity is the same as the outgoing Vinfinity. If it were solely 2 body interactions, we'd never capture a comet moving above escape velocity. However if a comet passes in front of a planet, the planet's gravity can throw it backwards thus reducing it's velocity with regard to the sun.
HopDavid
CC BY-SA 3.0
2014-06-02T15:31:01.280Z
Let us [continue this discussion in chat](http://chat.stackexchange.com/rooms/14870/discussion-between-hopdavid-and-localfluff).
HopDavid
CC BY-SA 3.0
2014-06-02T14:39:47.427Z
2014-06-02T14:39:47.427Z
HopDavid
<p><strong>TL;DR  There is an encounter within 1 light year about once every half a million years. So about 9000 close encounters during the lifetime of the Sun</strong></p>
<p>The answer to the second part of your question is that the stars near the Sun are extremely well mixed, with a range of ages from a few million years to 12 billion years. None of them are identifiably &quot;following the Sun&quot;.</p>
<p><strong>Some details</strong></p>
<p>Using the re-reduction of the Hipparcos astrometry, <a href="http://arxiv.org/abs/1412.3648" rel="noreferrer">Bailer-Jones (2014)</a> has integrated orbits for 50,000 stars to look for objects that might come or might have come close to the Sun.</p>
<p>The K-dwarf <a href="http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Hip%2085605&amp;NbIdent=1&amp;Radius=2&amp;Radius.unit=arcmin&amp;submit=submit%20id" rel="noreferrer">Hip 85605</a> is the winner on that timescale, with a &quot;90% probability of coming between 0.04 and 0.20pc between 240,000 and 470,000 years from now&quot;.</p>
<p>The next best is GL710 a K-dwarf that will come within about 0.1-0.44pc in 1.3 million years.</p>
<p>On a statistical basis, some work has been done by <a href="http://adsabs.harvard.edu/abs/2001A%26A...379..634G" rel="noreferrer">Garcia Sanchez et al. (2001)</a>. They estimate, using the Hipparcos data, that encounters within 1pc occur every 2.3 million years. However, the Hipparcos data isn't complete for low-mass, faint objects. Making a correction for this, the authors estimate a <span class="math-container">$&lt;1pc$</span> encounter every 100,000 years. The probability for closer approaches scales as the inverse of the square of the separation. i.e. the timescales for approaches closer than 0.1pc is 100 times longer.</p>
<p>There are plenty of big error bars in all this work - recall that an error of 1km/s in velocity leads to a 1pc error in position after 1 million years. All this should be nailed down really well by Gaia results in the next 2-3 years.</p>
<p><strong>Update</strong></p>
<p>There is a new paper based on Gaia DR3 astrometry by <a href="https://arxiv.org/abs/2207.06258" rel="noreferrer">Bailer-Jones (2022)</a>. The star GL710 is now much more firmly constrained, coming within 0.060-0.068 pc of the Sun  in 1.26-1.33 million years.</p>
<p>A new contender on the scene is HD7977, a G3 dwarf that may have come even closer at 0.02-0.12 pc about 2.72-2.80 million years <em>ago</em>.</p>
<p>There are a total of about 50 stars that will or have passed within 1 pc of the Sun in the last <span class="math-container">$\pm 6$</span> million years - so about one <em>known</em> encounter every 240,000 years. But note that this cannot possibly be a complete list and the figure is a lower limit even over this period, because faint stars and high velocity stars can move in and out of Gaia's sensitivity range on these sorts of timescales and thus would not be in the current catalogues.</p>
<p><a href="https://arxiv.org/abs/1708.08595" rel="noreferrer">Bailer-Jones (2018)</a> (using Gaia DR2 data) attempted to correct for this incompleteness and estimated an encounter rate within 5 pc of <span class="math-container">$545 \pm 59$</span> per million years. With the inverse-square scaling mentioned above (and also advocated by Bailer-Jones), this becomes <strong>about 1 encounter within 1 pc every 50,000 years or about 1 encounter within 1 light year every 0.5 million years.</strong></p>
<p>Assuming this rate is roughly correct over the whole lifetime of the Sun then this would yield 9000 close encounters over its 4.5 billion year lifetime (arguably there might have been a slightly smaller number of stars earlier in the Galaxy's history, but this might be outweighed by the Sun probably gradually migrating outwards from a radial position with a higher stellar density.)</p>
<p>Projecting this further, then if you are prepared to wait 50 million years, then you might expect an encounter within 0.1 light years, which is in the Oort cloud and really starting to get interesting as far as the timescales of mass-extinction events go.</p>

CC BY-SA 4.0
2014-12-14T23:01:04.843Z
2022-07-25T17:40:07.547Z
2022-07-25T17:40:07.547Z
ProfRob
<p>Interstellar exchange of massive objects is difficult across several light years. But as the stars orbit the galaxy the distances between them change. I don't find data for neighbor star distances which covers more than a couple of a hundred thousand years. I can imagine that it is difficult to trace their orbits further given todays uncertainties in distance measurements. But I'm interested in a purely statistical/geometrical estimation of how frequent nearby Sun-star passages have been.</p>
<ul>
<li>How many times has a star come closer than, for example, 1 light year from the Sun since it formed?</li>
<li>How well mixed have the nearest stars become over time? What fraction of them have followed the Sun since formation and made multiple passages?</li>
</ul>
<p>I suppose that 1 light year's distance is enough to cause disturbances in the Oort cloud and likely encourage comets.</p>
<p><img src="https://i.sstatic.net/2BYKf.png" alt="From Wiki" /></p>

CC BY-SA 4.0
2024-11-08T12:48:12.327Z
I was struck by the inspiration and have been haunted by the idea ever since... that if we have an Oort cloud, so do other systems!

On the graph above, the 'Outer border of the Oort cloud' grey area, should have a counter-part 'width' (perhaps varying widths based on mass of parent star) surrounding (or either side of) each orbit of every other star.

The question then, isn't how often another 'star' comes within 1.5 light years of our Sun, but how often another Oort cloud gravitationally 'connects' with our Oort cloud?

It is likely that we're looking at a 4+ body problem xD
equilibriator
CC BY-SA 3.0
2015-06-05T10:16:52.523Z
Small point to add to an old question.   http://www.space.com/28611-star-flew-through-solar-system.html
userLTK
<star><the-sun><distances><movement>
How often do stars pass within 1 light year of the Sun?
LocalFluff
# How often do stars pass within 1 light year of the Sun?

(Asked by: username_0 on 2014-06-02)

Interstellar exchange of massive objects is difficult across several light years. But as the stars orbit the galaxy the distances between them change. I don't find data for neighbor star distances which covers more than a couple of a hundred thousand years. I can imagine that it is difficult to trace their orbits further given todays uncertainties in distance measurements. But I'm interested in a purely statistical/geometrical estimation of how frequent nearby Sun-star passages have been.

* How many times has a star come closer than, for example, 1 light year from the Sun since it formed?
* How well mixed have the nearest stars become over time? What fraction of them have followed the Sun since formation and made multiple passages?

I suppose that 1 light year's distance is enough to cause disturbances in the Oort cloud and likely encourage comets.

![From Wiki](https://i.sstatic.net/2BYKf.png)

**Tags:** star, the-sun, distances, movement
> **Comments:**
> **username_2**: Small point to add to an old question.   http://www.space.com/28611-star-flew-through-solar-system.html  
> **username_1**: I was struck by the inspiration and have been haunted by the idea ever since... that if we have an Oort cloud, so do other systems!  On the graph above, the 'Outer border of the Oort cloud' grey area, should have a counter-part 'width' (perhaps varying widths based on mass of parent star) surrounding (or either side of) each orbit of every other star.  The question then, isn't how often another 'star' comes within 1.5 light years of our Sun, but how often another Oort cloud gravitationally 'connects' with our Oort cloud?  It is likely that we're looking at a 4+ body problem xD  

---

## 2 answers

### Answer by username_4 on 2014-12-14. Score: 23 | Accepted answer

**TL;DR There is an encounter within 1 light year about once every half a million years. So about 9000 close encounters during the lifetime of the Sun**

The answer to the second part of your question is that the stars near the Sun are extremely well mixed, with a range of ages from a few million years to 12 billion years. None of them are identifiably "following the Sun".

**Some details**

Using the re-reduction of the Hipparcos astrometry, [Bailer-Jones (2014)](http://arxiv.org/abs/1412.3648) has integrated orbits for 50,000 stars to look for objects that might come or might have come close to the Sun.

The K-dwarf [Hip 85605](http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Hip%2085605&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit%20id) is the winner on that timescale, with a "90% probability of coming between 0.04 and 0.20pc between 240,000 and 470,000 years from now".

The next best is GL710 a K-dwarf that will come within about 0.1-0.44pc in 1.3 million years.

On a statistical basis, some work has been done by [Garcia Sanchez et al. (2001)](http://adsabs.harvard.edu/abs/2001A%26A...379..634G). They estimate, using the Hipparcos data, that encounters within 1pc occur every 2.3 million years. However, the Hipparcos data isn't complete for low-mass, faint objects. Making a correction for this, the authors estimate a $<1pc$ encounter every 100,000 years. The probability for closer approaches scales as the inverse of the square of the separation. i.e. the timescales for approaches closer than 0.1pc is 100 times longer.

There are plenty of big error bars in all this work - recall that an error of 1km/s in velocity leads to a 1pc error in position after 1 million years. All this should be nailed down really well by Gaia results in the next 2-3 years.

**Update**

There is a new paper based on Gaia DR3 astrometry by [Bailer-Jones (2022)](https://arxiv.org/abs/2207.06258). The star GL710 is now much more firmly constrained, coming within 0.060-0.068 pc of the Sun in 1.26-1.33 million years.

A new contender on the scene is HD7977, a G3 dwarf that may have come even closer at 0.02-0.12 pc about 2.72-2.80 million years *ago*.

There are a total of about 50 stars that will or have passed within 1 pc of the Sun in the last $\pm 6$ million years - so about one *known* encounter every 240,000 years. But note that this cannot possibly be a complete list and the figure is a lower limit even over this period, because faint stars and high velocity stars can move in and out of Gaia's sensitivity range on these sorts of timescales and thus would not be in the current catalogues.

[Bailer-Jones (2018)](https://arxiv.org/abs/1708.08595) (using Gaia DR2 data) attempted to correct for this incompleteness and estimated an encounter rate within 5 pc of $545 \pm 59$ per million years. With the inverse-square scaling mentioned above (and also advocated by Bailer-Jones), this becomes **about 1 encounter within 1 pc every 50,000 years or about 1 encounter within 1 light year every 0.5 million years.**

Assuming this rate is roughly correct over the whole lifetime of the Sun then this would yield 9000 close encounters over its 4.5 billion year lifetime (arguably there might have been a slightly smaller number of stars earlier in the Galaxy's history, but this might be outweighed by the Sun probably gradually migrating outwards from a radial position with a higher stellar density.)

Projecting this further, then if you are prepared to wait 50 million years, then you might expect an encounter within 0.1 light years, which is in the Oort cloud and really starting to get interesting as far as the timescales of mass-extinction events go.


---

### Answer by username_3 on 2014-06-02. Score: 15

I was curious about the same things. I believe it was in the astronomy stack exchange I was referred to an online data base that gives position and velocity vectors for neighboring stars. From those I put together a spreadsheet. Here's a screen capture:

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

I only entered 48 of the closest stars so it's by no mean an exhaustive list.

It looks like your graphic matches my estimates which is reassuring. I don't know why the Ross Stars aren't in my list, possibly the omission is an error on my part when I was entering data to the spreadsheet.

Looks like the closest approaches are around 3 lightyears.

If each star has an Oort cloud, I believe the comets' velocity with regard to our sun would be pretty close to the star's relative velocity. Slowest star wrt our solar system seems to be Gliese 729 which is moving ~14 km/s wrt the sun.

If, for example, some of Van Maanen's Oort cloud came within a light year of our sun, they would have been moving 270 km/s. Those snowballs would have zoomed in and out of our neighborhood.

With these distances and relative velocities I don't see much opportunity for swapping comets.

It's speculated that our sun swapped comets with neighboring stars when our solar system was being formed. From [Wikipedia](http://en.wikipedia.org/wiki/Oort_cloud#Origin):

> Recent research has been cited by NASA hypothesizing that a large
> number of Oort cloud objects are the product of an exchange of
> materials between the Sun and its sibling stars as they formed and
> drifted apart, and it is suggested that many—possibly the majority—of
> Oort cloud objects were not formed in close proximity to the Sun.

> **Comments:**
> **username_0**: On the scale of 0.0002 billion years, movements of stars near the Sun is available. And interaction with siblings since the beginnings of earthly time is only one part of the equation. The other part is how the Sun might have interacted with comets (or spaceships?) from unrelated much older or younger stars, during the 16 or so times the Sun has orbited the Milky Way like once every 0.25 billion years. How frequently _should_ mass exchanging encounters have occured, is really my question. (Wiki's lack of dates makes terms like "recently" mean little in quickly developing fields of astronomy).  
> **username_0**: But I think you argue that stars and their comets are most likely to have higher than capture velocity relative to the solarsystem? And for that reason they are unlikely to stick around.  
> **username_3**: That's my thinking. Even if perihleion were 1/2 a light year, 14 km/s Vinf would be a hyperbola of very high eccentricity -- the path would look like a straight line. That's how I modeled the paths -- as straight lines.  
> **username_3**: It's my understanding they get the stars' distance by Parallax -- which is much more error prone as the stars go further. Radial velocity is measured by blue shift. And then distance plus angular velocity are used to get the other velocity components. So estimates are more prone to error with more distant stars.  
> **username_0**: So distances don't say everything, it's the delta-v that matters for exchanging stuff like comets. It might as well be easier for a more distant star to happen to launch a comet our way that our Sun is more likely to trap.  
> **username_3**: Our vision is murky but I imagine you're right. Over 4 billion years we might have swapped stuff with other stars doing slow fly bys.  
> **username_3**: Hyperbolas are symmetric. The incoming Vinfinity is the same as the outgoing Vinfinity. If it were solely 2 body interactions, we'd never capture a comet moving above escape velocity. However if a comet passes in front of a planet, the planet's gravity can throw it backwards thus reducing it's velocity with regard to the sun.  
> **username_3**: Let us [continue this discussion in chat](http://chat.stackexchange.com/rooms/14870/discussion-between-hopdavid-and-localfluff).  

---

