<p>I don't think we have a good estimate of the number of stars not in galaxies -- they're actually pretty hard to find unless they're really bright, since even if you spot it, how do you know that it's extragalactic?</p>

<p>But galaxy mergers (which are <em>very</em> common) tend to throw out large numbers of stars from the merging galaxies, so it seems likely there are many stars in space between the galaxies.</p>

<p>There is no reason why such stars can't have planets, but it's likely that the planets formed along with the star inside a galaxy and got ejected from the galaxy along with the star.</p>

<p>Stars (and planets) form when Giant Molecular Clouds collapse under their own gravity, the collapse frequently being triggered by tidal forces from a passing star or a nearby supernova.  It seems unlikely that there are many GMCs in intergalactic space, and there are certainly fewer encounters which might trigger collapse. So it seems likely that relatively few stars (or planets) actually form in intergalactic space.</p>

<p>Life does not seem to have evolved because of the existence of stars in the sky, so once a planet exists in a habitable zone, life should form regardless of where it is.</p>

<p>The Gaia mission is providing us with superb position and velocity data for billions of stars, and it has identified a handful (maybe more by now) of stars which seem clearly to have come from intergalactic space.  <a href="https://www.sciencedaily.com/releases/2018/10/181002103000.htm" rel="noreferrer">Here's an article</a> which covers the report last year.</p>

CC BY-SA 4.0
2019-02-13T21:57:26.823Z
2019-02-14T14:35:41.970Z
2019-02-14T14:35:41.970Z
Mark Olson
<p>I don't think it is common at all to <em>find</em> stars outside of galaxies, but most astronomers would agree that they must exist.</p>

<p>The problem is that individual stars are not very bright. We can see stars in our own Galaxy, but beyond that (in other galaxies or in the space between galaxies) we wouldn't be able to detect a single star on its own. We would generally observe star clusters (hundreds or thousands or tens of thousands of stars) in other galaxies as one bright blob, we can't see each star inside it.</p>

<p>Can you give us a source for the star with a planet that entered our Galaxy? I haven't heard of this so would need to see the details to explain where it came from.</p>

CC BY-SA 4.0
2019-02-15T00:27:49.037Z
The comment above yours from mistertribs, addressed the question of the Star with a Planet, that entered our Galaxy.
Peter U
CC BY-SA 4.0
2019-02-14T14:20:53.147Z
2019-02-14T14:20:53.147Z
FJC
<p>How common or rare is it to find stars outside of a galaxy? Can such a star still have planets that contain life? Recently, I think that I heard of a star with a planet that entered our Galaxy. Did this star come here from another galaxy or was it originally from the Milky Way?</p>

<star>
Stars located outside of a galaxy
Peter U
# Stars located outside of a galaxy

(Asked by: username_0 on 2019-02-13)

How common or rare is it to find stars outside of a galaxy? Can such a star still have planets that contain life? Recently, I think that I heard of a star with a planet that entered our Galaxy. Did this star come here from another galaxy or was it originally from the Milky Way?

**Tags:** star


---

## 2 answers

### Answer by username_1 on 2019-02-13. Score: 8

I don't think we have a good estimate of the number of stars not in galaxies -- they're actually pretty hard to find unless they're really bright, since even if you spot it, how do you know that it's extragalactic?

But galaxy mergers (which are *very* common) tend to throw out large numbers of stars from the merging galaxies, so it seems likely there are many stars in space between the galaxies.

There is no reason why such stars can't have planets, but it's likely that the planets formed along with the star inside a galaxy and got ejected from the galaxy along with the star.

Stars (and planets) form when Giant Molecular Clouds collapse under their own gravity, the collapse frequently being triggered by tidal forces from a passing star or a nearby supernova. It seems unlikely that there are many GMCs in intergalactic space, and there are certainly fewer encounters which might trigger collapse. So it seems likely that relatively few stars (or planets) actually form in intergalactic space.

Life does not seem to have evolved because of the existence of stars in the sky, so once a planet exists in a habitable zone, life should form regardless of where it is.

The Gaia mission is providing us with superb position and velocity data for billions of stars, and it has identified a handful (maybe more by now) of stars which seem clearly to have come from intergalactic space. [Here's an article](https://www.sciencedaily.com/releases/2018/10/181002103000.htm) which covers the report last year.


---

### Answer by username_2 on 2019-02-14. Score: 1

I don't think it is common at all to *find* stars outside of galaxies, but most astronomers would agree that they must exist.

The problem is that individual stars are not very bright. We can see stars in our own Galaxy, but beyond that (in other galaxies or in the space between galaxies) we wouldn't be able to detect a single star on its own. We would generally observe star clusters (hundreds or thousands or tens of thousands of stars) in other galaxies as one bright blob, we can't see each star inside it.

Can you give us a source for the star with a planet that entered our Galaxy? I haven't heard of this so would need to see the details to explain where it came from.

> **Comments:**
> **username_0**: The comment above yours from mistertribs, addressed the question of the Star with a Planet, that entered our Galaxy.  

---

