<p>This became too long for a comment, but shouldn't be considered a definite answer:</p>
<p>Until recently, almost every rogue planet was found with microlensing, meaning that they're only observable for a brief moment, and hence making follow-up spectroscopy — to reveal the constituents of their atmosphere — very hard.</p>
<p>Moreover, by far the most rogue planets are &gt;Jupiter-sized planets, implying that they're probably not habitable (it's just gas…). An exception is <a href="https://ui.adsabs.harvard.edu/abs/2020ApJ...903L..11M" rel="noreferrer">Mróz et al. (2020)</a>, but since the microlensing event is shorter for low masses, this event only took some 40 minutes.</p>
<p>A recent paper by <a href="https://ui.adsabs.harvard.edu/abs/2022NatAs...6...89M" rel="noreferrer">Miret-Roig et al. (2022)</a> reported the detection of ~100 planets. But these were discovered with imaging due to the heat they give off, and hence are also all an order of magnitude more massive than Jupiter.</p>
<p>As PM 2Ring comments, life on a planet without an energy-infusing star is difficult to imagine. However, the idea is not completely crazy, since a planet ejected from its solar system could maintain an acceptable temperature from geothermal heat (from radioactive isotopes) and geological activity for billions of years, and even though its atmosphere may escape, it could still have an ocean. Such possibilities have been discussed e.g. by <a href="https://ui.adsabs.harvard.edu/abs/1999Natur.400...32S" rel="noreferrer">Stevenson (1999)</a> and <a href="https://ui.adsabs.harvard.edu/abs/2011ApJ...735L..27A" rel="noreferrer">Abbot &amp; Switzer (2011)</a>. The latter article concludes that such a planet would need to come within the order of 1000 AU in order to be detectable, which is probably not very likely.</p>
<p>So while I can't really say how active this field is, it definitely <em>is</em> a field. But my uneducated guess is that we're still quite far from being able to study the habitability of rogue planets (but note that I'm not a rogue planet hunter).</p>

CC BY-SA 4.0
2022-03-07T21:36:10.067Z
This is an interesting background information, but it doesn't address the question.  Moreover the question has now been changed...invalidating this answer.
James K
CC BY-SA 4.0
2022-03-08T11:55:19.910Z
@JamesK Sorry, I got a bit carried away. But I think answering "_How_ active is the field of XX" is not answerable in a definite way (because on which scale?), so instead I tried to demonstrate that 1) it makes sense to ask the question, and 2) the field _is_ active to some degree, but 3) it is not something readily accessible for us to study observationally.
pela
CC BY-SA 4.0
2022-03-08T12:00:00.630Z
Aw man, sorry once again, I just realized the question was about inter*galactic* planets, not inter*stellar*  I suppose I should delete this answer…
pela
CC BY-SA 4.0
2022-03-07T17:02:56.743Z
2022-03-07T17:02:56.743Z
pela
<p>Disclaimer, I've never worked in this field of astrophysics. However, intuitively, I would be skeptical of:</p>
<blockquote>
<p>the habitablity of planets AROUND interglactic rogue STARS, and how those planets form</p>
</blockquote>
<p>Because for rogue stars to end up in intergalactic space, they were presumably ejected from their galaxies. This then ceases to be very interesting because star formation within galaxies is relatively well-understood, and galaxy dynamics leading to a star being ejected is also relatively well-understood, and we have no reason to think that the habitability of stars which are ejected are likely to be different from stars that aren't ejected.</p>
<p><strong>However</strong>: you really should check out the literature and discuss it with your supervisor. It won't be trivial, but it should be something that a person of your caliber can do. Go to Web of Science or Google Scholar and search for example for &quot;intergalactic habitability&quot;. A quick try myself didn't find anything (which is good, since it indicates this could be something to write a thesis on), but did find <a href="https://www.researchgate.net/profile/Joseph-Breeden/publication/325688096_Catching_a_ride_using_hypervelocity_stars_for_interstellar_and_intergalactic_travel/links/5b1e531ba6fdcca67b698958/Catching-a-ride-using-hypervelocity-stars-for-interstellar-and-intergalactic-travel.pdf" rel="nofollow noreferrer">this related paper</a>. I quote the abstract:</p>
<blockquote>
<p>Hypervelocity stars are unique among the stars in the galaxy for their extreme
velocities relative to the galactic center, in some cases achieving galactic escape
velocity. Dozens of hypervelocity star candidates have been identified so far. One
population includes B-type stars apparently ejected from the galactic core. A second
population has been identified within the plane of the galaxy with no single origin. As a fast-moving energy source, hypervelocity stars could be uniquely valuable property in the galaxy for advanced civilizations. Given their potential for transportation and exploration across the plane of the galaxy or to other galaxies, they could serve as prime objects for Search for Extraterrestrial Intelligence (SETI) searches or as long-duration transports for far-future human exploratory missions. In addition, some hypervelocity stars may be entering our galaxy from extragalactic sources, making them possible mechanisms by which intelligences in neighboring galaxies could be exploring ours.</p>
</blockquote>
<p>It's then up to you to see if there's any reason these hypervelocity stars should be different from the ones that remain bound to our galaxy. For example, is the ejection process somehow correlated to whether the star is likely to have habitable planets? <em>A priori</em> the answer is &quot;no&quot; (per what I wrote above), but there could be. You won't know till you search the literature.</p>
<p>Then, after you have a reasonable sense of what's already in the literature, you can talk to your supervisor about the feasibility of the topic.</p>

CC BY-SA 4.0
2022-03-08T02:52:25.487Z
2022-03-08T02:52:25.487Z
Allure
<p>Is the habitability of intergalactic rogue planets something that has been studied a lot?</p>
<p>The reason that I'm asking is that I've just started my PhD program last year and I'm wondering/concerned if such a project would ensure that I was competitive when applying for postdoc position.</p>
<p>Is this topic very common? Should I even be thinking about this as the topic of my dissertation?</p>
<p>How active is this field of research?</p>
<p>Let me rephrase: the habitablity of planets AROUND interglactic rogue STARS, and how those planets form</p>

CC BY-SA 4.0
2022-03-06T20:55:27.080Z
This kind of question is far better discussed with your supervisor.  I don't think that members of this forum are in the right place to tell you what you should and should not be thinking.
James K
CC BY-SA 4.0
2022-03-07T01:59:39.577Z
@JamesK I think "Is X studied a lot" can be answered here. Questions about future projects *can be* well received and answered here, e.g. [Current topics on Radio Astronomy and looking for advice](https://astronomy.stackexchange.com/q/41209/7982) though it seems that [this one](https://astronomy.stackexchange.com/q/28907/7982) got prematurely closed preventing anyone from posting an answer :-( I think it could have been reworded slightly and kept open.
uhoh
CC BY-SA 4.0
2022-03-07T02:05:59.287Z
Hi @figureskater and *Welcome to Stack Exchange!* I've made some edits to your question to improve the fit for this site. Do you really only want to ask about the *habitability* of *intergalactic* expoplanets? You could ask about *interstellar* rogue planets for which there may be more research, and if you open up your question to other aspects besides habitability there may be even more. But maybe that's exactly your point!
uhoh
CC BY-SA 4.0
2022-03-07T02:07:38.720Z
See for example [Estimates for "undetectable" planets in extra-solar systems](https://astronomy.stackexchange.com/q/28383/7982) and all of the 46 other questions tagged with [rogue-planet](https://astronomy.stackexchange.com/questions/tagged/rogue-planet)
uhoh
CC BY-SA 4.0
2022-03-07T03:15:18.307Z
I'm intrigued by the notion that an intergalactic or even interstellar rogue planet could be considered habitable. There's generally not much energy available on such bodies, apart from the heat of formation, and energy released by long-lived radioactive isotopes.
PM 2Ring
CC BY-SA 4.0
2022-03-07T06:07:55.723Z
@uhoh I'm not sure that your edit really helps much.  I really think that we (ie this internet community) are the wrong people to answer this, and so any answer would be at best irrelevant and at worst harmful. We don't know the OP well, we don't their supervisor. So answers would be opinion based.
James K
CC BY-SA 4.0
2022-03-07T07:37:19.020Z
@JamesK "Is the habitability of intergalactic rogue planets something that has been studied a lot?" and "How active is this field of research?" seem readily answerable in a fact-based way without telling anybody to or not to do anything. There's no need to pre-block the entire community for an opportunity to post an answer just because a few people won't. Why not just give it a day or two for the OP to check the edits and especially [respond to this comment](https://astronomy.stackexchange.com/posts/comments/106918) before the answer-blocking begins?
uhoh
CC BY-SA 4.0
2022-03-07T09:02:40.023Z
Habitability of objects in deep space outside the influence of any star and its insolation... not sure it's worth spending much time on after considering the energy balance.
planetmaker
CC BY-SA 4.0
2022-03-07T11:14:08.247Z
Probably not exactly what you’re going for, but rouge brown dwarfs are a fairly significant point of research amongst some; there are several reasons why, but a couple have to do with them acting as dark matter candidates, the stark contrast in how many there should be vs how many we observe, etc
Justin T
CC BY-SA 4.0
2022-03-07T21:40:03.680Z
The point of "answer blocking" is to prevent answers like Pela's - which is interesting and well written, but doesn't answer the question that the OP asked  "Should I study this for my PhD?"  Now the OP has edited, and invalidated Pela's answer.  Being "welcoming" doesn't mean never closing first time questions.  And this question should have been closed.
James K
<astrophysics><exoplanet><habitable-zone><rogue-planet>
Astrophysics Ph.D. thesis on intergalactic rogue planets and their habitability; how active is this field of research?
figureskater
# Astrophysics Ph.D. thesis on intergalactic rogue planets and their habitability; how active is this field of research?

(Asked by: username_0 on 2022-03-06)

Is the habitability of intergalactic rogue planets something that has been studied a lot?

The reason that I'm asking is that I've just started my PhD program last year and I'm wondering/concerned if such a project would ensure that I was competitive when applying for postdoc position.

Is this topic very common? Should I even be thinking about this as the topic of my dissertation?

How active is this field of research?

Let me rephrase: the habitablity of planets AROUND interglactic rogue STARS, and how those planets form

**Tags:** astrophysics, exoplanet, habitable-zone, rogue-planet

> **Comments:**
> **username_1**: This kind of question is far better discussed with your supervisor.  I don't think that members of this forum are in the right place to tell you what you should and should not be thinking.  
> **username_2**: @JamesK I think "Is X studied a lot" can be answered here. Questions about future projects *can be* well received and answered here, e.g. [Current topics on Radio Astronomy and looking for advice](https://astronomy.stackexchange.com/q/41209/7982) though it seems that [this one](https://astronomy.stackexchange.com/q/28907/7982) got prematurely closed preventing anyone from posting an answer :-( I think it could have been reworded slightly and kept open.  
> **username_2**: Hi @username_0 and *Welcome to Stack Exchange!* I've made some edits to your question to improve the fit for this site. Do you really only want to ask about the *habitability* of *intergalactic* expoplanets? You could ask about *interstellar* rogue planets for which there may be more research, and if you open up your question to other aspects besides habitability there may be even more. But maybe that's exactly your point!  
> **username_2**: See for example [Estimates for "undetectable" planets in extra-solar systems](https://astronomy.stackexchange.com/q/28383/7982) and all of the 46 other questions tagged with [rogue-planet](https://astronomy.stackexchange.com/questions/tagged/rogue-planet)  
> **username_3**: I'm intrigued by the notion that an intergalactic or even interstellar rogue planet could be considered habitable. There's generally not much energy available on such bodies, apart from the heat of formation, and energy released by long-lived radioactive isotopes.  
> **username_1**: @username_2 I'm not sure that your edit really helps much.  I really think that we (ie this internet community) are the wrong people to answer this, and so any answer would be at best irrelevant and at worst harmful. We don't know the OP well, we don't their supervisor. So answers would be opinion based.  
> **username_2**: @JamesK "Is the habitability of intergalactic rogue planets something that has been studied a lot?" and "How active is this field of research?" seem readily answerable in a fact-based way without telling anybody to or not to do anything. There's no need to pre-block the entire community for an opportunity to post an answer just because a few people won't. Why not just give it a day or two for the OP to check the edits and especially [respond to this comment](https://astronomy.stackexchange.com/posts/comments/106918) before the answer-blocking begins?  
> **username_4**: Habitability of objects in deep space outside the influence of any star and its insolation... not sure it's worth spending much time on after considering the energy balance.  
> **username_5**: Probably not exactly what you’re going for, but rouge brown dwarfs are a fairly significant point of research amongst some; there are several reasons why, but a couple have to do with them acting as dark matter candidates, the stark contrast in how many there should be vs how many we observe, etc  
> **username_1**: The point of "answer blocking" is to prevent answers like Pela's - which is interesting and well written, but doesn't answer the question that the OP asked  "Should I study this for my PhD?"  Now the OP has edited, and invalidated Pela's answer.  Being "welcoming" doesn't mean never closing first time questions.  And this question should have been closed.  

---

## 2 answers

### Answer by username_6 on 2022-03-07. Score: 5

This became too long for a comment, but shouldn't be considered a definite answer:

Until recently, almost every rogue planet was found with microlensing, meaning that they're only observable for a brief moment, and hence making follow-up spectroscopy — to reveal the constituents of their atmosphere — very hard.

Moreover, by far the most rogue planets are >Jupiter-sized planets, implying that they're probably not habitable (it's just gas…). An exception is [Mróz et al. (2020)](https://ui.adsabs.harvard.edu/abs/2020ApJ...903L..11M), but since the microlensing event is shorter for low masses, this event only took some 40 minutes.

A recent paper by [Miret-Roig et al. (2022)](https://ui.adsabs.harvard.edu/abs/2022NatAs...6...89M) reported the detection of ~100 planets. But these were discovered with imaging due to the heat they give off, and hence are also all an order of magnitude more massive than Jupiter.

As username_3 comments, life on a planet without an energy-infusing star is difficult to imagine. However, the idea is not completely crazy, since a planet ejected from its solar system could maintain an acceptable temperature from geothermal heat (from radioactive isotopes) and geological activity for billions of years, and even though its atmosphere may escape, it could still have an ocean. Such possibilities have been discussed e.g. by [Stevenson (1999)](https://ui.adsabs.harvard.edu/abs/1999Natur.400...32S) and [Abbot & Switzer (2011)](https://ui.adsabs.harvard.edu/abs/2011ApJ...735L..27A). The latter article concludes that such a planet would need to come within the order of 1000 AU in order to be detectable, which is probably not very likely.

So while I can't really say how active this field is, it definitely *is* a field. But my uneducated guess is that we're still quite far from being able to study the habitability of rogue planets (but note that I'm not a rogue planet hunter).

> **Comments:**
> **username_1**: This is an interesting background information, but it doesn't address the question.  Moreover the question has now been changed...invalidating this answer.  
> **username_6**: @JamesK Sorry, I got a bit carried away. But I think answering "_How_ active is the field of XX" is not answerable in a definite way (because on which scale?), so instead I tried to demonstrate that 1) it makes sense to ask the question, and 2) the field _is_ active to some degree, but 3) it is not something readily accessible for us to study observationally.  
> **username_6**: Aw man, sorry once again, I just realized the question was about inter*galactic* planets, not inter*stellar*  I suppose I should delete this answer…  

---

### Answer by username_7 on 2022-03-08. Score: 0

Disclaimer, I've never worked in this field of astrophysics. However, intuitively, I would be skeptical of:

> the habitablity of planets AROUND interglactic rogue STARS, and how those planets form

Because for rogue stars to end up in intergalactic space, they were presumably ejected from their galaxies. This then ceases to be very interesting because star formation within galaxies is relatively well-understood, and galaxy dynamics leading to a star being ejected is also relatively well-understood, and we have no reason to think that the habitability of stars which are ejected are likely to be different from stars that aren't ejected.

**However**: you really should check out the literature and discuss it with your supervisor. It won't be trivial, but it should be something that a person of your caliber can do. Go to Web of Science or Google Scholar and search for example for "intergalactic habitability". A quick try myself didn't find anything (which is good, since it indicates this could be something to write a thesis on), but did find [this related paper](https://www.researchgate.net/profile/Joseph-Breeden/publication/325688096_Catching_a_ride_using_hypervelocity_stars_for_interstellar_and_intergalactic_travel/links/5b1e531ba6fdcca67b698958/Catching-a-ride-using-hypervelocity-stars-for-interstellar-and-intergalactic-travel.pdf). I quote the abstract:

> Hypervelocity stars are unique among the stars in the galaxy for their extreme
> velocities relative to the galactic center, in some cases achieving galactic escape
> velocity. Dozens of hypervelocity star candidates have been identified so far. One
> population includes B-type stars apparently ejected from the galactic core. A second
> population has been identified within the plane of the galaxy with no single origin. As a fast-moving energy source, hypervelocity stars could be uniquely valuable property in the galaxy for advanced civilizations. Given their potential for transportation and exploration across the plane of the galaxy or to other galaxies, they could serve as prime objects for Search for Extraterrestrial Intelligence (SETI) searches or as long-duration transports for far-future human exploratory missions. In addition, some hypervelocity stars may be entering our galaxy from extragalactic sources, making them possible mechanisms by which intelligences in neighboring galaxies could be exploring ours.

It's then up to you to see if there's any reason these hypervelocity stars should be different from the ones that remain bound to our galaxy. For example, is the ejection process somehow correlated to whether the star is likely to have habitable planets? *A priori* the answer is "no" (per what I wrote above), but there could be. You won't know till you search the literature.

Then, after you have a reasonable sense of what's already in the literature, you can talk to your supervisor about the feasibility of the topic.


---

