Astrophysics Ph.D. thesis on intergalactic rogue planets and their habitability; how active is this field of research?

Question

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

Answer 1

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), 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) 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 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 Stevenson (1999) and Abbot & Switzer (2011). 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).

Answer 2

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. 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.