I just watched this SpaceRip video on YouTube which shows pictures taken by Hubble while looking into the disk of the Andromeda galaxy to study a certain type of variable star. It occurred to me that if we can study individual stars in Andromeda, we must be able to use most of our methods of exoplanet detection on them.

Is this true? If so, why not look for exoplanets in another galaxy? My logic tells me that since it's difficult to look through the disk of our own galaxy (since we're inside it), we're limited to a relatively local region for detecting exoplanets. But if we turned our sights to Andromeda, couldn't we study the differences in planet formation between different regions of a galaxy?

I also suspect that the reason we're not doing this is because we would need much more sensitive instruments for this, and because we already have one planet finding telescope in space (so it'd be tough to justify sending up another one).

Am I mostly right in this reasoning? Are we capable of searching for exoplanets in Andromeda? Would there be a benefit to doing it (besides the obvious cool factor)? Is cost pretty much the only limiting factor?


3 Answers 3


Unfortunately, it's highly highly unlikely. We're barely even capable of identifying individual stars within the Andromeda galaxy (and the ones we can identify, if any, are almost all supergiants - these are pretty much the only stars we can identify in the Local Magellanic Clouds, which are closer than Andromeda). Even if we were able to identify individual stars in Andromeda, effects like transits could be the result of dwarf stars rather than actual planets (since dwarf stars are far too small to be seen with any telescope that we can get in the near-future)

  • $\begingroup$ Good point, I hadn't considered other junk floating around in the galaxy that we couldn't detect $\endgroup$ Jul 21, 2011 at 20:42
  • $\begingroup$ This answer seems to be wrong, see e.g. slate.com/technology/2015/01/… $\endgroup$
    – Allure
    Feb 19, 2019 at 23:42

In fact, the first exoplanet in the Andromeda Galaxy may have already been discovered:



  • $\begingroup$ holy crap - that's amazing $\endgroup$ Jul 21, 2011 at 23:33
  • $\begingroup$ Was that based on a single microlensing event or several? $\endgroup$ Jul 22, 2011 at 1:06
  • $\begingroup$ @PeterMortensen Nothing actually came of that hype, and for good reason. They are using a small, ground-based telescope that can't even resolve individual stars - so they are looking for variations in things they can't see. The furthest exoplanet discovered to date is 8500 pc away (sort by distance), just 1% the distance to Andromeda. Also, unfortunately, microlensing is non-repeatable - you get one chance, and after that you will never see it again. $\endgroup$
    – user10851
    Feb 1, 2013 at 2:11
  • $\begingroup$ Second that, this answer should not be the accepted one to this question, as it is misleading though not exactly wrong. $\endgroup$
    – Thriveth
    Jun 6, 2013 at 13:16

"It is one of the deepest and most detailed images ever taken of a galaxy outside our own." Just imaging those stars required Hubble-level capabilities, and that's child's play compared to exoplanet detection. I don't think that doing even the Doppler-shift spectroscopic method would be feasible for a long time to come. Plus, there's decades of work already queued up in studying near-Earth exoplanets, which are much more intrinsically interesting to the human race anyway. Extragalactic exoplanet study doesn't seem to add much besides degree of difficulty.

  • $\begingroup$ The RadioAstron is due to launch this year, which will have a resolution 10,000 times that of Hubble (New Scientist 23 July 2011) - I'm not sure of the technicalities here, and therefore the practicalities - but I wondered how well this might help the case. $\endgroup$ Jul 21, 2011 at 20:00
  • $\begingroup$ I would still worry about the photometric depth, even if its resolution is spectacular. I don't have enough expertise on the subject to comment on visible vs. radio, but my last two sentences definitely still apply. $\endgroup$
    – Andrew
    Jul 21, 2011 at 20:23
  • $\begingroup$ Oh, I couldn't disagree with your answer. I was just throwing it out there, so to speak; and thanks for the further input. $\endgroup$ Jul 21, 2011 at 20:29
  • $\begingroup$ I see, thanks for the info. As far as what's intrisically interesting, personally I'd find extragalactic exoplanets way more awe inspiring than nearby ones, since we can't really get to even the nearby ones anyway -- and it's unlikely that we'd be able to detect radio signals from them unless they were directed at us. Mostly I thought it'd be interesting to see the patterns of planet formation in different regions of a spiral galaxy, but I guess we're a long way off of being able to do that. $\endgroup$ Jul 21, 2011 at 20:46

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