Are there only planets in the Milky Way galaxy, or are there other planets in other galaxies?

If planets are only in the Milky Way, why aren't there planets in others?

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    $\begingroup$ Come on... Really? You couldn't bother to google 'planet'? Here you go... en.wikipedia.org/wiki/Planet. $\endgroup$ – Danu Aug 22 '14 at 16:41
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    $\begingroup$ This question appears to be off-topic because it does not meet the minimum level of research effort to make for an acceptable question. $\endgroup$ – Danu Aug 22 '14 at 16:42
  • $\begingroup$ I would agree that there is little research effort, but I can understand the motivations for asking it. After all, how many extra-galactic planets have we found? Logic says that there should be as many in every other galaxy as there are in the Milky Way, but we haven't had much success finding them. But yes, logic would answer the question pretty well. By the way, why not ask this on the Astronomy SE in the first place (note: not a suggestion)? $\endgroup$ – HDE 226868 Aug 22 '14 at 16:50
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    $\begingroup$ @Danu I couldn't find this specific point on the Wikipedia page either. This is a reasonable enough (if simple question). $\endgroup$ – Emilio Pisanty Aug 22 '14 at 17:24
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    $\begingroup$ @Mehrdad: I think the fascination is twofold. (1) we've only just developed technology to detect them, so it's a growth industry just now. (2) we're interested to know how common earth-like planets are, because we're interested in life and the only kind of life we can confidently predict is possible, is the kind of life we see on earth. Note that finding extrasolar gas giants is already pretty boring to general news reporters, they want a better chance of liquid water than ever before or they're not running it. Planets with exotic chemistry also stand a chance, e.g. giant diamonds ;-) $\endgroup$ – Steve Jessop Aug 22 '14 at 23:01

Observing planets in other galaxies is really hard to do because they are so far away and planets are so small. One of our closest neighbors, the Andromeda Galaxy (also called M31), is about $10^{19}$ km away (just under 780 kpc), so finding a planet the size of Jupiter (roughly $10^5$ km diameter) is pretty tough (radius to distance is very small). Even the closer neighbors, the Magellanic clouds at distances of 50 and 60 kpc (LMC & SMC respectively), are still over $10^{18}$ km away so finding planets orbiting stars there are equally challenging.

It has been proposed that one can use microlensing to detect (large) planets orbiting stars. Some people have claimed to have found a star with an gravitational lensing aberration that could be the result of a planet. This is really a guessing game because the resolution is, at best, a few pixels on a CCD.

We do not believe that the Milky Way galaxy is unique in its development of planets, especially given the findings of the Kepler mission (prior to its failure) that there's roughly 1.6 planets per star. But we can't really say that there is indeed planets in other galaxies simply because we haven't had observations confirming the hypothesis.

  • $\begingroup$ It's difficult to take this question seriously, but it does have merit, I suppose, if we hadn't observed planets from other galaxies, but hasn't measuring the effects of planetary transit on the light emitted from far away stars shown that they do have objects orbiting them? How about the effect planets have on the motion of the stars? $\endgroup$ – stephenbayer Aug 22 '14 at 20:05
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    $\begingroup$ The light transit is unobservable due to the extreme distances, the only hypothetical way is the gravitational lensing mentioned in the 2nd paragraph. I don't know that we can even really track the positions of the stars well enough to apply Kepler's laws. $\endgroup$ – Kyle Kanos Aug 22 '14 at 20:08
  • $\begingroup$ Using gravitational lensing is still giving inconsistent results. Recently there was some hype over finding several exo-planets in a single solar system (Gliese 581 star system) that were later decided to be calculation errors (at least for the most promising exo-planets). Here's a link to the article I read: arstechnica.com/science/2014/07/…. $\endgroup$ – Jasper Aug 22 '14 at 23:43
  • $\begingroup$ @Jasper: Yes, it is unreliable because it is basically a one-pixel shift in the CCD image. $\endgroup$ – Kyle Kanos Aug 22 '14 at 23:44
  • $\begingroup$ Our closest neighbors are Magellanic Clouds (LMC&SMC) and some other dwarf galaxies. M31 is much farther than those. I don't want to downvote your answer but please fix this mistake. By the way, some methods for finding extrasolar planets would probably work for Magellanic Clouds as well. $\endgroup$ – firtree Aug 23 '14 at 0:00

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