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I was wondering what kinds of stars would be visible in M31 and other nearby galaxies using a moderate (~8") telescope and a CCD camera. This would be a limiting magnitude of around 17.

I have heard of LBVs (Luminous Blue Variables). However, I would like to know what spectral class of stars would most likely be visible (would it be mainly OBA type stars)?

The context of this question is in the search for extragalactic transiting exoplanets. Current amateur detectors have the capability to detect planets with host star apparent magnitudes of 14. I was wondering if it might be possible to find exoplanets in M31, M33 or even the LMC/SMC with these limits. An advantage to searching for planets in M31 would be the very limited search region--an amateur astronomer would only have to perform photometry on the M31 region over several months or years to find a planet, rather than having to search a much larger region.

Are there any other issues with detecting transiting planets around very luminous stars? Would the percent dip in brightness be far less for a Jupiter sized planet? How would this affect our ability to detect extragalactic transiting exoplanets?

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  • $\begingroup$ This question has an answer which suggests maximum magnitude around 11.4. Might be useful. $\endgroup$ – Andy Jun 10 '16 at 8:53
  • $\begingroup$ Also there's a paper which attempted to catalogue the brightest stars. (Follow the link to the PDF.) It seems to distinguish reasonably well between foreground stars and actual members of M31. $\endgroup$ – Andy Jun 10 '16 at 8:56
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LBV stars are very powerful, very large and bright, and even though they may be near the end of their life, still young - since they don't survive for long. These are problematic.

Their powerful solar winds will clear the region around them, meaning that planets are less likely to form close to the star. It is much less likely for a LBV star to be aligned correctly to allow a transit to be visible from Earth. A hot Jupiter close to such a star would rapidly lose mass due to the extreme temperature.

If a transit does occur, because the star is so large, it obscures a small proportion of the light, so a the change in brightness is small. And of course they are intrinsically variable themselves, so detecting the very small signal is harder.

Since they are still young so planets may not have finished forming by the time they go supernova.

We haven't discovered any planets orbiting LBVs in the Milkyway.

Most stars that have known planets are F,K and G spectral type, this is partly sample bias (we look for planets around sun-like stars) But such stars would not be visible in M31 using amateur equipment.

An 8in telescope has a limiting magnitude of about 14, (E. Hubble was observing Cepheids with a mag of 16+) but you are already close to the limit of the telescope, and distinguishing the very small changes is consequently is harder. Given the difficulties, it is perhaps unsuprising that we haven't found many planets more that 10000 ly away.

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  • $\begingroup$ This is an excellent answer, thank you. As an extension to this question, do you think this would be possible using Kepler (especially K2 if it points to galaxies in the Local Group)? $\endgroup$ – AstronomyGeek Jun 13 '16 at 9:11

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