When it comes to transmission spectroscopy of exoplanet atmospheres, the limiting factor is the host star's apparent brightness, not the distance. If a star in the Andromeda Galaxy had a transiting planet, and was bright enough to be V = 10, we could study it's planet just as well as we could if a V = 10 star in the solar neighbourhood had a transiting planet.
The second major factor is the radius ratio. The planet-to-star radius ratio should be fairly high to ensure a greater signal-to-noise ratio.
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