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In the following discussion I will refer to Earth-like exoplanets planets orbiting in the habitable zone of sunlike stars simply as “Earth-like exoplanets."

NASA is currently developing or considering three space telescopes that would be able to directly image Earth-like exoplanets:

  • The WFIRST space telescope (if it’s equipped with a starshade).

  • The HabEx space telescope, which would be equipped with both a starshade and a coronagraph.

  • The LUVOIR space telescope, which would use a coronagraph to block the light of an exoplanet’s star.

I can think of three possible reasons why it might not be feasible to build a ground-based telescope that can (like the three space-based telescopes) directly image Earth-like exoplanets:

  • Reason 1: A starshade is one of the best technologies for blocking the light of an exoplanet’s star, and it seems very difficult to use a starshade with a ground-based telescope.

    Reason 1 seems valid, but it doesn’t seem that it can be the whole story because (as discussed above) the LUVOIR telescope will only use a coronagraph to block the light of an exoplanet’s star, and the HabEx telescope will use a starshade for some observations and a coronagraph for other observations.

  • Reason 2: Maybe the type of coronagraphs planned for the LUVOIR and HabEx telescopes wouldn’t work for a ground-based telescope.

  • Reason 3: One of the main reasons why scientists want to directly image Earth-like exoplanets is to spectroscopically examine their atmospheres to look for possible signs of life such as oxygen and methane. Maybe spectroscopically examining the atmospheres of Earth-like exoplanets requires imaging in the infrared spectrum, which isn’t possible for ground-based telescopes.

Are these three reasons valid, and do they imply that it isn’t feasible to build a ground-based telescope that can directly image Earth-like exoplanets? Are there other factors that need to be considered? Or is it actually possible to build a ground-based telescope that can directly image Earth-like exoplanets?

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  • $\begingroup$ Check eso.org/sci/facilities/eelt/science and you will see that the prospects from ground for the next generation of giant telescopes is not great for what you want. Direct Imaging so close in needs next-gen coronography and image stability that can only be provided in space. For your point 3.) taking a spectrum requires vastly different equipment and is in a way simpler, particularly now that high-resolution radial velocity spectrographic techniques are being developed. Those can give you an atmospheric spectrum from the ground that's RV-separated from the stars, if strong enough $\endgroup$ – AtmosphericPrisonEscape Nov 21 '19 at 8:14
  • $\begingroup$ How much money do you have? In seriousness - large effective aperture scopes with high-density adaptive optic systems, plus all the blocking and filtering you refer to are technologically possible but at a cost (I believe) exceeding that of a space-based system including launch. $\endgroup$ – Carl Witthoft Nov 21 '19 at 16:37

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