Are images of exoplanets' surfaces technically possible?

Question

While already even taking an image of an extra solar planet sounds like science fiction to me, is it technically possible by what we know now to take also more detailed images of extraterrestrial planet surfaces?

For simplicity, let's take Earth-like exoplanets.

Gravitational lenses and the FOCAL spacecraft are first hints I could find so far, but there are seem to be some limitations:

• You can see only objects which are exactly behind the Sun, for specific exoplanets this limits possible view angles, but as this is so far distance, this should not be a problem?
• Magnification up to 10^15, which resolution of the surface does this allow for for say the closest Earth-like exoplanet (12 light years)
• The Wikipedia article on FOCAL says, "images would be hard to interpret" so does this mean pixels would be so random that combined with low res you can't tell what you see? Would we be even able to disambiguate cloud vs. surface boundaries?

UPDATE: NASA has funded a proposal to use solar gravity lens with a set of small satellites.

UPDATE2

https://futurism.com/nasa-telescope-vegetation-exoplanets

Answer 1

With a resolution of 1E-09 radians, Jupiter would be 1 pixel wide at 7 light years.

If you had 1E-10 radians, you could resolve the largest features.

Visible light has a wavelength of 5E-07 meters, so if you wanted to use a cluster of telescopes to synthesize an aperture with some kind of coherent combination, "all you would need" is a 200 meter diameter aperture. It wouldn't have to be solid or filled, a sampled aperture would do it.

In fact, a sampled aperture can be optimized to improve contrast in some areas by loosing efficiency and contrast in other areas. See this answer to the question "What makes small interferometers useful? Like NIRISS on JWST"

You can also read more about it in:

• JWST's NIRISS Aperture Masking Interferometry
• ArXiv NIRISS aperture masking interferometry: an overview of science opportunities
• The JAM Team's: Non-Redundant Aperture Masking Interferometry (AMI) and Segment Phasing with JWST-NIRISS

In June 2018 Science there is a short article talking about the use of optical interferometry from multiple telescopes for imaging. Optical interferometers sharpen views of the sky.

above: "Boiling cells of plasma were seen on a distant red giant star using an optical interferometer in Chile." Source Presumably this is the VLA.

See also 1, 2, 3

---

There is research (beyond the scope of this answer) investigating the use of quantum entanglement to dramatically simplify the interferometric combination of broadband signals from multiple telescopes as well. Here's the paper I was thinking of:

Longer-Baseline Telescopes Using Quantum Repeaters: ArXiv and Phys. Rev. Lett. 109, 070503 – Published 16 August 2012, Daniel Gottesman, Thomas Jennewein, Sarah Croke

We present an approach to building interferometric telescopes using ideas of quantum information. Current optical interferometers have limited baseline lengths, and thus limited resolution, because of noise and loss of signal due to the transmission of photons between the telescopes. The technology of quantum repeaters has the potential to eliminate this limit, allowing in principle interferometers with arbitrarily long baselines.

Answer 2

As @uhoh has pointed out, optical interferometers only a little bigger than the ones we have now would allow imaging of large-scale features of large planets around nearby stars.

For a more detailed view of smaller or more distant planets we either need much larger interferometers (the optical equivalent of VLBI techniques used for radio) or to use something exotic like a gravitational lens.

We can do the numbers for optical VLBI. Suppose for example we could build an array of telescopes distributed across the Moon's disk for an aperture of 3000km (and no atmospheric interfernce), at (for convenience of arithmetic) 300nm in the near UV. We would have a resolution of about $10^{-13}$ radians, which gives a resolution of 1km at 1 light year, or 1000km (a continent) at 1000 ly.

Somewhat more speculatively, if we could put our array into solar orbits the same size as that of Saturn and somehow figure out how to combine the signals, we would have an aperture of 3 bn km, and resolution of 1mm at 1ly or a few km at Andromeda.

Using the Sun's gravitational lens is tricky, but potentially very powerful. There is a very readable article assessing some of the problems:

Abstract The gravitational field of the sun will focus light from a distant source to a focal point at a minimal distance of 550 Astronomical Units from the sun. A proposed mission to this gravitational focus could use the sun as a very large lens, allowing (in principle) a large amplification of signal from the target, and a very high magnification. This article discusses some of the difficulties involved in using the sun as such a gravitational telescope for a candidate mission, that of imaging the surface of a previously-detected exoplanet. These difficulties include the pointing and focal length, and associated high magnification; the signal to noise ratio associated with the solar corona, and the focal blur. In addition, a method to calculate the signal gain and magnification is derived using the first-order deflection calculation and classical optics, showing that the gain is finite for an on-axis source of non-zero area.

One big issue is that to image an object in a certain direction, you need to go 550AU from the Sun in the exactly opposite direction, and the field of view is very narrow, so much so that an exoplanet would move across the field of view of a reasonable sized spacecraft in a fraction of a second due to its motion around its star.

While it might be possible to gain useful scientific information from a mission like this, it will take a lot of post-processing to produce anything recognisable as an image to a human eye.