This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Within just a few years, astronomers may at last find a planet that shows signs of life as we know it, in the form of atmospheric gases that betray signs of biological activity. This would be a transformational event for our civilization. But, what would we do next? How could we explore this alien world? With the current state of our technology, sending a robotic spacecraft to visit a planet that is light-years away is simply not possible. And no telescope in existence or even in the design stage would be capable of imaging such a world except as a pinpoint of light—a single pixel in the most advanced detector, which would give no details at all about what the surface of this exoplanet actually looks like.
That is true of human-built telescopes, at least. But Nature has gifted us with a powerful magnifying instrument that existed for billions of years before the human race evolved. It’s the Sun, whose intense gravity warps spacetime in its immediate neighborhood, bending the path of light rays passing nearby. In 1919, this light-bending was seen to alter the apparent positions of distant stars during a solar eclipse, vindicating Albert Einstein’s recently-published General Theory of Relativity. And in the 1930s, Einstein calculated that if two stars were lined up just right along our line of sight, the light-bending effect would allow the closer star to magnify the image of the more distant one.
Using galaxies and galaxy clusters rather than stars as the magnifiers, astronomers have used this so-called gravitational lensing effect to observe distant cosmic structures that would otherwise be too faint to see. But to date our Sun’s magnifying effects have scarcely been exploited. Soon, that could change. We have calculated that a modest telescope located approximately 50 billion miles from the Sun, at the focus of its lensing effect, could magnify the image of an exoplanet 100 light-years away by a factor of 100 billion.
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The result would be more than a single pixel—it would be an image a thousand pixels wide. That’s vastly more detailed than pictures of Pluto taken with the Hubble Space Telescope prior to the New Horizons mission—detailed enough to see surface features such as continents, oceans, mountain ranges and deserts. To get this resolution without the magnifying power of the Sun, we calculate that you would need a telescope with a diameter of about 75,000 kilometers, or about six times the diameter of the Earth. This is, to put it mildly, impractical.
It sounds simple enough, but such a mission would face significant challenges. To start with, it would have to stare directly at the Sun. Therefore, the small space telescope would require the ability to block most of the Sun’s light. This can be done with a help of an onboard instrument called a coronagraph, which creates what amounts to an artificial solar eclipse. No existing coronagraph could do this (they are mostly used to block the pinpoint light of distant stars, not the blazing close-up glare of the Sun).
But given the rapid development of coronagraphic technology, such capabilities are probably not far off. Alternatively, the spacecraft could use a starshade—an independent spacecraft, positioned precisely to block the Sun. These are also currently in development. Starlight from the parent star, which causes trouble for more conventional planet-imaging schemes, will be a factor of 10 million times weaker than light from the planet and much dimmer than our own Sun’s corona.
The image seen by our space telescope will not look like a planet, however. It will look like a ring of light surrounding the blotted-out Sun—and that ring (known as an Einstein ring) will contain the reflected light not of the entire planet, but only from a small region on its surface. So imaging the planet’s entire starlit surface would be done on a pixel-by-pixel basis, by moving the spacecraft in a spiral fashion as it slowly corkscrews its way around the Sun’s far-distant gravitational focus. At each position in the spiral, the telescope would sample slightly different Einstein rings containing amplified images of different areas of the remote planet’s surface.
Although very powerful, the Sun is not a very good lens in a traditional sense; its magnified images will be highly blurred, with any given pixel containing light reflected from adjacent regions on the surface of the exoplanet. This type of aberration would require correction through modern image reconstruction techniques. Fortunately, the planet’s rotation would provide periodic changes that would be helpful for guiding that reconstruction.
Clearly, this mission is daunting in every area of mission design and operations. Direct imaging of an exoplanet, in general, requires overcoming several key technological challenges. However, most of these challenges could be addressed with already existing capabilities, and engineers are making great progress in small-spacecraft development. To reach the necessary distances, our mission would employ a Jupiter flyby followed by a low-perihelion escape maneuver near the Sun.
Alternatively, propellant-free propulsion techniques such as sunlight-reflecting “solar sails” could allow high escape velocities with perihelia of 20 solar radii, but these would require sail area-to-mass ratios larger than the current state-of-the-art. Either approach could reach the solar lens’ focal line in 25-30 years. Although it seems feasible, the engineering aspects of building an astronomical facility on scales this large are still unexplored; only recently did we begin to consider such concepts.
While all currently envisioned NASA exoplanetary concepts aim at getting just a single pixel to study an exoplanet, a mission such as this opens a breathtaking possibility for directmegapixel high-resolution imaging and spectroscopy of a potentially habitable exoplanet at a distance of up to 100 light years, with resolution of a few kilometers on its surface over a broad range of wavelengths. This could provide a powerful diagnostic for the atmosphere, surface material characterization and biological processes on a distant twin or close sibling of our own familiar Earth. This would be the next major step, possibly the biggest step in the 21st century for exoplanet exploration.