We now know of more than 5,000 exoplanets beyond the solar system. What we really understand about each of these worlds, though, is barely anything at all. Most of them have been seen only indirectly from their shadows as they cross in front of the stars they orbit. The few that researchers have managed to actually take a picture of—that is, to directly image using light emanating from the planets themselves—appear as little more than monochromatic dots even in the very best current telescopes. And so far all of those directly imaged worlds are among the brightest, largest and least Earth-like exoplanets known.
The far future may be a different matter. How detailed could a picture of a distant exoplanet be—especially one that is small and rocky like Earth? The answer is that someday astronomers could obtain images revealing continents, clouds, oceans, ice caps and even vegetation on some remote Earth-like world orbiting an alien star.
The problem is that the most powerful telescope for this task can’t be built—not exactly, anyway. Instead it must be conjured into existence using the tenets of Einstein’s general theory of relativity to transform our sun itself into a star-sized magnifying glass. Albert Einstein’s key insight—that gravity can be understood as the curvature of spacetime—means that stars and other massive objects act as natural “gravitational lenses” that warp and amplify the light from background objects.
Astronomers today routinely use galaxies and galaxy clusters as gravitational lenses, but the prospect of using this technique for our sun poses so many challenges that few researchers have taken it seriously. Most notably, the approach requires precisely positioning a conventional telescope—something like Hubble, for instance—at the point where any given target’s lens-amplified light comes to a focus. For the sun, those focal points are found at the extreme outskirts of the solar system—at least 14 times farther out than Pluto.
Now a new study by astronomers at Stanford University shows that a simplifying shortcut could exist for the still arduous task of imaging exoplanets using our sun as a cosmic telescope. The study, published in the Astrophysical Journal, suggests astronomers could eventually achieve exoplanet imaging with a resolution 1,000 times greater than that of the Event Horizon Telescope, which has been used to capture the historic first images of supermassive black holes. “It’s just neat to think of this as kind of the ultimate end game of the process of studying exoplanets,” says Bruce Macintosh, a Stanford astrophysicist, who co-authored the paper, “or at least the end game short of actually visiting them.”
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