We now know of more than 5,000 exoplanets beyond the solar system. We know very little about each of these planets. They are mostly seen only indirectly through their shadows, which they cross in front the stars they orbit. Researchers have only managed to take a few pictures of the few that they can actually see using the light from the planets. These images are monochromatic dots even with the most advanced telescopes. All of the directly imaged worlds have been among the brightest, most massive and least Earth-like exoplanets.
The far future could be a different story. How detailed could a picture from a distant exoplanet be, especially if it is small and rocky as Earth? Astronomers may one day be able to obtain images that reveal continents, oceans, clouds, oceans, and even vegetation on distant Earth-like worlds orbiting an alien star.
The problem is that the most powerful telescope to accomplish this task cannot be built. It must be conjured using Einstein’s general theory relativity to transform the sun into a magnifying glass of star-sized size. Albert Einstein’s key insight, that gravity can be understood to be the curvature spacetime, means that stars and other large objects act as natural “gravitational lens” that warps and amplifies the light from the background.
Astronomers use galaxies, clusters of galaxies, and other objects as gravitational lenses. However, using this technique to find our sun presents so many challenges that very few researchers have taken it seriously. The most difficult aspect of the approach is precisely positioning a telescope, such as Hubble, at the point where the target’s lens-amplified sunlight comes to a focal point. For the sun, those focal points are found at the extreme outskirts of the solar system–at least 14 times farther out than Pluto.
A new study by astronomers from Stanford University suggests that there may be a shortcut to the difficult task of imaging exoplanets with our sun as a cosmic observatory. 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. Bruce Macintosh (a Stanford astrophysicist who co-authored this paper) said, “It’s just neat that you think of this kind of the ultimate goal of the process of studying other planets.”
Alex Madurowicz is Macintosh’s graduate student and co-author. He first fed satellite images of Earth into a computer modeling program that reduced the world to what it would look like if it were seen from far away through a stellar gravitational telescope. The resulting image would look like an “Einstein Ring” in most cases. This is a distortion of the star’s light that creates a circular, distorted circle around the lensing star. An earlier work done by Slava Turyshev, NASA’s Jet Propulsion Laboratory had shown that to correct those distortions, one would need to move a conventional light-gathering telescope back and forth within a region at the solar system’s edge. The resulting pixel-by-pixel scan of the planet’s warped projection, somehow choreographed from Earth upward of 80 billion kilometers away, could take thousands of hours and consume enormous amounts of fuel.
Madurowicz, Macintosh, and Macintosh recognized that this harsh calculus could be changed, however, since the sun is slightly oblong than perfectly spherical. This minor detail means that the target exoplanet must align perfectly with the sun’s Equator as seen through the focal-region telescope. The product is not an Einstein-ring, but a “cross”, four asymmetrical copies the planet around its perimeter. Madurowicz discovered that by exploiting this asymmetry the scanning process to reconstruct an exoplanet’s undistorted image can be eliminated. He says, “You don’t have to move [your telescope], around in the image.” “You can stay in one spot .”
Turyshev was not part of this latest study and is skeptical that the tedious process of scanning can be eliminated. He says that the idealized method for image reconstruction Macintosh & Madurowicz propose would need to overcome interference from the sun’s brightness and its seething atmosphere, known as corona. Turyshev states, “It would be nice to have the sun just be dark.” It isn’t, and even the most expensive equipment couldn’t stop a small amount from trickling into a telescope. This is especially true if the telescope is looking directly at our star. He adds that although their paper is beautiful, it’s only a theory.
Although scanning could be automated, there are still limitations. Each exoplanet that is being targeted for solar gravitational lensesing would need its own Hubble-like space telescope, sent to the solar system’s outer limits. For example, for such an observatory to image a second exoplanet just 10 degrees off from its original target, it would need to shift its position around the sun by more than 14 billion kilometers. Madurowicz states that to use a solar gravitational lenses, one must align the telescope, the sun, and the planet very precisely. A single telescope cannot image more than one planet or a star system with many interesting worlds at once.
This limitation is the reason Jean Schneider, an astronomer at the Paris Observatory, has his eye on a different, perhaps more feasible alternative to solar gravitational lensing: the hypertelescope. This broad concept envisages the detection and analysis of the surface features of exoplanets using space-based fleets containing many meter-scale mirrors that fly in formation to create virtual telescopes much larger than any one of them. Schneider agrees that direct images of extraterrestrial vegetation could be “precious” as they would offer insights not possible through any other method of remote observation.
Aki ROberge, an astrophysicist with NASA’s Goddard Space Flight Center points out that astronomers aren’t even sure if there’s another world out there. “Not just ‘Earth-size,'” she says, “but ‘Earth-like,’ with oceans, continents, an atmosphere and a biosphere.” And direct imaging, it seems, is the only way to really find out.
A proposed observatory recommended in the National Academies of Sciences, Engineering, and Medicine’s report Pathways to Discovery in Astronomy and Astrophysics for the 2020s, otherwise known as the Astro2020 Decadal Survey, may offer the best near-term hope of giving Roberge and her peers the answers they need. This survey is a once-a decade roadmap that guides U.S. Astronomy. And topping its latest roadmap is a concept for a space telescope with a mirror more than six meters wide, something of a “super Hubble” tuned for gathering optical, infrared and ultraviolet light that is intended for launch as soon as the early 2040s.
According to Astro2020’s recommendations, one of the core capabilities of such a telescope would be directly imaging a diversity of exoplanets with the key objective of studying their atmospheres to make better guesses about their environmental conditions. Astronomers could then determine whether the chemical necessities or byproducts of life as we know them, such as water, organic compounds, and oxygen, exist on any of the target worlds. This proposed telescope might image fuzzy blobs, which could be the first step towards understanding whether an exoplanet is capable of harboring life. Most astronomers believe that only after such a mission can we make the big leap of building a hypertelescope, or using the solar gravitational lenses to obtain detailed surface images. “We have a path to the 2040s. After that, it’s the Wild West,” Roberge says.
Despite the remoteness of the solar gravitational lenses, Turyshev and Macintosh agree that it is worth considering its potential now. Advancements in solar sails, and other unconventional propulsion technologies, offer the possibility to speed up the journey to the outermost reaches of the solar system. Although the challenges are still daunting, using our star as a telescope may be closer than anyone realizes. By anticipating the theoretical and practical limits of the approach, when–or if–it finally lies within our grasp, the question will not have to be “Can we do this?” but rather “What planets should we image?”