by Alex Madurowicz
New research led by KIPAC PhD student Alex Madurowicz, published in The Astrophysical Journal, describes a novel technique to image Earth-like exoplanets in detail by using the Sun as a telescope. The gravity of the Sun lenses and magnifies light from a distant planet, but also distorts the image into what is now known as an Einstein ring. By tracing the path of light as it bends around the Sun, the Einstein ring can be deconstructed to recover an image of a distant planet. This concept would allow for observations in far greater detail than an ordinary telescope could ever possibly achieve, such as movies of the detailed surfaces of exoplanets.
A brief history of the solar gravitational lens
In 1936, Albert Einstein published a new prediction of his theory of relativity in a short note titled “Lens-like action of a star by the deviation of light in a gravitational field.” In this note, he identifies the formation of what we now call an Einstein ring, a circular image formed from a gravitational lens. At the time, this seemed but a trifling curiosity, and he writes “there is no hope of observing this phenomenon directly.”His calculations were correct of course, but his ambitions were limited by the realities of early twentieth-century technology.
After the historic space race of the 1960’s, interplanetary exploration became possible and the scope of human imagination expanded even further. Shortly after Voyager 1 departed Earth, Stanford Professor Von Eshleman recognized an important implication of Einstein’s predictions. In his 1979 work “Gravitational Lens of the Sun: Its Potential for Observations and Communications over Interstellar Distances,” Eshleman realized that the massive power of the solar gravitational lens could be exploited by moderately sized human spacecraft, amplifying their signals and enabling communications across distances that would be unfeasible otherwise. He identified a few practical problems, most notably that you would need to travel to the very edge of the solar system for it to work.
In the modern era, this concept has continued evolving. What was originally envisioned as an interstellar wifi router has become perhaps the most powerful telescope that humans may ever build. Investigations led by Slava Turyshev at JPL in 2020 have advanced our theoretical understanding of non-ideal gravitational lenses. His vision for a Hubble-sized telescope at the focus of the solar gravitational lens could resolve detailed images of the surfaces and atmospheres of Earth-like extrasolar planets.
The basic idea is this: the gravitational lens of the sun acts like a giant projector lens, creating an image of the target planet you could see if you hung a giant bedsheet at the location opposite the planet from the Sun. This projection is rather large, about a kilometer in diameter, and very far away, at a minimum of 550 times the distance between the Earth and Sun. Building a sensor, such as a CCD, a kilometer across is unfeasible; instead, one or multiple telescopes could move around inside this projection and at each location collect light like a single pixel in a detector would. Then multiple observations could be combined to reconstruct a complete image.
The most powerful telescope?
It is difficult to convey how remarkable this is. Recently, the Event Horizon Telescope (EHT) collaboration released their famous image of the supermassive black hole at the center of the galaxy M87. Using a vast interferometric array of radio dishes spread over the surface of Earth and combining all of the measurements to act like a single Earth-sized telescope, they were able to produce the highest angular resolution image of all time with an angular resolution of 25 microarcseconds. The solar gravitational lens could produce images with angular resolution of 25 nanoarcseconds, 1000 times more precise than the EHT. An ordinary telescope would need to be nearly the size of the Sun to achieve this, which is simply not feasible to build.
DIY: How to build a gravitational telescope
An ordinary lens such as a magnifying glass has a curved surface which bends incoming light rays and brings them to a focus to form an image. A gravitational lens is no different, except that it relies upon the curvature of spacetime to bend the light rays instead. However, a realistic astrophysical lens is not similar to a simple convex lens like a magnifying glass. Instead, the optical equivalent is nearly identical to the base of a wine glass, and so the images formed are heavily distorted.
This distortion could be either a blessing or a curse, depending on what planet you want to look at. Since the Sun is rotating, it is oblate—wider around its equator than along the polar direction. This has the consequence that the images formed using the solar gravitational lens also depend on the direction you wish to look, with extra distortion for certain directions. If the target planet is aligned with the Sun’s rotation axis, this extra distortion is minimized. Existing mission concepts prefer this, so that spacecraft scanning over the projection of the planet can easily distinguish differences in features across the planet.
If instead the planet is aligned with the Sun’s equator, this extra distortion is maximized and the new technique uses this to its advantage. In this situation, light from every location on the planet gets mixed into the location at the center of the projection. A single telescope could sit at this one location and observe the Einstein ring that forms around the edge of the Sun. At this location, the Einstein ring is not a perfect ring but rather four anti-symmetrically distorted copies of the planet, similar to the lensed supernova in the galaxy below.
We demonstrated that it is possible to directly reconstruct the image of the planet by analyzing the information in the ring without scanning the telescope around the projection. However, the two techniques are complementary and could be used together. Using both the information encoded in the structure of the Einstein ring while scanning the telescope across the projection could improve the reconstruction even further.
A number of instrumental engineering challenges still need to be overcome to make this vision a reality. First, a target planet must be identified and located on the sky with sufficient precision. Then, the telescope must navigate to align the orbits of the craft, sun, and target planet. Lastly, optical instrumentation strategies which can remove contaminating light from the sun, corona, host star, and background objects must be deployed to improve the signal to noise ratio.
None of these engineering challenges are sufficiently difficult to be considered impossible. Deploying a Hubble-sized telescope to the solar gravitational lens could enable observations that sound more like science fiction than science fact. By spatially and spectrally resolving the surfaces of extrasolar planets, one could investigate atmospheric composition and dynamics, and resolve details like oceans, continents, forests, and perhaps even extraterrestrial cities on other worlds. Who knows what fascinating and futuristic observations the twenty-second century will hold?
Stanford scientists describe a gravity telescope that could image exoplanets Stanford News Service
Integral Field Spectroscopy with the Solar Gravitational Lens (A. Madurowicz, B. Macintosh; arXiv link)
Integral Field Spectroscopy with the Solar Gravitational Lens (A. Madurowicz, B. Macintosh; ApJ link)
Direct Multipixel Imaging and Spectroscopy of an Exoplanet with a Solar Gravity Lens Mission (Turyshev, et al.; arXiv link)
Gravitational Lens of the Sun: Its Potential for Observations and Communications over Interstellar Distances (V.R. Eshleman; Science [paywall])