A futuristic telescope design could one day let scientists finally look at fine details of the atmosphere of Earth-sized exoplanets — confirming if some are potentially habitable worlds. It would do so using the gravity of the Sun and a quirk of Einsteinian physics.
The so-called “gravity telescope” would use the Sun to examine very distant worlds, perhaps as soon as a few decades from now if the funding, technology, and will come together in the right way, new research shows.
Co-author and exoplanet researcher Bruce Macintosh of Stanford University tells Inverse his team’s paper, published May 2 in The Astrophysical Journal, builds upon decades of research by engineers and scientists seeking to understand more about the 5,000 known planets outside of our solar system.
How does it work? — The concept of using the Sun as a telescope is also decades old, but papers like this newly published work can suss out more details on the initiative, he says.
The gravity telescope uses a long-established astronomical technique called gravitational lensing. The effect happens when a massive object in the foreground of view (like a galaxy) bends the light of a distant object in the background (like a planet). Einstein correctly predicted this effect at least as far back as 1936.
The paper imagines using a Hubble-class telescope at a great distance (550 astronomical units, or Sun-Earth distances). That is relatively close in astronomical terms but still daunting. “It's about two times further away than Pluto, or about seven or eight times further away than the Voyager spacecraft,” Macintosh says.
The distance of 550 AU is the focal region of the Sun’s gravitational lens, allowing the source requiring magnification (the exoplanet) and the lens of the Sun to align so that the telescope can see distant objects behind, refracted by the Sun's gravity.
Digging into the details — Macintosh cautioned that his team are not engineers, but the telescope would likely have to be equipped with a sunshield (a coronagraph) to protect it against any stray light and block out the light of the Sun. Coronagraphs are well-tested in space and are equipped on some instruments aboard the James Webb Space Telescope.
Gravitational lensing events of individual stars or planets are usually accidental. Astronomers typically don’t know the background objects even exist until they pop up in an archival image from a telescope happening to gaze in that area of the sky. However, NASA’s James Webb Space Telescope plans to deliberately exploit the technique for a recently-detected ancient star that will be moving behind foreground members of a star cluster.
In theory, it sounds like using the Sun as a gravitational lens would be even easier. After all, it is closer to us, and the gravitational lensing effect would therefore be considerably more robust. Also, the Sun would be used to examine planets already discovered, making the investigation process more efficient. That said, there are considerable technological questions astronomers and engineers must address before making this vision a reality, Macintosh said.
“Part of the point of this paper was to look really carefully at the math and physics, and try to understand how well it would work to have a picture you could make [of a planet], and how you would go about making that picture,” Macintosh, who is also deputy director of the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), says.
In theory, the futuristic telescope may be able to look at the spectrum of a distant rocky planet’s atmosphere to look at the signals of chlorophyll, try to see the “shiny and reflective” signal of water, or look at the chemical compositions of clouds, he added.
By comparison, the newly launched NASA James Webb Space Telescope will look in less detail at gas giant atmospheres. Macintosh says astronomers will task observatories 20 years from now to look at the signature of oxygen in rocky world atmospheres.
The gravity telescope would thus be the next logical step, he said, and a meaningful milestone. “I think there's also something just compelling about making that first picture of a life-bearing world,” he said.
Harkening back to pioneering 1600s telescopic studies of the moon, Saturn’s rings, and Jupiter with one of the first telescopes, he added: “It's like Galileo looking through the telescope for the first time.”
“One of the things about the Sun, as a lens, is that it's actually not a very good lens,” Macintosh joked. “If somebody tried to sell you that lens, we would send it back to the shop.”
The drawbacks — One principal hurdle is the sheer brightness of the Sun, which, if improperly managed, would easily wash out the subtle light of an exoplanet. The lensing may also be subject to spherical aberrations and astigmatism, as a 2021 study in Physical Review D discussed.
But feasibility studies of solar lensing have been ongoing for decades, including a key 1979 effort by Stanford professor Von Russel Eshleman, now a professor emeritus of electrical engineering. Eshelman pondered how both astronomers and spacecraft could use the lens, and his paper helped guide the latest study.
The Voyager spacecraft, successful as they have been, provide an excellent case study as to how technology might need to change to make this futuristic telescope more efficient. Their signals take nearly a day to reach Earth by radio, or more precisely, 21.5 hours for Voyager 1 and about 18 for Voyager 2. It also takes a NASA signal that long to send a command to the distant spacecraft via Earth’s Deep Space Network of radio dishes.
It was the best we could do with 1970s technology, but Macintosh said the new telescope would ideally leverage artificial intelligence and data advances in compression technology to speed communications back and forth, perhaps using lasers following numerous recent NASA feasibility studies with spacecraft. (This would speed up the data transfer rate, although lasers, like radio waves, are limited by the speed of light.)
“The telescope would have to be pretty autonomous, both to plan the observations and track the planet,” he said. “It would also have to make intelligent decisions about what data to send back, and how to compress it.”
This is all, of course, assuming that the telescope could get out to this region in the first place. Considering the Voyagers launched 45 years ago and just moved into interstellar space within the last decade, the new study calls for more progression in rocket technology to get the Hubble-sized exoplanet lensing telescope out there quicker.
But once the telescope is in place, lead author Alexander Madurowicz, a Ph.D. student at KIPAC, wrote that the telescope would be a “remarkable” milestone in astronomical imaging.
“Recently, the Event Horizon Telescope (EHT) collaboration released their famous image of the supermassive black hole at the center of the galaxy M87,” he said in a blog post. “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.”
The solar gravitational lens, he said, would produce images with an angular resolution of 25 nano-arcseconds, which would be much finer than the EHT’s resolution of 25 microarcseconds. In other words, the solar lens would be a big milestone in imaging detail.
What’s next — Madurowicz also provided a pathway for future engineers to tackle some of the greater challenges in the future. “First, a target planet must be identified and located in the sky with sufficient precision,” he says. “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,” he adds.
