As Einstein originally predicted with his general theory of relativity, gravity changes the curvature of spacetime. As a result, the passage of light changes when it hits a gravitational field. So the general theory of relativity was confirmed! For decades, astronomers have used this to perform Gravitational Lenses (GL), where a distant source is focused and amplified by a massive object in the foreground.
In a recent study, two theoretical physicists argue that the sun could be used in the same way to create a solar gravity lens (SGL). They argue that this powerful telescope would provide enough light amplification to enable direct imaging studies of nearby exoplanets. This would allow astronomers to determine if planets like Proxima b might be habitable long before we send missions to study them.
The study, which recently appeared online and is due to be published in the journal Physics Review D, was conducted by theoretical physicist Viktor Toth – formerly at the Perimeter Institute for Theoretical Physics – and Slava G. Turyshev, a physicist at NASA's Jet Propulsion Laboratory, who was also the Principal Investigator (PI) of the Laser Astrometric Test of Relativity (LATOR) mission.
Einstein rings captured by the Hubble Space Telescope during its lifetime. Photo credit: NASA / ESA / A. Bolton (CfA) / SLACS team
Gravitational lenses not only enable all kinds of in-depth astrophysical research, but have also resulted in some of the most spectacular images of the universe ever captured. These include so-called "Einstein rings", as the light from a distant object can sometimes look like when it hits a gravitational field between it and the viewer.
Depending on the orientation between the viewer, the source and the lens, the light from the source can also appear as an arc, cross or other shape. While any massive body can be used as a gravitational lens, the sun is in a favorable position for GL astronomy. For starters, it's the most massive body in the solar system, which makes it the most powerful lens available.
Second, the focus range of its lens starts at ~ 550 AU from the sun, which is a realistic distance for a future mission. The focus region of the next largest object (Jupiter) begins at a distance of over 2,400 AUs. In a nutshell, astronomers could develop a correct orientation towards the Sun to create an SGL and use that for astronomical observations – such as getting a good look at nearby exoplanets!
Direct imaging is a particularly promising method of characterizing exoplanets that future exoplanet studies will focus on like never before (as opposed to detecting exoplanets). By studying light reflected directly from a planet's atmosphere or surface, astronomers can obtain spectra that indicate what a planet's atmosphere is made up of, and possibly even see signs of vegetation on the surface.
Diagram of an SGL in action. By using the sun's gravitational field as a lens, future missions could capture high-resolution images of exoplanets and other celestial objects. Photo credit: Toth H. & Turyshev, S.G.
However, this method is difficult because current telescopes do not have the resolution required to directly image smaller planets orbiting closer to their stars (where rocky planets are). Therefore, most of the directly imaged exoplanets were gas giants, typically with long-period orbits. As Turyshev emailed Universe Today:
“In order to be able to observe and image an exoplanet directly, we need access to very large telescopes. So if we want to see our own earth from a distance of 100 light years in just one pixel, we need a telescope with a diameter of ~ 90 kilometers.
“The next larger telescopes that are being built on the ground (European Extremely Large Telescope) and flying into space (James Webb Space Telescope) are 39 meters and 6.5 meters, respectively. The concepts that are being considered to replace these great machines (LUVOIR and HabEx) are 16 and / or 24 meters. "
Based on that trend, Turyshev argues, no one alive today will see what an alien world looks like in their life up close (nor their children and grandchildren). With an SGL, observations of nearby exoplanets (such as Proxima b and c or the seven rocky planets orbiting TRAPPIST-1) could be made by the middle of this century.
What a simulated earth at the distance of Proxima Centauri (4.24 ly) would look like when projected from the SGL. Photo credit: Toth H. & Turyshev, S.G.
To determine whether SGL is possible, Toth and Turyshev relied on previous studies in which they developed a wave-theoretical description for SGL. In the end, they found that this was the case and even simulated which images of the earth would look like with a resolution of 1024 x 1024 pixels (see above) folded and with Gaussian noise (left) and after unfolding (right).
This is what Earth would look like if it were the same distance as Proxima Centauri (4.24 light years) and imaged by a telescope 650 AU from the Sun (and using it as a lens). If you look closely, you can see the cloud cover and the contrast between land masses – in this case the US, Baja California, Mexico. Toth and Turyshev estimate that the total exposure time required for this level of detail would be about a year.
Of course, the team also identified some challenges that needed to be tackled first. The distance to the focus region is the most important problem, which is located approximately 82.28 billion km from Earth. That's roughly four times the distance between Earth and the Voyager 1 probe, which holds the record for the most distant mission ever – 150 AU (22.44 billion km; 13.94 billion miles) as of 2020.
Second, they found that the lens would suffer from spherical aberrations and astigmatism that needed to be corrected. Eventually, the intense brightness of the sun would of course overwhelm any light received from distant objects. Said Toth:
"(D) The observations are necessarily long (the telescope sees a" pixel "at a time when it traverses a kilometer-wide image plane in the focal area, and enough data must be collected for each pixel to mitigate the effects of the noise, mainly from the solar corona) , during which a) the movement of the telescope relative to the image must be precisely known and b) the target exoplanet can move itself, change its appearance (clouds, vegetation, etc.) and its lighting. Some of these problems can be treated as noise, others can be eliminated through a clever image reconstruction strategy. "
Artist's impression of the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the solar system. Photo credit: ESO / M. Kornmesser
Fortunately, there are a few possible solutions that Toth and Turyshev recommend. For example, their conceptual study called for the use of a telescope with a 1 meter primary mirror, although a 2 to 2.5 m (6.5 to 8 foot) telescope might also be possible. This could be achieved by sending a small fleet of spacecraft imaging vehicles that could combine their resolution to correct for aberrations.
In order to cope with the disturbances from the sun, a correspondingly constructed coronagraph must also be developed. Fortunately, Toth and Turyshev estimate that, given the focal length of the Sun, a coronagraph about three feet in diameter will do. Much like the technology for a constellation of small spacecraft that together form a space telescope, this has to wait for future developments.
But the payouts that would contain resolved images of potentially habitable planets would be immeasurable! Imagine if you could photograph Proxima b, showing the size and shape of its continents alongside its vast oceans (assuming it has both). And how awesome would it be to take pictures of Proxima c, a gas giant believed to have a system of rings like Saturn?
There are also the three planets that orbit in the habitable zone of TRAPPIST-1, all of which could have huge oceans on their surface. There's also the extremely valuable scientific data we're about to gain, including spectroscopy that could show whether the atmosphere of nearby exoplanets contains the chemical signatures we associate with life (also known as "biosignatures").
A special SGL telescope would also be a suitable addition to the many next-generation telescopes that will be put into operation in the coming years. These include missions like the James Webb (JWST) and Nancy Grace Roman Space Telescope, which build on the achievements of Hubble and Kepler by finding thousands more exoplanets in nearby star systems.
Similarly, ground-based telescopes with adaptive optics and corona diagrams – such as ESO's Extrem Large Telescope (ELT) and Giant Magellan Telescope (GMT) – allow direct imaging studies of smaller, rocky planets orbiting closer to their stars. Most of the candidates for habitable planets are expected here, especially with darker M-stars (red dwarf).
In the end, there is little doubt that an SGL would be a worthwhile investment as we enter a new age in astronomy and astrobiology that goes beyond discovering exoplanets and focuses on characterizing and finding extraterrestrial life! As Turyshev summarized:
“In the next 10 to 15 years we will discover thousands of new exoplanets using indirect methods (transit spectroscopy, radial velocity, astrometry, microlens, etc.). Once we have a number of exciting goals, SGL helps us investigate them. We could launch a mission towards the SGL's focus region for a specific target and examine that preselected target or target system.
Be sure to check out this video that explains the concept of a solar gravity lens and how it could revolutionize astronomy (courtesy of former NASA astronomer Christian Ready and Launch Pad Astronomy):
Further reading: arXiv