In a new paper in the Astrophysical Journal, a duo of astronomers from the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University described a way to manipulate solar gravitational lensing to view extrasolar planets. By positioning a space telescope, the Sun, and an exoplanet in a line with the Sun in the middle, they could use the solar gravitational field to magnify light from the exoplanet as it passes by.
Gravitational lensing wasn’t experimentally observed until 1919 during a solar eclipse.
With the Moon obstructing the light from the Sun, scientists were able to see stars near the Sun offset from their known positions. This was unequivocal proof that gravity could bend light and the first observational evidence that Einstein’s theory of relativity was correct.
Later, in 1979, Stanford University Professor Von Eshleman published a detailed account of how astronomers and spacecraft could exploit the solar gravitational lens.
But it wasn’t until 2020 that the imaging technique was explored in detail in order to observe planets. Slava Turyshev of NASA’s Jet Propulsion Laboratory and colleagues described a technique where a space-based telescope could use rockets to scan around the rays of light from a planet to reconstruct a clear picture, but the technique would require a lot of fuel and time.
Building on that work, Kavli Institute Ph.D. student Alexander Madurowicz and Professor Bruce Macintosh invented a new method that can reconstruct a planet’s surface from a single image taken looking directly at the Sun.
By capturing the ring of light around the Sun formed by the exoplanet, their algorithm can undistort the light from the ring by reversing the bending from the gravitational lens, which turns the ring back into a round planet.
They demonstrated their work by using images of the rotating Earth taken by NOAA’s satellite DSCOVR that sits between Earth and the Sun.
Then, they used a computer model to see what Earth would look like peering through the warping effects of the Sun’s gravity.
By applying the algorithm to the observations, they were able to recover the images of Earth and prove that their calculations were correct.
In order to capture an exoplanet image through the solar gravitational lens, a telescope would have to be placed at least 14 times farther away from the Sun than Pluto, past the edge of our Solar System, and further than humans have ever sent a spacecraft.
But, the distance is a tiny fraction of the light-years between the Sun and an exoplanet.
“By unbending the light bent by the Sun, an image can be created far beyond that of an ordinary telescope,” Madurowicz said.
“So, the scientific potential is an untapped mystery because it’s opening this new observing capability that doesn’t yet exist.”
A Hubble-sized telescope in combination with the solar gravitational lens would be sufficient to image exoplanets with enough power to capture fine details on the surface.
“The solar gravitational lens opens up an entirely new window for observation,” Madurowicz said.
“This will allow investigation of the detailed dynamics of the planet atmospheres, as well as the distributions of clouds and surface features, which we have no way to investigate now.”
According to the team, it will be a minimum of 50 years before this technology could be deployed, likely longer.
In order for this to be adopted, astronomers will need faster spacecraft because, with current technology, it could take 100 years to travel to the lens.
Using solar sails or the Sun as a gravitational slingshot, the time could be as short as 20 or 40 years.
“Despite the timeline’s uncertainty, the possibility to see whether some exoplanets have continents or oceans drives them,” Professor Macintosh said.
“The presence of either is a strong indicator that there may be life on a distant planet.”
“This is one of the last steps in discovering whether there’s life on other planets.”
“By taking a picture of another planet, you could look at it and possibly see green swatches that are forests and blue blotches that are oceans — with that, it would be hard to argue that it doesn’t have life.”
Alexander Madurowicz & Bruce Macintosh. 2022. Integral Field Spectroscopy with the Solar Gravitational Lens. ApJ 930, 19; doi: 10.3847/1538-4357/ac5e9d