One of general relativity's key predictions is that a big object, like a star, galaxy, or black hole, may deflect light traveling close by. Light from faraway objects can therefore be gravitationally lensed by things closer to us. Gravitational lensing may operate as a natural telescope, intensifying and amplifying the light of distant objects under the correct conditions. Astronomers have used this approach to examine some of the universe's most distant galaxies. However, astronomers have considered utilizing this impact closer to home.
One proposal is to investigate neighboring exoplanets using the sun's gravity as a lens. The sun would gravitationally concentrate light from an exoplanet, having a focal point in the vicinity of roughly 550 AU to 850 AU, depending on how near the exoplanet's light passes by the sun. In theory, we could put one or more telescopes at that distance, resulting in a sun-sized telescope. For objects 100 light-years away, this would result in a resolution of around 10 square kilometers. The most distant spacecraft we've built so far is Voyager I, which is only around 160 AU from the sun, indicating that we still have a long way to go before this type of solar observatory becomes a reality.
However, it is a project that we may pursue in the future. It wouldn't require any miraculous technologies or new physics to accomplish. All it will take is a lot of engineering. Even then, using all of the data obtained to create an accurate picture will be a struggle. This solar lens telescope, like radio telescopes, would not be able to record a single image all at once. To picture exoplanets, a precise understanding of how the sun concentrates light is required, which is where recent research comes in.
No telescope is flawless. Diffraction is one of the limits of optical telescopes. The focusing effect can cause light waves to interfere with each other somewhat when they pass through a telescopic lens. Diffraction is an effect that can blur and distort your image. As a result, there is a limit to how crisp your picture may be for any telescope, known as the diffraction limit. While a gravitational lens telescope is unique, it does have a diffraction effect and a diffraction limit.
The scientists analyzed the sun's gravitational lensing to look at the diffraction effects it might have on a picture from an extended object like an exoplanet in a paper just published in the Monthly Notices of the Royal Astronomical Society. They discovered that a solar-lens telescope could detect a 1 Watt laser originating from Proxima Centauri b, which is around 4 light-years away. They discovered that in general, the diffraction limit is substantially lower than the telescope's total resolution. Depending on the wavelength measured, we should be able to discern features on the order of 10 km to 100 km.
The researchers also discovered that even at sizes below the diffraction limit, there might be items worth researching. Neutron stars, for example, would be too tiny for us to discern characteristics, but we could study things like surface temperature change. This study primarily indicates that exoplanets and neutron stars would be excellent candidates for a solar-lens telescope. It would be a game-changing instrument for astronomers in the future.
Journal Information: Sara Engeli et al, Optical properties of the solar gravity lens, Monthly Notices of the Royal Astronomical Society (2022). DOI: 10.1093/mnras/stac2522