In the last 25 years, astronomers have discovered over 4,000 exoplanets. Only a few of them were able to see directly in the pictures, but there were only single pixels. Are we doomed to it?
Over the past quarter-century, humans have discovered over 4,000 extrasolar planets, or planets orbiting stars other than the sun. It doesn’t change the fact that we didn’t see any of them directly, and the few that we were able to see are only single pixels. Is there really no way to properly photograph exoplanets so that it appears to us as a disk on which we could see oceans, continents or clouds?
Currently, the mirrors of the largest ground-based telescopes are 10 m in diameter. However, the situation will change significantly over the next few years. In Chile, the Extremely Large Telescope is being built, the mirror of which has a diameter of up to 39 meters, and yet next to it are also made other equally impressive telescopes such as TMT (Thirty-meter Telescope) or GMT (Giant Magellan Telescope). Scientists agree that these instruments, once they enter service, will revolutionize our knowledge of the universe. They will not help us, however, to take pictures of the continents on exoplanets located "in our cosmic environment". Why? Well, in order to photograph any details of the planet’s surface 100 light years away from us, we would need a telescope whose mirror is not 40 meters, but 90 kilometers. Currently, the creation of such an instrument is less likely than a successful manned landing on Mars.
However, a group of scientists appeared that decided to look for a solution to this problem. Researchers have developed a project that involves sending miniature probes with attached sails that would move away from Earth more quickly and faster than any previously sent space probe, turned toward the sun and used its gravity as a giant magnifying glass. Such a fleet of miniature probes would be able to photograph the surface of an exoplanet so accurately that we would be able to see on its surface structures of the order of 10 km.
The Solar Gravity Lens (SGL) project may seem completely unrealistic at first glance, and yet NASA, in collaboration with numerous universities, space companies and organizations such as the Planetary Society, is seriously considering such a project.
On the one hand, this project is extremely difficult to implement, but on the other hand, if it succeeded, the scientific "yield" would be enormous.
In our immediate surroundings, i.e. within 100 light-years of Earth, we have discovered several planets that can orbit the stars’ ecosphere – says Sława Turyszew, a physicist at Jet Propulsion Laboratory. So the question is, what could we do if we discovered any evidence that life could be on either of them? Could we fly there or at least see exactly such a planet?
The researchers decided to check whether the phenomenon of gravitational lensing could be used in this case. After all, this phenomenon has been used by astronomers to search for distant galaxies that we would never have been able to see were it not for the fact that the gravity of other galaxies between us and such a distant galaxy curves rays that radiate toward us picture like a magnifying glass. To see the enlarged object behind the lens, it is enough to be at a proper distance from it, in the so-called focus.
SGL researchers believe that exoplanets can be photographed the same way, just send the probe toward the focus of the lens. For example, in the case of an exoplanet 100 light years away from us, the focus of the lens, which would be the Sun, is 97 billion km away, i.e. 16 times farther than Pluto.
For the sake of perspective, the Voyager 1 probe, which launched from Earth in 1977 and is now the farthest of all objects ever sent by man from Earth, only 20 miles away from the sun.
How to speed up the probe?
Scientists propose the use of solar sails that, pushed by the pressure of the sun’s radiation, could accelerate continuously for a very long time. At the very beginning of the mission, the probes would be directed toward the Sun. Flying near it would use its gravity to accelerate and begin a journey outside the solar system. In this way, they could reach their destination within "only" 25 years.
Instead of massive, hard-to-accelerate probes, researchers would prefer to send a squadron of small probes that could be carried into space in a way "by the way" during the launch of other satellites. One idea is to use cubesats, satellites of the order of 10 x 10 x 10 cm, which could then be combined into a larger optical instrument already in space. If the costs of producing such satellites could be reduced, they could be sent into lens foci for many different exoplanets.
Upon reaching the bonfire and turning toward the sun, the detector would see the light coming from the exoplanet, blurred in an arc, the so-called Einstein Ring consisting of two parts. One part would consist of a single 10 x 10 km fragment on the surface of the observed planet, which would be 1 pixel in our image. The second part would contain radiation coming from the rest of the planet. When passing through the focus of the lens, the probe would have to move slightly using for example Miniature ion motors to change its position in the focus. Assuming we had the right optical instruments at our disposal, taking a million images of rings from different locations would allow us to take an image of an exoplanet resembling an image of the Earth taken from the Moon in 1968.
Is it even possible?
The technological challenges facing engineers who will undertake such a mission will be enormous. It is enough to mention, for example, extremely precise navigation and communication at such large distances, or the need to use apertures that will shield instruments from the radiation of not only the solar disk, but also the radiation of the host star of the planet we observe.
Still, NASA believes it’s worth exploring the subject further. For this reason, the agency decided to allocate $ 2 million to the authors of the project. a grant from the NIAC (NASA Innovative Advanced Concepts) program, in which the most innovative ideas are supported. We probably will not see such a mission during our lifetime, or at least its scientific results, but current teenagers have quite a good chance.
A telescope that could photograph continents and oceans on an exoplanet