It is not difficult for astronomers to find a star in the night sky, but it is very difficult to find an Earth-like planet next to it. Earth-like planets do not emit their own light in visible light; we can see them mainly because they reflect sunlight. Push the line of sight toward a distant star system, and this reflected light will be attenuated by distance and obscured by the dazzling light of the host star. The problem faced by telescopes is like trying to find a firefly next to a searchlight: it's not that the firefly doesn't exist, but that it is easily drowned out by bright light.
But scientists don’t only have the option of “seeing directly”. The transit method can capture the periodic dimming caused by the planet passing in front of the star, and the radial velocity method uses subtle changes in the star's spectrum to find traces left by the planet's gravity pulling the star. These methods can prove the existence of planets and help measure their size or mass, but they are not equivalent to actually seeing the planets. If you want to further study its atmosphere, it is particularly important to directly receive the light from the planet. This is the value of direct imaging.
However, direct imaging first faces a problem of disparity in brightness. Young giant planets still retain part of the heat from their formation and can emit easily observable radiation in the infrared band. They are already relatively "conspicuous" targets in direct imaging. Even so, the brightest ones are only one hundred thousandth as bright as their host stars. Earth-like planets are smaller, cooler, and capture even fainter light, making it even more difficult to find them.
To see planets clearly, astronomers use a coronagraph to block some of the starlight and then try to subtract the remaining glare from the image. But starlight is not a fixed pattern that can simply be erased. The mirrors and instruments of the telescope will undergo slight changes, causing the glare pattern to change accordingly. Traditional methods rely on reference images to simulate starlight. Once the reference image does not completely correspond to actual observations, the remaining starlight may still drown out the planetary signal.
Scientists at Northwestern University are now trying to use a FLIP image processing framework to provide a more sophisticated "polishing" method. It not only compares the reference image, but also combines it with the physical model of the telescope to simulate the process of starlight passing through the mirror and reaching the detector, and then uses calculation methods to continuously adjust the model to make the simulated glare as close as possible to the real observation. This allows scientists to more accurately remove starlight and compensate for small instabilities in the instrument.
Scientists used FLIP to process observation data from the near-infrared camera coronagraph of the Webb Space Telescope and detected the giant planet 51 Eridani b for the first time in the 4.3-micron band. The planet has been previously discovered by ground-based telescopes, but in this wavelength band, carbon dioxide in its atmosphere absorbs some of the infrared light, making the signal even weaker. Other tests show that FLIP is 2 to 15 times more sensitive than traditional stellar glare removal technology.
But this does not mean that humans will be able to directly photograph "another Earth" right away. FLIP's current capabilities are mainly to help find giant planets with lower temperatures, smaller masses, and older ages. In the future, it may be used in planned missions such as ground-based observatories and habitable world observatories to help image Earth-like planets. Among them, there may be difficulties in reducing computational costs and accepting more observational data tests.
Having said that, why is it so difficult to find a planet similar to the Earth? The difficulty is that the light of terrestrial planets is too weak, and the light of stars is too strong, and it is impossible for our observation instruments to stay focused on one area forever. But with the update of technical methods, every time more starlight is eliminated, astronomers are one step closer to the tiny world hidden in the light.
Reporter Zhang Mengran
