With the help of "direct imaging", exoplanets can be "believed"

Produced by: Science Popularization China

Author: Dong Zhichuan (Nanjing Institute of Astronomical Optics and Technology, National Astronomical Observatories, Chinese Academy of Sciences)

Producer: China Science Expo

In daily life, we tend to believe in things that we can see directly and more intuitively. In fact, this is not only true in life, but also in scientific research. Scientists are eager to achieve "seeing is believing" in research results. This is not only a pursuit of scientific truth, but also has practical application significance. It is also an urgent need to achieve grand scientific goals.

For example, in astronomy, “seeing is believing” is sometimes necessary.

In 2008, scientists used the "direct imaging method" to photograph an extrasolar planetary system - the HR8799 system. After 12 years of continuous observation, Jason Wang, an astrophysicist at Northwestern University in the United States, recently released a time-lapse video that clearly shows how the four planets in the HR8799 system orbit their star.

Video of 4 planets orbiting the star in the HR8799 planetary system

(Image source: ScienceNet)

This is just one of the application scenarios of "direct imaging". One of the important goals of astronomers for a long time to come is to keep a close eye on exoplanets and take direct images of them.

What is the difficulty in directly imaging exoplanets?

For scientists, direct imaging of exoplanets is technically very challenging. Because planetary light is very weak and starlight is very strong, usually the planetary light will be submerged under the strong starlight and difficult to detect directly. The difficulty of directly detecting exoplanets is like looking for a faint firefly next to a searchlight.

Therefore, to directly "see" the signal of the planet, a special method is needed - high-contrast imaging coronagraph technology. The coronagraph can suppress the strong light of the star through various modulation techniques, obtain high-contrast imaging in a specific area of ​​the final image plane, and enable the imaging of faint planets.

Urgent pursuit of "direct imaging, seeing is believing"

Why do we always have an obsession and an urgent desire to "directly image and see is believing"? This is because sometimes, if we cannot directly image, we may not be able to solve more fundamental problems.

In many movies, some shots may raise questions.

In the movie "Jurassic Park," the high-tech fighter planes seemed to lose their aim when they needed to shoot down pterosaurs. In the movie "Godzilla," infrared missiles were unable to lock onto the cold-blooded reptile Godzilla, causing them to miss their targets frequently.

Pterosaur

Image source: https://www.earth.com/

These high-tech fighter jets that appear in science fiction movies should have the most advanced aiming systems. Why can't they always hit dinosaurs or giant beasts that the human eye can easily aim at?

This is because the original intention of designing aircraft and missiles is not to target pterosaurs or Godzilla. In the eyes of radar, it only focuses on the metal reflective surface on the target. In the eyes of infrared detection and guidance missiles, perhaps its target is just a heat source.

Infrared thermal imaging

(Photo source: veer)

Humans can clearly see these large creatures with their eyes, so why do sophisticated weapon identification technologies such as radar and thermal imaging not work? Aren’t there missiles that are guided by visual or image recognition?

In fact, there are also visual or TV-guided missiles, but most of them are used to attack slow targets such as armored vehicles and tanks, or attack fixed targets on the ground. Moreover, if we only rely on human eyes to attack flying targets, the range of the "attack window" is really limited. The "seeing is believing" we pursue is not limited to the range of our eyes, but also to show us the places that our eyes cannot reach with intuitive images.

The difficulty of "shooting Wing Loong with missiles" brings us the lesson that "direct imaging and seeing is believing" are still very reliable means of target identification and method of feature inspection.

Indirect Sensing: Current Methods of Detecting Exoplanets

The same is true for astronomical research. Our current more mature methods for planetary detection include occultation, radial velocity, astrometry, gravitational microlensing, and direct imaging. Except for a few, these methods are all indirect perception and detection of targets.

Indirect perception and detection can still be explained by scenes in movies. The pterosaur shown on the radar screen of a fighter jet is not the pterosaur itself, but only the radar reflection wave signal or infrared heat source image of the pterosaur. These are just the detection of the "subsidiary effects" of the pterosaur's body, which is an indirect detection and perception method. Most of the exoplanets we have successfully discovered so far use these indirect effects.

The first discovery of an exoplanet was in 1992, when Polish scientists used the "pulsar timing method". Using this method, scientists discovered two planets orbiting a pulsar called PSR B1257+12. Although this was a breakthrough for humans, pulsars are not strictly stars like the sun.

Artistic image of the PSR B1257+12 pulsar

(Image source: wiki)

The first exoplanet discovered by humans in the true sense should be 51 Pegasus b, which was discovered by scientists using the radial velocity method in 1995. The discoverer Michel Meyer and his student Didier Queloz won the 2019 Nobel Prize for this.

The occultation method is currently the most common way to discover exoplanets, and more than 70% of exoplanets have been discovered using this method.

Although astrometry was the earliest method used to search for exoplanets, as of 2018, only HD 176051b, discovered in 2010, is the only exoplanet confirmed to have been discovered by astrometry.

But our research is by no means limited to this. We always want to "see is believing" so that we can better understand the object of our research and then study it. This is not an obsession, but a need for further scientific analysis.

Direct imaging: Astronomers want to “see is believe”

If humans can successfully image an exoplanet directly and see it with our own eyes, we will be able to better analyze its shape and elemental composition, and obtain the most direct and rich information on its luminosity, temperature, atmosphere, orbit, etc., and even directly examine its topography (although achieving this goal is quite difficult).

If this can be achieved, it is undoubtedly equivalent to taking a clear, high-pixel "ID photo" of a distant, unfamiliar exoplanet, thus achieving "filing and registration". In this way, it will be easier for scientists to further determine whether there is life on a distant exoplanet.

Jason Wang, the physicist mentioned at the beginning, has tried to use this intuitive method to show people the trajectory of planetary motion. He once said, "Astronomical events either happen too fast or too slow to be shown through videos. But this 'clearly visible to the naked eye' video intuitively shows the planets moving on the human time scale. I hope it will allow people to enjoy something wonderful."

High-contrast imaging coronagraph experimental platform helps "seeing is believing"

Fortunately, Chinese researchers have achieved some results on the road to direct imaging. The Exoplanet Detection Technology Laboratory of the Nanjing Institute of Astronomical Optics, Chinese Academy of Sciences, has established a universal high-contrast imaging coronagraph experimental platform.

To further improve the imaging contrast of the system, the researchers conducted research on speckle noise elimination technology and wavefront reconstruction algorithms based on "core functional devices". The initial goal is to increase the imaging contrast of the system by one (to two) orders of magnitude to directly image young giant planets; the long-term goal is still to strive to make the imaging contrast of the system achieve direct imaging of terrestrial planets.

The James Webb telescope, which is more familiar to everyone, can better accomplish the mission of direct imaging because there is no atmospheric interference in space. Moreover, the Webb telescope has already been tested and demonstrated its ability to directly image exoplanets. Our Chinese scientists have also begun to learn from the same idea and transplanted the system into space, "moving" it to the space station that is currently under construction and is becoming more and more perfect, in order to achieve "quiet" imaging under high-contrast conditions without atmospheric turbulence interference.

Webb Telescope

(Image source: Wikipedia)

I believe that in the future, researchers will be able to achieve higher imaging contrast and apply this system more widely, and even help humans find a habitable second home.

Editor: Guo Yaxin