When astronomers announced that they had discovered the new exoplanet , maybe they didn't intentionally title the party, but the way they found the planet must be different from what you think. Because in 99% of the cases, astronomers do not have the ability to "see" the planet they just discovered directly.
In many people's imagination, the process of astronomers discovering extraterrestrial planets is as follows: astronomers use a super invincible telescope (such as Hubble Telescope floating in Earth's orbit) to carefully observe a certain area in the universe. After searching for several months, they finally discovered a planet in the camera! Astronomers are very excited, and quickly export the photos and post a big news! Then we saw this picture in the news:
or this picture:
But what you need to know is that all these pictures are imaginary pictures drawn by artists.
Then why not use the pictures seen in the astronomer's telescope? It's very simple, because in most cases, astronomers simply cannot "see" these planets directly in a telescope.
Many people may not know that it is very, very difficult for humans to directly observe planets outside the solar system.
We all know that the celestial bodies in the universe can be divided into two categories: stars and planets. Among them, the stars are the ones that do not shine and rotate around the stars are the planets.
It is too simple to observe stars. Most of the stars we see in the night sky are stars outside the solar system. It is no exaggeration to say that from the day the eyes evolved, humans have begun to observe stars.
So, when did humans first observe planets outside the solar system?
The answer is 1992.
Yes, I did not write it wrong and you did not read it wrong. Only 23 years after humans landed on the moon did we have the ability to observe planets outside the solar system.
Why is this so difficult? First of all, of course, it is because other galaxies are too far away from us. You may have heard of the name Centaur α in many science fiction novels and games. It is the closest star to us, so it is also the first stop on the interstellar colonization journey of humans in many science fiction works.
Centauri α is "only" 4.3 light years away from us, but this is already a very long distance. It's so far away that it's meaningless to convert it into kilometers, because humans can't imagine such a big number. We can only use analogy to understand how far this distance is:
If the distance between the earth and the sun is analogous to the distance from the living room to the bathroom (10 meters), then the distance from the earth to the α-star Centaurus is equivalent to the distance from Beijing to Urumqi. Well, this is still the closest star to us. The diameter of Milky Way is at least 20,000 times that distance.
But the long distance is not the main reason why we find it difficult to observe exoplanets in the solar system. The biggest difficulty is that these planets are not only far away, but also do not shine. Although they reflect the light of stars like , Jupiter, and Venus in the solar system, this faint reflected light is easily blocked by the dazzling light of the star next to it. Some people say that if you want to see a planet next to a star on Earth, it is like looking for a firefly flying next to a few kilometers away.
When looking up at the starry sky, countless people must have guessed that in the dark between these stars, are there many planets orbiting Jupiter like the Earth? Are some planets also possess life or civilization?
But this curiosity can only stay in the conjecture stage, because people simply cannot determine whether those stars are planets-until 1992.
In this year, a Polish astronomer and a Canadian astronomer discovered two planets 2300 light years away from us in a very tortuous way. Note that the word here is to "discover" the existence of two planets rather than "seeing" the two planets.
In 1990, Polish astronomer Wolszczan was working at the Arecibo Observatory in Puerto Rico. When he used the radio telescope here to observe a pulsar 2,300 light years away, he noticed some strange phenomena. The so-called pulsar (Pulsar) is a special neutron star . With its own rotation, its electromagnetic wave radiation will intermittently sweep across the earth. In the eyes of observers on Earth, it is constantly sending pulsed signals to the Earth at fixed time intervals, hence the name. For example, Wolszczan observed a pulsar that rotates 161 times per second in 1990. The biggest feature of
pulsar is that the pulse signal it sends is very stable and can be used as a natural timer in the universe. After Wolszczan observed for a period of time, he found that the signal sent by this pulsar was not that stable. Sometimes it would rise a little faster, and sometimes it would rise a little slower.
This confuses Wolszczan because the signal period of the pulsar should be exactly the same. He was unable to determine whether the deviation was caused by the error of the radio telescope, so he asked his colleague, Canadian astronomer Frail, to observe the pulsar at the Observatory in New Mexico, USA.
After two years of observation, calculation and analysis, the two astronomers believe that the signal deviation they observed is because the pulsar has two planets orbiting it around the periphery of the pulsar, and the existence of these two planets has had an impact on the signals sent by the pulsar. They published the result in Nature magazine, which is the first time that humans have confirmed the existence of exoplanets in the scientific community. Is
different from what you think? From beginning to end, people have never "see" these two planets, but have indirectly derived their existence through some data analysis.
Later astronomers invented some other methods to find exoplanet , but most of them were very tortuous ways. These methods include:
Transit method: When a planet just runs between its parent star and the earth, it will briefly cover part of parent star , making the light that the parent star reaches the earth weaker. This change in light intensity often takes less than one percent, so it requires precise measurements to discover. For example, if an alien observes the transit of the Earth in a distant place, the Earth's blocking of the Sun will only reduce the intensity of the Sun's rays by 0.008%, less than one ten thousand.
Transit Star Method Schematic diagram. In fact, we cannot see the above scene. We can only speculate through the changes in the received light intensity to speculate that a planet blocks the star
See this, you may also find that this transit Star Method has an innate disadvantage. That is, the observed planet, its parent star, and the earth must be in a straight line. Even if it is slightly higher, the transiting star phenomenon cannot be observed on Earth. At the same time, the size of this planet cannot be too small, otherwise the impact on the light intensity of the parent star is too small and cannot be observed on the earth.
In the vast universe, it is obvious that only a few planets just meet such observation conditions.
Although this method sounds like a lot of flaws, so far, the vast majority of exoplanets discovered by humans have been found in this way. The picture below is a classification diagram drawn by all the more than 5,000 exoplanets discovered by humans by 2022 according to the detection method. The green part was discovered by transit method:
As of 2022, the number of exoplanets discovered each year. Among them, the green part was discovered by transit method using transit method. In addition to transit method, there are many other methods, but most of them are also this roundabout way, such as radial velocity method. This method first measures the speed of a star moving relative to the earth and . If the measured speed changes slightly over time, it can be inferred that there are planets around it that we cannot see interfering with its orbit.
, etc., does this feel completely different from what is said in the news? In the news, astronomers will describe in detail the size and weight of the planets they discovered, and say that some of them may have liquid water, right?
is very simple, they calculate these. By observing the perturbations received by stars, the size, weight and orbit of the planets can be calculated - although we don't see them directly. Astronomers can also measure the heat emitted by stars, and then combine the distance between the planet and the star to calculate the average temperature on the planet's surface and whether liquid water may exist.
Sometimes astronomers will announce the atmospheric components of a certain exoplanet they have observed. How did this be done? When a transit occurs, part of the light emitted by the star passes through the planet's atmosphere and then arrives on Earth (if the planet has an atmosphere). After receiving such light, astronomers will perform spectral analysis on it. Since each molecule leaves special traces in the spectrum, astronomers can calculate the atmospheric composition on this distant planet by analyzing such traces in the spectrum.
has said so much, it seems that all our understanding of exoplanets is obtained through indirect means. Haven’t there been exoplanets that were directly “see” by astronomers?
has, but very few. Moreover, the appearance of these planets being seen by astronomers is likely to be very different from what you imagined.
What you imagine might be like:
or no matter how bad it is, it should be like this:
But in fact, the exoplanet that astronomers can directly "see" is like this:
Don't rush to comment first. The highlight in the middle is not the planet that astronomers "see" but a star. The planet in the photo is the small gray dot located on the upper left of the star . You can wipe the screen with your hands to make sure it is not a small stain that sticks to the screen. I believe this picture will give you a better understanding of how big the brightness contrast between planets and stars is.
As mentioned earlier, it is as difficult as finding a firefly next to a searchlight a few kilometers away. Astronomers have thought of many ways to overcome this difficulty. First, they used infrared for observation, which greatly reduced the brightness contrast between planets and stars. For example, in visible light, the sun is billions of times brighter than Jupiter. But if we observe it with infrared rays, the brightness comparison between the two will drop to only 100 times.
In addition, astronomers can also use corona meter to block the light of the star, so as to better observe the areas around the star. But even so, only a small number of planets far enough away from their parent planet can be directly "see" by astronomers.
Among the more than 5,000 exoplanets discovered by humans so far, only about 1% were found using this direct observation method, and the remaining 99% were all discovered by indirect methods.
Whether it is an indirect or direct method, only planets that just meet various conditions may be discovered by astronomers. For example, the transit method requires that the planet being observed, its parent star, and the earth be connected in a line; the radial velocity method requires that the planet being observed is large in size and is very close to the parent star, so that it can disturb the movement of the parent star; the direct observation method requires a less bright star plus a planet that is very far away from it, etc.
We can reasonably infer that due to the limitations of human current observation methods, the 5,000 exoplanets we have discovered are only a very small part of the planets in the universe.
So, how many planets are there in the universe? No one knows this, but some astronomers estimate that there are at least 10 billion planets similar to Earth in the Milky Way alone.
If we summarize it, it means that among the 10 billion potential planets in the Milky Way, humans "detected" 0.00005% of them, and then in these 0.00005% were "detected" into the planets, and only 1% of the planets were truly "seeing" by us.
This is the current human ability to observe exoplanets.
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