Do you know how hard scientists have worked to find aliens?

Not long ago, a fake "alien" body was brought into the Mexican Congress, sparking a wave of heated discussions. There was even a rumor that "Mexico became the first country in the world to recognize the existence of aliens" (click on the title to review ~ note that it has been debunked ).

The dry and hard creature in the related video screenshot is the so-called "alien" this time.

In fact, if you do some research, you will find that this "alien" was skinned two years ago and is made of a mix of various bones. For example, its left thigh is a human tibia, its arms and right thigh are human femurs (the right thigh is upside down), and the skull comes from the back of an alpaca. It can be said to be a very mixed bag.

Well, let’s not talk about this fake “alien” thing for now, after all, if you look at the liar for a second, you lose. Now let’s talk seriously about the various efforts of the scientific community to find extraterrestrial life (referred to as aliens, but it should be understood that it may not be human-shaped)!

Some friends may ask: Since we are trying hard to find aliens, does that mean we admit that aliens exist?

It is not difficult to understand if you think about it. The Milky Way alone has at least 200 billion stars. The universe is so vast that it would be absurd if intelligent life evolved only on Earth. However, we must make a clear distinction here. It is one thing to believe that there is intelligent life on other planets, but it is another thing to know whether aliens have the ability to come to Earth or whether they have come to Earth. This requires solid evidence. If you clarify these concepts, you will not fall into the language trap of obsessed alien enthusiasts.

Unfortunately, we have not found aliens yet. After all, it is so difficult to make discoveries in the field of science, much more difficult than the two-piece alpaca man. But this does not prevent researchers from using all kinds of methods to search for aliens. Let's take a look at what methods they are.

Waiting for aliens to come

So far, only a handful of man-made probes have flown out of the solar system, so if the Earth and aliens really have contact, the people on Earth would probably feel ashamed: Sorry, we are too backward to travel far, and can only wait for you to come. So how do we wait specifically?

There are folk ways to wait, for example, many alien civilization enthusiasts will seriously report UFO incidents. Unfortunately, until now, most UFOs have been proven to be nothing more than airplanes, Venus, rocket clouds or other atmospheric optical phenomena, and those that cannot be identified have not yet reached the point where they must be explained by aliens.

Murchison Meteorite (part) Source: Melbourne Museum, Australia

A more proven method is to look for clues of life from meteorites or nearby asteroids.

On September 28, 1969, a meteoroid rushed into the atmosphere over Murchison, Australia, and turned into a pile of meteorites in a deafening roar. Studies on this pile of meteorites, which are 2.5 billion years older than the Earth, have found that they contain many organic compounds, including aromatic hydrocarbons, carboxylic acids, sulfonic acids, fullerenes, aliphatic hydrocarbons, purines, pyrimidines... and 70 kinds of amino acids! These molecules that are closely related to life can't help but make people imagine.

At the end of 2020, Hayabusa 2 returned gravel samples from the Ryugu asteroid to Earth, from which more than 20 amino acids were detected. In March 2023, uracil and vitamin B3 were also detected. All this evidence shows that, at least for life forms on Earth, there are indeed some familiar basic building blocks in space. Perhaps alien life has some similarities with our "chemical composition".

Turn the solar system upside down

Although our space capabilities are limited now, we can still visit nearby planets and even satellites within the limits of our capabilities to see if there are any surprises in these unknown corners.

Mars, which has a similar rotation period and inclination to the Earth and has contributed inspiration to countless science fiction novels, Jupiter's satellites that may have huge under-ice oceans, Titan with a dense atmosphere rich in hydrocarbons, and Enceladus, which has ice volcanoes and can spew out a variety of intriguing compounds... are all targets of human eager exploration.

A huge plume erupts from the ice volcano on Enceladus. Source: NASA

The rovers sent to Mars have successively discovered hydrated minerals, lake traces and evidence of existing liquid water. The European Space Agency's icy Europa probe is on its way to Jupiter. The Huygens lander landed on the surface of Titan, showing a surface made of a mixture of water and hydrocarbon ice and landforms eroded by fluids. The Cassini probe personally passed through the plume of Enceladus' eruption and "tasted" the chemical composition of the particles in it.

These are the progress that humans have made in recent decades. Maybe there is really no life in these places, maybe we haven't turned to the right corner, but we have been working hard.

"Posting small advertisements" everywhere

We can also distribute flyers into deep space with a little self-introduction attached, in case intelligent life picks them up, they might come to visit. Pioneer 10 and 11, Voyager 1 and 2, respectively, carried "Earth Business Card" gold-plated aluminum plates and "Earth Voice" golden records to the stars, both of which marked the location of the Earth in a way that "if you can guess the design idea, you may be able to understand it."

The "Earth business card" on the Pioneer. Source: NASA

But we don’t believe that aliens will pick up this kind of drifting bottle. After all, the universe is really empty. Even the collision between the Andromeda Galaxy and the Milky Way will be like two ghosts passing through each other. Therefore, these probes may not be picked up by any civilization in the end. They will drift in interstellar space for 100,000 years or 10 billion years, becoming an indelible monument to Earth civilization (if they don’t collide with any celestial body).

A message in a bottle can also be a faster radio wave. On November 16, 1974, at the celebration of the renovation of the Arecibo radio telescope in Puerto Rico, a 23×73 pixel binary image was sent to the M13 star cluster 22,000 light years away.

This interstellar broadcast is not intended to directly contact aliens (technically impossible), but is only to show the achievements of mankind to the starry sky. However, if aliens receive it more than 20,000 years later, it is hard to say how they will understand it (I have to say that this picture is somewhat abstract...).

Arecibo information. Source: Public sources

However, many people of insight have also pointed out that this practice of actively exposing Earth information carries significant risks. Think about ourselves. If there are intelligent creatures other than Homo sapiens on Earth, can we really coexist peacefully? With whom have we humans ever coexisted peacefully? If we see creatures on Mars that seem to be bullied, can we really control our hands from collecting specimens? We are not confident in our own ethical standards, so how can we have unfounded fantasies about aliens? If you have some doubts about this, think about the "Don't answer" in Liu Cixin's "The Three-Body Problem".

Listen quietly

If we want to search for aliens safely, the best way is probably not to send out signals, but just listen. In fact, this is also a kind of "waiting at home", but aliens may not come in person, what we are waiting for is electromagnetic waves. However, we have not received any signals from intelligent life so far, and we often make mistakes.

For example, in November 1967, when Jocelyn Bell of Cambridge University was examining the signals received by the radio telescope, she noticed a series of regular pulses with a stable period of 1.337 seconds. At first, she thought it was a communication message from an alien civilization and named it "Little Green Man", but she and her mentor soon discovered a signal with a period of 1.2 seconds, which was very similar to the previous signal characteristics, but this signal came from a completely different area of ​​the Milky Way. Therefore, they ruled out the hypothesis of alien civilization and believed that these signals might come from similar stars. Later, people confirmed that this was the neutron star predicted by physics.

There are many more signals coming from deep space, including other neutron stars and various radio bursts, but the radio waves currently received are either monotonously repetitive or disappear forever. From the perspective of the information science mastered by humans on Earth, there is no "human" smell in them.

There have been even bigger blunders in this regard. For example, in 1998, the Parkes Observatory in Australia received a strange radio signal that lasted for a long time and was very regular. The ecstatic scientists studied the signal for 17 years, but finally found that the signal came from a microwave oven...

Listen quietly

Another most intuitive method is to just “look” – literally.
Assuming that aliens also live on planets, we can use the light of their stars to discover clues about them.

As of September 2023, humans have confirmed 5,506 exoplanets. There are four or five main methods for detecting exoplanets, one of which is the transit method, which has been very fruitful and is particularly suitable for discovering aliens.

The transit of Venus in 2012. Photo by the author

A transit is when a planet passes between its star and the Earth, just like our Mercury and Venus sometimes transit the Sun. When a planet transits a star, the star will appear a little darker because the planet blocks part of the star. For example, when Venus transits the Sun, the Sun will appear about 1‰ darker. In the eyes of sensitive space telescopes, this little bit is enough!

Why is the transit method particularly suitable for discovering aliens? Because when a transit occurs, the starlight passes through the planet's atmosphere, and the substances in the atmosphere absorb specific wavelengths. By analyzing the spectrum, people on Earth can know its atmospheric composition. If there are substances that are difficult to produce in nature, such as Freon, we can be very confident that:

"I think this star is worth a look."

—In the depths of the universe, an alien scientist observing our sun muttered to himself.

Planning and production

Author: Qu Jiong Popular Science Writer

Review丨Liu Qian, Researcher at Beijing Planetarium

Planning｜Ding Zong

Editor: Ding Zong