In the search for extraterrestrial life, why do scientists use Earth life as a reference?

Image caption: The ocean world beneath Europa's ice layer is thought to be potentially suitable for life

With the launch of the Europa Clipper probe, humanity has taken another important step in the search for extraterrestrial life. This mission will explore the ocean beneath the icy surface of Europa to find potential habitable environments for life on this planet.

Some people may ask: Why do astronomers always look for signs of alien life based on life forms on Earth? Isn't it possible that alien life is completely different from us?

For scientists, searching for extraterrestrial life is like playing hide-and-seek in an unfamiliar place. If you play hide-and-seek in your familiar home, you can easily think of common hiding places such as under the bed, in the closet, and behind the sofa. But in an unfamiliar environment, the challenge lies in those unimaginable hidden corners.

Although scientists are also working hard to find new ways to detect extraterrestrial life, they are currently still exploring based on the life forms on Earth, because this is the scope of scientists' imagination. So, in this interstellar hide-and-seek, can scientists uncover the mystery of those unknown life forms?

0 1 Closer Exploration

Searching for extraterrestrial life on planets within the solar system is undoubtedly the most direct and simplest way.

As early as 1976, NASA's "Viking 1" mission embarked on a journey to search for life on Mars. As Earth's closest neighbor, Mars, looking for signs of life on it is the primary and most important scientific mission of "Viking 1". Soon after the lander landed steadily on the surface of Mars, it began to try to dig out any traces of life in this foreign soil.

Image caption: A photo of Mars taken by the Viking 1 lander in August 1976

Scientists knew that life on Mars would likely be very different from life on Earth, so they didn’t limit themselves to looking for specific organisms or molecules. Instead, they designed a range of experiments designed to capture subtle signs of life, not just physical forms.

For example, plants on Earth draw energy and thrive through photosynthesis, a process that requires the participation of sunlight and carbon dioxide. Therefore, scientists on Viking 1 cleverly designed an experiment to test whether the Martian soil has the ability to absorb carbon dioxide from the air.

In the experiment, the lander collected soil samples from the surface of Mars, irradiated them with light, and precisely measured whether carbon dioxide was absorbed by the soil. However, the experimental results did not show any trace of photosynthesis in the Martian soil.

The lander also carries two other experiments: one uses carbon dioxide gas, and the other uses sugars and amino acids, both of which are "delicacies" favored by life on Earth, to test whether there are signs of microbial growth in the Martian soil.

Ultimately, the results of these three experiments led most scientists to conclude that there is probably no life on the surface of Mars, at least no life that can photosynthesize or grow on sugars. However, scientists still cannot determine whether there are traces of life hidden beneath the surface of Mars, or even whether there is life lurking deep under the surface of Mars.

0 2 Long-distance exploration

Compared with the exploration within the solar system, the search for life outside the solar system is undoubtedly more difficult and requires completely different technical means.

The nearest exoplanet to Earth, Proxima B, is about 4.2 light-years away. These distant planets are out of reach, and with the current level of technology, humans are unable to send probes for on-site exploration.

Searching for life on exoplanets is like playing hide-and-seek in your neighbor's house, but you can only peer through the window and never step in. You might find a hiding place from the right perspective, but you have no idea how many hidden corners you haven't yet seen.

Image caption: Proxima Centauri photographed by the Hubble Space Telescope. It is about 4.2 light-years away from us and is the closest star to the sun. Is there life on the planets orbiting it?

Top observation tools like the James Webb Space Telescope can reveal the size of exoplanets, their distance to their central stars, and may even detect the gas composition of their atmospheres, but this is only limited to obtaining these basic information. How can scientists use this information to search for traces of life?

Astronomers initially searched for life by looking for oxygen, because on Earth, life activities produce most of the oxygen in the atmosphere, and perhaps the oxygen on a planet is also produced by alien life.

However, as the research deepened, scientists discovered that oxygen can also be produced by non-living means. Therefore, astronomers are no longer limited to looking for oxygen, but are looking at the combination of gases such as oxygen, water, methane and carbon dioxide. Because scientists believe that planets without life are unlikely to have a combination of these gases at the same time. Of course, this judgment is not absolute!

Image caption: Because specific elements emit specific wavelengths of light, scientists can determine what a distant planet's atmosphere is made of based on its spectrum.

Looking for these combinations of gases is like looking behind the couch in a game of hide-and-seek. Scientists don't know for sure what they'll find, but there are only so many places they can look, so every possible place is worth trying and exploring. Where else can scientists look?

0 3How to search for extraterrestrial life?

There are two essential differences between playing hide and seek and searching for extraterrestrial life.

First, when playing hide-and-seek, you usually know that someone is playing with you. However, in the search for extraterrestrial life, scientists do not know whether there is other life in the universe. Perhaps there is nothing in the universe, or perhaps extraterrestrial life exists quietly near the stars around us.

Until they find definitive evidence of life beyond Earth, scientists won't be able to determine how common life is in the universe.

Second, most scientists do not believe that alien life is deliberately hiding, we just haven't found it yet. Some theories suggest that more advanced civilizations may deliberately avoid detection, but researchers believe this is unlikely within the solar system.

Most astronomers and astrobiologists know that if you limit your search to life forms similar to Earth, you may miss signs of life that are very different. However, after all, scientists have never actually detected the existence of alien life, so how to start the search is indeed a huge challenge. In this case, starting with a feasible direction (i.e., using the forms of life on Earth as a reference) is much better than doing nothing.

Experiments with probes like Viking 1 or searching for oxygen may not guarantee that scientists will find extraterrestrial life, but doing so may help them find it. Even if not, scientists can eliminate some obvious possibilities and focus on more elusive clues. That's why scientists are currently looking for extraterrestrial life based on Earth life.

References

https://theconversation.com/why-do-astronomers-look-for-signs-of-life-on-other-planets-based-on-what-life-is-like-on-earth-227658

Compiled by: Wen Xing

Planning: Liu Kun, Li Peiyuan, Zhang Chao, Yang Liu

Reviewed by: Li Xin, Research Librarian of Beijing Planetarium