More than 20 types of amino acids found, extraterrestrial life really exists?

Recently, Japan's Ministry of Education, Culture, Sports, Science, and Technology said scientists detected more than 20 amino acids in samples collected by the asteroid probe "Hayabusa 2". These samples were taken from the asteroid "Ryugu" by "Hayabusa 2" in 2020. According to Professor Emeritus of Astrobiology Kenshasa Kobayashi of Yokohama National University, the discovery of multiple amino acids on celestial bodies beyond Earth is "unprecedented" and may even suggest the existence of life beyond Earth.

Is this discovery of amino acids so amazing? In response to China Science News, several experts pointed out that this is not the first time that human beings have found amino acids in extraterrestrial materials, and it is difficult to summarize discoveries in extraterrestrial exploration by saying "there is no life". However, the samples brought back by Hayabusa 2 can help scientists understand the important scientific issue of the origin of life on early Earth, what the Earth might have been like during the pre-life period, and how the first proteins were formed without the involvement of life.

Not the first discovery

In response to the detection of amino acids on the surface of the "Dragon Palace", Ji Jianghui, a researcher at the Purple Mountain Observatory of the Chinese Academy of Sciences, said that this is not the first time scientists have found amino acids outside the Earth.

Scientists found more than 100 amino acids in the Murchison meteorite that fell in Australia in 1969, but there is insufficient evidence for the existence of extraterrestrial amino acids because the meteorite may have been contaminated by the Earth. In addition, scientists have found the presence of the simplest amino acid, glycine, in Sagittarius B2, Orion KL, and W51.

In 2015, the Philae lander carried by the Rosetta spacecraft found nitrogen and amino acids, key components that makeup life, on the comet Churyumov-Gerasimenko. Scientists have also found amino acids in asteroid meteorite samples several times before.

However, compared to previously studied meteorites, these samples collected from extraterrestrial bodies in their natural state are very special. Moreover, the Ryugu samples came from beneath the surface of the asteroid, which is not exposed to sunlight or cosmic rays, and the samples were analyzed without being exposed to Earth's air.

Amino acids are the building blocks of proteins. There are two theories as to where the original amino acids on Earth came from, Earth-formed and space-formed. One theory suggests that amino acids were brought to Earth from space via meteorites, where they have previously been detected; however, there are also claims that amino acids were originally present on Earth and that the amino acids found on meteorites were attached after the meteorites fell to Earth.

Zhu Jin, honorary director of the Beijing Planetarium and chief scientist of the Science Communication Center of the Beijing Academy of Science and Technology, said that water and amino acids on Earth could have come to the Earth's surface in the form of asteroids (small celestial bodies) hitting the Earth. "The discovery of Hayabusa II is some kind of support for this view.

Not the only criterion

Amino acids are not the only criterion for judging whether life exists or not.

According to Shen Jianxun, a postdoctoral fellow at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, there are two types of amino acids: those that can synthesize biological proteins and those that can't. As long as there are amino and carboxylic acid functional groups, they can be called amino acids in a broad sense.

"The amino acids found in biological proteins so far are 23 alpha-amino acids possessing left-handed spin and specific branched chains, and thousands of other amino acids do not provide the constituent components for proteins of earthly organisms, except for some post-translational modifications of proteins that form derived amino acids." Shin emphasized.

According to the report, the amino acids found in the "Dragon Palace" include isoleucine and valine, which are essential for humans.

In addition, scientists also found glycine, which is used as a collagen material, and glutamic acid, which is widely known as a refreshing ingredient, as well as a variety of non-protein amino acids.

"Structural proteins known to live on Earth are synthesized from levorotatory amino acids as basic building blocks, and this selection for left- and right-handed chirality occurs almost exclusively in living organisms; amino acids formed by non-living processes in nature or the laboratory are usually 'levorotatory' and 'right-handed' 50/50." Just as the discovery of bricks does not necessarily mean there must be a building there, the detection of chemicals that can form life does not mean there is life, nor can it be inferred how long it will take for life to form, Shin said.

He noted that given that we only know the characteristics of life on Earth, this may limit our prediction of the existence of alien forms of life. Therefore, it is difficult to summarize new discoveries in extraterrestrial exploration in terms of the presence or absence of life, and we can only describe these phenomena based on the complexity of the chemical system.

Asteroid detection is accelerating

With so many asteroids in space, why are scientists focusing on the "Ryugu"?

"Ryugu" is a carbonaceous asteroid (C-type asteroid), with a diameter of about 900 meters, formed by a large number of small and large rocks gathered together by their gravity. Ji Jiang Hui said, according to the observation of "Ryugu" by Hayabusa 2, there are traces of hydrated minerals inside the asteroid, which may be evidence that "water" was brought to Earth from interstellar space. The origin of life may have come from the asteroid.

Jianxun Shen pointed out that the high amino acid diversity of extraterrestrial objects and possible interstellar transport pathways are really exciting in the astrobiology community, suggesting that Mars, which is very similar to Earth, may also have a greater diversity of organic molecules below the surface. Therefore, the discovery of the "Dragon Palace" has given scientists more motivation and a sense of mission to detect the signals of life on Mars.

Asteroid detection has always been a popular research direction. In addition to Japan's "Hayabusa 2", in 2023, the U.S. probe Osiris-REx will collect samples from the asteroid "Bennu" and send them back to Earth.