The Venus life incident almost brought this emerging research field into a dead end!

Scientists who are tirelessly searching for extraterrestrial life in the distant stars have always relied on biosignatures as important indicators. Although extraterrestrial life has remained elusive to this day, criticism of biosignatures, especially doubts about their uncertainty, seems to have brought more challenges to the search for extraterrestrial life, making the direction of astrobiology more confusing.

Compiled by Xiaoye

In September 2020, a discovery announced by an international team of astronomers caused a huge sensation around the world.

They published a paper in the journal Nature Astronomy[1], reporting the detection of high concentrations (100 ppb) of phosphine gas absorption lines in the atmosphere of Venus, a planet the size of the Earth. After analyzing the possible causes, they concluded that there are previously unknown photochemical reactions on Venus, or that the phosphine in the atmosphere may be a mark left by some kind of life.

The latter speculation quickly made headlines around the world with headlines like “Life on Venus.” As The New York Times reported, “Scientists know that phosphine cannot be produced except by biological processes, so they assert that living organisms are the only explanation for the chemical’s origin.”

But that's not true.

The existence of life in the atmosphere of Venus is still a matter of great controversy even today, with the scientific community unable to agree on whether phosphine exists on Venus, let alone whether it is strong evidence for extraterrestrial life.

How to search for signs of extraterrestrial life?

Image source: pixabay

When the news of "the existence of life on Venus" spread around the world, it soon attracted doubts from academic experts.

First, the natural environment of Venus is a veritable "hell": the temperature of the rocky surface can reach 480 degrees Celsius during the day, the atmosphere is filled with carbon dioxide, and there are clouds composed of concentrated sulfuric acid, and sulfuric acid rain comes from time to time. In such an environment, how can life survive for a long time? [2]

Secondly, although the phosphine content in the Earth's atmosphere is very low, it mainly comes from human activities, the release of anaerobic microorganisms, and photochemical reactions such as the reduction and decomposition of organic matter. However, in the solar system, phosphine generated by non-biological means exists in the turbulence of the atmospheres of Saturn and Jupiter: after it is formed under the high temperature and high pressure conditions inside the planet, it is continuously transferred into the atmosphere. A paper published in the Proceedings of the National Academy of Sciences (PNAS) in 2021 [3], through the analysis of data from multiple radio telescopes, proposed that the phosphides in the mantle of Venus entered the atmosphere through active explosive volcanic activity, and reacted with sulfuric acid in the atmosphere to produce phosphine, explaining the possible non-biological source of phosphine.

Finally, some experts criticized the team’s handling of data fitting. The team used detection data from the Atacama Millimeter/Submillimeter Array (ALMA), but the ALMA data has high noise, so the researchers used a large number of variable modeling to eliminate the noise. Other experts pointed out that this rather radical data repair method is likely to produce false positive results [4].

Two months after the paper was published, the team also published a statement through editing on the paper page, admitting that the data processing method was incorrect and that data calibration did affect the research conclusions [1].

In addition, the team of Geronimo Villanueva, a planetary astronomer at NASA's Goddard Space Flight Center, questioned the basic data of the study. In a comment submitted to Nature Astronomy, they pointed out that the phosphine absorption line mentioned by the original paper's authors may actually be sulfur dioxide gas that makes up the clouds of Venus[5]. Therefore, whether the original paper team discovered phosphine is still controversial and requires further observation and analysis, not to mention whether this gas is strong evidence of the existence of alien life on our "friendly neighbor".

So why are scientists so concerned about the composition of planetary atmospheres?

First, we need to understand how scientists search for extraterrestrial or even extrasolar life. For planets in the solar system, we can also use telescopes for remote sensing observations, or launch probes and landers to conduct "field investigations" to search for evidence of life. However, for distant exoplanets light years away, the main and even the only means at present is to use various large astronomical telescopes on land or in space for remote sensing observations, first find exoplanets with atmospheres, and then analyze the composition of their atmospheres.

Specifically, when a planet in a galaxy orbits in front of its host star, the light emitted by the star will penetrate the planet's atmosphere, and astronomers can use the changes in the color of the light at this time to detect the composition of the planet's atmosphere. Because starlight is composed of many different colors (as shown in Figure 1 below), and the molecules in the planet's atmosphere can absorb specific color spectra of starlight, the continuous star spectrum will become intermittent (as shown in Figure 2 below). Large telescopes such as the James Webb Space Telescope (JWST) can observe the absence of spectra, thereby helping astronomers infer the composition of planetary atmospheres.

Figure 1: The continuous spectrum of starlight

Figure 2: After starlight is absorbed by different molecules in the atmosphere of a planet similar to the Earth, a discontinuous spectrum appears.

After determining the composition of the atmosphere, scientists further traced the source of these gases to determine whether they were from biological or non-biological activities, and ultimately determine whether there might be signs of life on the observed planet. Even the most primitive and simple life activities are enough to inspire people.

In the final stage, biomarkers became a basic but broad indicator for determining whether there are signs of life. Earth is currently the only planet known to scientists to have life, and therefore has become the only benchmark for searching for extraterrestrial life. Any material component related to past or present biological products can be considered a biomarker. For example, oxygen, a large amount of stable oxygen on Earth is the result of continuous production by biological photosynthesis; and methane, most of the methane on Earth is produced by biological activities and the product of human industrial activities [6, 7].

Uncertainty of biomarkers

However, does finding biomarkers confirm the presence of signs of life?

Things are not simple. From the perspective of the universe, even the situation on Venus, which is so close to us, is difficult to judge, let alone the exoplanets that are several light years away from us.

If scientists detect a putative biomarker gas on an exoplanet, they then use a statistical method called Bayesian inference to calculate the chances of life being present based on three major probabilities, two of which are relevant to biology and have significant uncertainty: the first probability is the probability of life being present, taking into account all known information about the planet; the second is a "priori" probability, assuming that life exists on the planet, and then calculating the probability of detectable life signals based on our current background knowledge of planetary science.

The third probability involves the probability of a lifeless planet producing detectable life signals. In recent years, more and more researchers have realized that this is precisely the daunting challenge they face.

Figure 3: Astrobiologists use Bayesian reasoning to calculate the probability of life on an exoplanet, P(life|D,C), using three major probabilities as parameters:

P(D|C, life): Calculate the probability of the emergence of life by taking into account all known information about the planet: data (DATA) and planetary science background knowledge (CONTEXT)

P(life|C): The “prior” probability that a planet could produce detectable life signals, given our current background knowledge of planetary science, assuming that life exists on the planet.

P(no life|C) : The probability that a lifeless planet will produce detectable life signals

Source: 10.1089/ast.2017.1737[8]

Last year, Arizona State University astrobiologist Cole Mathis and Tokyo Institute of Technology Earth-Life Science Institute Harrison Smith published a paper in BioEssays[9], which discussed the biggest difficulty in identifying biomarkers on exoplanets, which is to distinguish between biological processes and non-biological processes. Because many other biomarkers proposed in studies may also appear in non-biological systems, thus leading to false positive results of signs of life, it is very important to be able to identify clear biomarkers that are unique to biological activity.

The theory of "unconceived alternatives"[10] proposed by Peter Vickers, a philosopher of science at Durham University, is closely related to the third possibility. In short, whether it is the geological and chemical environment of exoplanets or extrasolar life, there are actually many mysteries that we have yet to solve. So even if the presence of a certain gas is detected, how can scientists be sure that all possible non-biological sources have been ruled out before determining that it is a biomarker of biological origin?

“We are always exploring new ideas. There may be non-biological sources, but we have not thought of them yet,” said Vickers. “This is the unexpected alternative explanation in astrobiology. We cannot responsibly say that it is completely impossible. The probability is between 0 and 1. Anything is possible.”[11]

NASA's Webb Space Telescope was launched in 2021 and sent back data on the atmospheric composition of the exoplanet K2-18 b during its operation. Some people interpret this as a possible phenomenon of life, but controversy also exists. K2-18 b is 120 light-years away from Earth and is between the size of Earth and Neptune. It is considered a "super-Earth" and orbits a red dwarf star K2-18, located in the habitable zone. In 2023, data from the Webb Space Telescope showed that there were statistically weak traces of dimethyl sulfide (DMS) in the planet's atmosphere [12]. On Earth, marine life produces DMS, so researchers explained that this planet may be a "water world" with a habitable ocean surface.

However, other scientists have given a different explanation for the same gases: this atmospheric composition can also prove that this is a gas planet similar to Neptune and is not habitable.

Vickers' theory of "unexpected alternative explanations" has repeatedly forced astrobiologists to revise their views on what substances are definite and appropriate biomarkers. The oxygen and phosphine mentioned above are both uncertain. Before the 2010s, oxygen was considered a strong biomarker gas, but researchers including Victoria Meadows of NASA's Astrobiology Institute began to discover that rocky planets also have ways to accumulate oxygen without a biosphere, such as through sulfur dioxide, which is abundant on Venus and Europa.

As long as all non-biological sources cannot be ruled out, finding a single so-called "biosignature" gas cannot be equated with a sign of life.

Combined gas enhances certainty

In fact, astrobiologists have largely given up on the idea of ​​using just a single gas as a biomarker. Instead, they are focusing on identifying “ensembles,” or groups of gases that cannot coexist in the absence of life, to increase the certainty of a biomarker and strengthen its association with signs of life.

Today, if there is a gold standard for biomarkers, it is a combination of oxygen and methane. In an oxygen-rich atmosphere, methane degrades quickly. On Earth, the two gases can coexist because the biosphere provides a constant supply of oxygen and methane.

So far, scientists have not found an abiotic explanation for the oxygen-methane biomarker. But Vickers, Smith, and Mathis remain cautiously skeptical that this pair of gases (or any other mixture of gases) will ever be completely convincing. "We have no way to be 100% sure that what we see is the product of biological activity and not the result of some unknown planetary abiotic chemistry," Smith said.

Mathis pointed out a limitation of remote sensing observations: "JWST is not a life detector. It is a telescope. It can only tell us what gases are in the atmosphere of the planet."

Sarah Rugheimer, an astrobiologist at the University of York, is more optimistic, however, and is actively investigating non-biological alternative explanations for combined biomarkers such as oxygen and methane. Nevertheless, she said: "If we see oxygen, methane, water and carbon dioxide on an exoplanet, it would be worth popping the champagne."

NASA's Webb telescope obtained a molecular spectrum of the atmospheric components of K2-18 b, including carbon dioxide, methane and dimethyl sulfide. So the question is, can such a combination of gases be used as a combined biomarker to determine the existence of life on the planet? Source: NASA

Academia establishes unified standards?

Faced with the current significant uncertainty in biomarkers, different opinions and interpretations of a test result by different experts in the academic community are likely to stir up various public opinions and may also undermine the public's trust in science. As the Venus life event reaches a climax in 2021, NASA managers and scientists call on the astrobiology community to establish standards for determining biomarkers.

In 2022, hundreds of astrobiologists organized an online workshop to discuss this issue and develop a common framework for biomarker evaluation [13]. The framework consists of five questions aimed at promoting communication between different disciplines in the field of astrobiology and the public:

Did you detect a real signal?

Have you fully confirmed the signal?

Do your test results have a biological origin?

Is it possible for life to express such signals in such an environment?

Is there independent evidence supporting a biological (or abiotic) explanation?

Despite this, there are still no official standards or even definitions for biomarkers in academia.

However, Vickers believes that despite the uncertainty, research should continue to move forward. In emerging fields such as astrobiology, it is not uncommon to encounter dead ends and to go back and start over. Boldly proposing various biomarkers can encourage scientists to actively verify and look for unexpected non-biological explanations to rule out various possible false positive results.

After the uproar over the Venus life incident, academic experts have calmed down and are thinking about the next research direction.

Clara Sousa-Silva, an astrochemist at Bard College in the United States, is an expert on phosphine and has also participated in the study of atmospheric detection on Venus. For her, it is time to reconsider our friendly neighbor Venus. The huge controversy over biomarkers has prompted scientists to launch new research projects, not only to discover and rule out non-biological sources of phosphine, but also to provide stronger evaluation criteria for the precise exploration of signs of life on exoplanets in the future.

References

[1] https://www.nature.com/articles/s41550-020-1174-4

[2]https://piyao..cn/h5/rumordetail?id=qpjX

[3]https://www.pnas.org/doi/10.1073/pnas.2021689118

[4] https://www.cas.cn/kj/202011/t20201119_4767449.shtml

[5] https://www.nature.com/articles/s41550-021-01422-z

[6]https://zh.wikipedia.org/wiki/%E7%94%9F%E5%91%BD%E5%8D%B0%E8%BF%B9

[7]https://www.zhihu.com/question/421069854/answer/1477780574

[8]https://www.liebertpub.com/doi/10.1089/ast.2017.1737

[9]https://onlinelibrary.wiley.com/doi/10.1002/bies.202300050?af=R

[10]https://www.liebertpub.com/doi/10.1089/ast.2022.0084

[11] https://www.quantamagazine.org/doubts-grow-about-the-biosignature-approach-to-alien-hunting-20240319/

[12]https://www.nasa.gov/universe/exoplanets/webb-discovers-methane-carbon-dioxide-in-atmosphere-of-k2-18-b/

[13]https://nap.nationalacademies.org/read/26621/chapter/4#8

This article is supported by the Science Popularization China Starry Sky Project

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.

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