Shulou(Shulou.com)11/24 Report--

In June, astronomers were disappointed to find that the James Webb Space Telescope (James Webb Space Telescope,JWST) failed to find a thick atmosphere around the rocky planet TRAPPIST-1C. TRAPPIST-1C is an exoplanet in the TRAPPIST-1 galaxy, and the TRAPPIST-1 galaxy is the most likely to find extraterrestrial life.

Before that, another planet in the TRAPPIST-1 system, TRAPPIST-1 B, was found to have a similar situation. The system with dim red stars has seven rocky planets, several of which are located in the habitable zone. The habitable zone refers to the distance around a star at which there may be liquid water on their surface, where life in other worlds may thrive.

If life exists on a planet, what tools do we need to verify it? this is a question that has perplexed astronomers for a long time. If life exists, what is needed to discover it is not a new question. But thanks to the James Webb Space Telescope, these problems can finally be solved. Over the next few years, JWST will be able to detect the atmospheres of several planets that may give birth to life in distant galaxies. Chemical signals hidden in the atmospheres of these planets may be the first signs of life outside the solar system. This raises another question: what chemical signals can be detected to indicate the existence of extraterrestrial life.

"you try to use very little information about one planet to come to a conclusion that can be quite profound-to change our view of the universe as a whole." Said Joshua Krissansen-Totton, a planetary scientist at the University of Washington.

In order to detect such a biological feature, scientists must find an ingenious way to deal with the limited information they collect about exoplanets.

The chemicals in the background, even the most powerful telescopes, including JWST, have almost never really "seen" exoplanets-in general, astronomers can only learn about these distant worlds by flickering.

Instead of observing the planets directly, astronomers point their telescopes at the stars and wait for the planets to "transit", that is, to pass between their stars and their telescopes. When a planet transits, light from some stars passes through its atmosphere and darkens the star at specific wavelengths, depending on the chemicals in the atmosphere. The resulting peaks and troughs of stellar brightness are like chemical bar codes for transit planets.

When the planet is in front of the star, part of the starlight is absorbed by molecules in the planet's atmosphere. Telescopes aimed at stars can detect light that is not absorbed; this forms a spectrum, with pits where the light is missing. In the image above, each concave spot is characteristic of a specific molecule in the spectrum of a hypothetical Earth-like planet. | Source: Knowledge Magazine, source: adapted from National Aeronautics and Space Administration (NASA), National Aeronautics and Space Administration (ESA), National Space Administration of China (CSA), National Academy of Sciences (STSC), Joseph Olmsted (Joseph Olmsted). Perhaps the most direct way to collect information about the existence of organisms in these barcode-like planetary spectral signals is to detect the presence of chemical signals of gases produced by life. For a time, scientists thought that the abundance of oxygen on the earth due to light cooperation was a unique feature. But oxygen may also be produced in other processes, such as sunlight that can decompose water in the earth's atmosphere.

This problem also exists for other gases, and most of the gases produced by organisms are also produced without life. As a result, today's scientists tend to consider these gases in combination with specific circumstances, rather than regarding a single gas as a biometric feature.

For example, methane is produced regardless of the existence of life. It is not a convincing biological feature in itself. But finding methane and oxygen at the same time "would be very exciting", said Robin Wordsworth, a planetary scientist at Harvard University; it would be difficult to produce such a combination without life. Similarly, recent research by Krissansen-Totton and his colleagues shows that if there is no life, it is difficult to explain the discovery of methane along with appropriate amounts of other gases, such as carbon dioxide.

Observations of the atmospheric changes of exoplanets over time may also provide valuable background information to further confirm the biological characteristics of the planets. For example, a 2018 study showed that seasonal changes in ozone concentrations may be the fingerprints of life.

Abnormal, rather than hypothetical, of course, "if you're looking for a particular gas, such as oxygen or methane, then you've assumed life types elsewhere," Christensen-Thornton said. Therefore, some scientists assume that there are no similarities between extraterrestrial biochemistry and earth biochemistry on the premise of unknown biological characteristics.

One possible indicator of unknown biological characteristics is the extent of chemical "anomalies" in the atmospheres of exoplanets-what scientists call a chemical imbalance.

A near-equilibrium atmosphere is chemically boring, like a closed gas flask in a laboratory. Of course, no planet is really as boring as a flask in the lab. Stars can power chemical reactions in a planet's atmosphere, and geological processes such as volcanic activity can exacerbate imbalances, exacerbating certain chemical anomalies in the atmosphere.

The existence of life can also cause the planet to deviate from equilibrium. Assuming that extraterrestrial life produces some kind of gas, they will move the planet's atmosphere away from equilibrium. However, the mere imbalance "is not a clear indicator," says Christensen Thornton.

In 2016, he and his colleagues calculated the thermal imbalance between each planet in the solar system and Titan's atmosphere. By comparison, it can be found that the Earth's atmosphere is extreme-but only if the influence of the ocean is taken into account in the calculation. If the interaction between the atmosphere and the ocean is ignored, the Earth's atmosphere is actually closer to equilibrium than the Martian atmosphere.

But even if these anomalies may not point to the existence of life, the discovery that the atmosphere of an exoplanet is far from equilibrium still means that something interesting is happening. "some event is changing the atmosphere in a wonderful way worth studying," Christensen-Thornton said.

Recently, David Kinney, a philosopher of science at Yale University, and Chris Kempes, a biophysicist at the Santa Fe Institute, tried to develop a new way to detect possible unknown biometrics. Their idea is simple: to find life, you have to look for the strangest planets.

Telescopes such as JWST, which specialize in observing distant planets, can record the characteristics of specific elements or molecules in the planet's atmosphere, which show up in the peaks and troughs of transmitted light. But molecular characteristics do not explain its origin: completely different planetary processes, such as active volcanoes, frozen oceans and biological activity, may produce similar spectral characteristics. If no assumptions are made about extraterrestrial life, almost all gases can become biometric in the right environment. In 2016, Sara Seager, an astrophysicist at the Massachusetts Institute of Technology, and her colleagues proposed a list of about 14000 molecules as possible biological features. Kinney and Kempes used the list and machine learning algorithms to develop their assessment methods to screen for abnormal data in observations. Such a method can accurately assess the "anomalies" of the atmospheres of exoplanets, and can score and compare hypothetical atmospheres of different exoplanets.

Ginny and Kempes believe that the strangest atmospheres in a group of planets are most likely to give birth to life. This inference is based on the following basic assumptions: life in the universe is rare and they leave traces in the planetary atmosphere that are difficult to imitate without life. Of course, these assumptions may prove wrong, Ginny said. But "if we don't want to make any assumptions," he added, "then I think it will be difficult to make any form of scientific progress, let alone in an area where there is so much uncertainty.

First of all, to understand inanimate systems in order to reduce this uncertainty, scientists need a vital explanation that can accurately rule out any potential physical characteristics. This requires a thorough understanding of extraterrestrial geology and chemistry. As a result, some scientists believe that rather than focusing on whether planets are habitable or not, studying apparently lifeless planets will help us find extraterrestrial life.

"I think we need to understand a lot of very basic things about planets before we start asking the question of habitability," said Laura Kreidberg of the Max Planck Institute for Astronomy in Germany. He co-authored with Wordsworth an overview of the astronomy of rocky exoplanets, published in the 2022 Annual Review of Astronomy and Astrophysics.

Whether these potentially rocky planets can be observed by JWST will have an atmosphere is a more important question to consider. Within the range of telescopes, only red dwarfs, such as TRAPPIST-1, have habitable zone planets. An annoying feature of these stars is that they emit intense radiation, which many scientists believe will inevitably take away the atmosphere of any potentially habitable planet, which may be why TRAPPIST-1 B and TRAPPIST-1 C atmospheres are rare or even non-existent.

Red dwarfs are also the most common stars in the Milky way-so if their rocky planets fail to maintain their atmosphere, the number of potentially habitable worlds will be greatly reduced.

If we can observe enough exoplanets, "we can learn more about the meaning of biological characteristics," Wordsworth said. "one of the very powerful things that exoplanets bring to us is statistics."

Scientists first confirmed the existence of exoplanets in 1992, and since then, more and more exoplanets have been discovered with the word "biological characteristics" that may lead to direct and conclusive evidence. But "the discovery of exoplanet life requires a gradual accumulation of evidence," Christensen-Thornton said. As the evidence accumulates, scientists can begin to test their assumptions about rocky planets in a more rigorous way, and perhaps re-evaluate them.

"Astronomy is essentially a science of discovery," Credberg said. "even if we have extremely careful plans, frameworks and systems, once we start to get data and observe things, everything will be turned upside down."

Author: Elise Cutts

Translation: September 1st & K.Collider (

Revision: * 0

Original link: What would signal life on another planet?

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop), author: Elise Cutts

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