アストロバイオロジー

tl;dr 宇宙の生命を研究する科学研究分野。宇宙の生命は決定的に観察されていないため、宇宙生物学は、どのような条件下で地球外生命が可能になるのかに最も関心を持っています。

Astrobiology is a multidisciplinary field of study that investigates the origin, evolution, distribution, and future of life in the universe. It combines knowledge from astronomy, biology, geology, and other sciences to understand the conditions necessary for life to exist, both on Earth and elsewhere. To research the possibility of extraterrestrial life, astrobiologists use various techniques and tools such as searching for evidence of past or present life on other planets, moons, or asteroids in our solar system, looking for signs of life on exoplanets around other stars, and studying the chemical and physical conditions necessary for life to form and survive. This research helps us to better understand our place in the universe and answer one of the most fundamental questions in science: are we alone in the universe?

The search of extraterrestrial life

Astrobiology, as a field of study, has been steadily growing over the past decades, driven by the increasing number of exoplanet discoveries and our ability to probe deeper into the universe. As we continue to uncover the vast array of planetary bodies within our own solar system and beyond, the potential for finding life, in some form or another, grows increasingly possible.

Notably, NASA’s Mars rover missions, such as the Mars Science Laboratory mission with the Curiosity rover, have been pivotal in the search for life beyond Earth. These missions have focused on understanding whether Mars, our closest planetary neighbor, has the conditions necessary to support life, either currently or in its past. As part of this ongoing research, rovers have identified traces of water and complex organic molecules, key elements necessary for life as we understand it.

Additionally, the study of extremophiles, organisms on Earth that thrive in extreme conditions, broadens our understanding of what constitutes a habitable environment. Microorganisms thriving in conditions of extreme heat, cold, or toxicity, demonstrate that life can potentially exist in environments far removed from our conventional understanding of habitability. Such insights have significant implications when considering potential extraterrestrial life, given the extreme conditions often encountered on other planets and moons.

One of the significant developments in astrobiology is the search for exoplanets in the habitable zone (or “Goldilocks zone”) around other stars, where conditions might be “just right” for liquid water and thus life to exist. The Kepler mission, for example, has discovered a multitude of such planets, expanding the range of possible locales for extraterrestrial life.

Methodology

The methods of astrobiology include:

  • Observational astronomy: This refers to the direct observation of celestial bodies using telescopes and other detection devices. These observations can provide valuable information about the composition, temperature, and atmospheric properties of distant planets and other celestial bodies. With the aid of spectroscopy, astrobiologists can detect the presence of certain elements or compounds, such as methane or water, that might indicate the potential for life.
  • Laboratory simulations and experiments: Astrobiologists often use laboratory experiments to simulate conditions on other planets or in outer space. This can help to determine what kind of life (if any) could survive under those conditions. For example, scientists might simulate the high-radiation environment of Mars or the high-pressure environment of a gas giant to see how certain organisms or organic compounds react.
  • In situ measurements and sample analysis: The term “in situ” means “in place,” and in the context of astrobiology, it refers to measurements or experiments conducted directly on the celestial body of interest. For example, NASA’s Mars rovers perform in situ analysis of Martian soil and rocks to detect signs of past or present life. Future missions might return samples to Earth for further analysis.
  • Theoretical modeling and computer simulations: These are used to predict what types of life might exist on other planets based on a variety of factors, including the type of star, the planet’s distance from its star, its size, its geological activity, and its atmosphere. Theoretical models also help understand how life might arise and evolve under different conditions.
  • Analysis of meteorites and other extraterrestrial materials: The study of meteorites can provide valuable information about the early solar system and the potential for life on other planets. Some meteorites have been found to contain amino acids, the building blocks of proteins, suggesting that the basic ingredients for life could be common throughout the universe.
  • Study of Earth’s extremophiles and their potential habitability elsewhere: Extremophiles are organisms that can survive in extreme conditions, such as deep-sea vents, high-altitude lakes, or the arid Atacama Desert. By studying how these organisms survive and thrive, astrobiologists can infer what types of life might be possible on other planets with similarly extreme conditions. This research also helps define the limits of what we consider habitable environments.

Each of these methods provides unique insights into the potential for life beyond Earth. Together, they form a comprehensive approach to astrobiology that combines direct observation, laboratory and field research, and theoretical modeling.

Requirements for proof of evidence

Findings that would be considered strong indications of extraterrestrial life would include the discovery of a reliable and repeatable proof of the following conditions:

  1. The existence of organic compounds, a key building block of life, beyond Earth: Organic compounds are those that contain carbon, and they form the basis of all known life. The discovery of such compounds in places other than Earth would suggest that the conditions necessary for life as we know it may exist elsewhere. This evidence could come in the form of complex organic molecules in the atmosphere or on the surface of other planets or moons, or in interstellar clouds of dust and gas.

  2. The presence of liquid water, a vital requirement for life as we know it: All known organisms require liquid water to live and reproduce. The discovery of liquid water on other celestial bodies, such as the recent discoveries of liquid water under the surface of Mars or the ocean worlds of Jupiter’s moon Europa and Saturn’s moon Enceladus, significantly boosts the chances that life could exist there.

  3. Evidence of metabolic activity or biogenic substances produced by living organisms: This could include the detection of gases in a planet’s atmosphere that are typically produced by biological activity, such as oxygen or methane. For example, Earth’s atmosphere contains a high level of oxygen because of the photosynthetic activity of plants and some bacteria. Unusual concentrations or fluctuations of these gases could indicate metabolic activity. Additionally, the discovery of substances like fossil fuels or certain types of rock formations that we know are produced by life on Earth could also indicate the presence of life.

  4. The detection of radio or optical signals that are indicative of intelligence: The Search for Extraterrestrial Intelligence (SETI) is an ongoing scientific effort to detect signals from space that could indicate the existence of technologically advanced civilizations. These could be radio signals, optical pulses, or other types of signals that are unlikely to be produced by natural phenomena and that show signs of being artificially produced, such as patterns or sequences that suggest a coded message.

  5. Direct contact with extraterrestrial beings or discovery of artifacts: This would be the most definitive and compelling evidence of extraterrestrial life, but it is also the most unlikely to occur, given the vast distances between stars and the technological challenges of interstellar travel. However, should we encounter extraterrestrial beings or find artifacts clearly made by an intelligent non-human entity, whether on Earth, in our solar system, or on a distant planet, this would constitute unambiguous proof of life beyond Earth. It’s worth noting that while there have been numerous claims of such contact throughout history, none has been sufficiently substantiated with evidence that satisfies the scientific community.

According to Carl Sagan

Carl Sagan made the following statement in his book “Pale Blue Dot: A Vision of the Human Future in Space” published in 1994.

Since, in the long run, every planetary civilization will be endangered by impacts from space, every surviving civilization is obliged to become spacefaring–not because of exploratory or romantic zeal, but for the most practical reason imaginable: staying alive… If our long-term survival is at stake, we have a basic responsibility to our species to venture to other worlds.

The quote implies that space exploration and colonization is necessary for the survival of humanity in the long run. Sagan argues that the threat of impacts from space is a natural and inevitable danger that every civilization must eventually face, and becoming a spacefaring civilization is the only way to protect ourselves from this threat and ensure the survival of our species.

Astrobiology remains a developing field, with its subject matter largely speculative until definitive proof of extraterrestrial life is found. As Carl Sagan once wrote, “extraordinary claims require extraordinary evidence.” To date, in the broad public domain, this evidence remains elusive, but the search continues.

According to Jean Sendy

In his book The Coming of the Gods, Jean Sendy writes the following fundamental observation about astrobiology:

“Astrophysics” is a neologism that was coined until about 1920, because until then the means of “studying tile phenomena of outer space from the standpoint of physics” were too embryonic to constitute a discipline. The word “exobiology,” which designates “the study of the phenomena of outer space from the standpoint of biology,” is a more recent neologim that appears only in the Dewest dictionaries. The domain of exobiology. however, is still nearly empty: the first hypotheses are heing fomulated, but they rest on nothing concrete. As long as we know nothing about tile evolution of life in the fest of the Galaxy, our biologists will be limited to one particular case: earthly life and evolution.

Jean Sendy provides an insightful observation on the fields of astrophysics and astrobiology (referred to as exobiology in this context). He notes that the term “astrophysics” only came into use around 1920, as the study of outer space phenomena through the lens of physics was still in its infancy.

Even more recent, according to Sendy, is the term “exobiology,” which is the study of life in the universe from a biological perspective. This field is primarily concerned with understanding the conditions that may make life possible elsewhere, how such life might evolve, and how we might detect it.

Sendy underscores the inherent challenges of this discipline. At the time of his writing, he describes the field of exobiology as being “nearly empty,” with hypotheses only beginning to be formulated. The main challenge Sendy identifies is that our understanding of life is based solely on one example: life on Earth. Consequently, our ability to theorize about life elsewhere in the universe is limited by our understanding of earthly life and evolution.

This statement underscores the inherent complexity and speculative nature of astrobiology. Without concrete examples of life beyond Earth, our understanding and theories are based primarily on our knowledge of how life works here. As our ability to explore other planets and star systems improves, and as we make more detailed observations of our own planet and life, our understanding of what is possible in terms of life elsewhere in the universe will inevitably evolve and expand.

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