All the answers to the question is there life in space?

The possibility that there is life, and perhaps even civilizations, outside the earth has always fascinated the human imagination. In the article we will try to evaluate the possibilities for existence of life and culture in other worlds. First article in the series

The possibility that there is life, and perhaps even civilizations, outside the earth has always fascinated the human imagination. The astronomical number of stars in the universe makes us wonder if it is possible that among the billions of stars in our galaxy, the Milky Way, Earth is the only one where life and intelligence have appeared. From time to time a scientific discovery related to extraterrestrial life is published, capturing the headlines and igniting the public's imagination.
In the 90s, a meteorite from Mars that was found in Antarctica made headlines, with tiny patterns inside that resembled microscopic fossils found on Earth; and evidence of the presence of water under the ice cover on Jupiter's moon Europa. Although the many research missions to Mars have not found evidence of the existence of life, Mars and Europa (and perhaps also Titan, a moon of Saturn) remain the last corners of the solar system where there is still a chance of finding biological life, even if simple.
On April 24, 2007, the discovery of an Earth-like planet, orbiting a dim red sun at a distance of about twenty light years from Earth, made headlines. Although apparently, this discovery does not constitute direct proof of the existence of life, it causes great excitement among astronomers. Finding planets outside the solar system, and in particular a planet whose conditions are similar to the conditions prevailing on Earth, means a step up in examining the question "Are we alone?".
There is no doubt that finding an alien life form, even a primitive one, let alone an advanced culture, will be one of the most important discoveries in human history, and will have far-reaching consequences for our understanding of the human race's place in the universe, and perhaps also for its future development.
In this article we will try to evaluate with scientific tools the conditions and the probability of the development of extraterrestrial life and culture, and the means and chances of discovering them.

The conditions for the development of lifeAlien life may be very different in form and even in essence from the life we ​​know. Even on the surface of the earth there is a huge variety, such as bacteria, plants, insects and mammals. Alien life on other worlds may be even more different - not only in form but also in essence, for example, life based not on the element carbon but on another element, such as zinc, which has an electronic structure similar to that of carbon, or life that uses not water, but a biochemical solvent Another, such as ammonia (NH3), methane (CH4) or ethane (C2H6), which are in a liquid state at temperatures much lower than those we consider suitable for life.
The Huygens research probe, launched in 2006 from the Cassini spacecraft, which studies the planet Saturn, discovered lakes and rivers of methane on the surface of the moon Titan, and evidence of the existence of a mixture of water and ammonia under the surface.
The definition of the conditions for existence of life obviously depends on the question, what kind of life is involved. For the development of life similar to the life we ​​know on Earth (based on organic molecules), a relatively narrow temperature range is necessary, approximately between the freezing point of water and slightly above the boiling point (thermophilic bacteria - "heat-loving" - have been found in hot springs at the bottom of the oceans at temperatures of 120 degrees Celsius and even more).
It is common to assume that the basic condition for the development of life is the presence of liquid water, therefore in other planets the presence of planets in the appropriate area is required. The area where the temperature on the surface of the planets allows the presence of liquid water is called the "habitable zone" The size and location of the "seated area" depend on the radiation intensity of the parent star (the "Sun") and the distance of the planet from the parent star, as described in the diagram.

Where is life possible in the solar system?As mentioned, the temperature on the surface of a planet depends first and foremost on its distance from its sun. In our solar system, the satyr region extends approximately in the area between Venus and Mars. However, in practice liquid water exists only on the surface of the Earth, because the temperature on the surface of a planet depends not only on its distance from the Sun, but also on the properties of its atmosphere and other factors.
On Venus, for example, the atmosphere, which is mostly made up of CO2 and is a hundred times more compressed than that of the Earth, causes a strong greenhouse effect, raising the temperature to 500 degrees Celsius, far beyond the range that allows the existence of organic life as we know it. Mars, however, has long since lost most of its atmosphere and water due to its low gravity, and the average temperature on its surface is well below the freezing point of water. The water in it is permanently frozen, in the ground and at the poles.
Many research missions to Mars have tried to uncover life on the red planet, but so far mostly evidence has been found of the existence of liquid water in large quantities on the surface of Mars in the distant past. Detailed photographs of the surface show flow marks, dry stream channels and alluvial fans. Two Viking spacecraft that landed on the surface of Mars in the XNUMXs performed biochemical tests of soil samples, looking for evidence of metabolism, that is, the activity of microorganisms.
Evidence of metabolism was indeed found in some of the experiments, but another experiment (mass spectrograph) showed that there are no organic substances in the soil, so the activity found was probably inorganic.

Recently, evidence was found for the presence of water on the surface of Mars in our time, such as the flow channels in the Newton Crater (see photo). One of the explanations is that these channels were created due to the seasonal melting of water that was frozen in the ground. On August 4, 2007, NASA launched a spacecraft named Phoenix, which will land in the North Pole region of Mars a research robot searching for signs of life in the ice cover.
What is special about this project is that, unlike the previous projects, which were mainly intended for surface mapping and studies of a geological nature, Phoenix's main task is to find water, with the help of a robotic arm that will dig in the frozen ground in the polar region. Phoenix will try to confirm findings by the Mars Odyssey spacecraft from 2002, which support the theory that large amounts of frozen water are near the surface in the arctic plains of Mars, and that seasonal melting may create suitable conditions for the development of microorganisms.
Water in a liquid state has also been found outside the Yeshiv region of the solar system, in the ocean that is probably under the ice sheet of Europa, a moon of Jupiter, and possibly also on two other moons - Callisto and Ganymede. The heat needed to keep the water in a liquid state is created by the tidal forces exerted by Jupiter's heavy gravity. NASA planned a robotic research mission to Jupiter's moons - JIMO ((Jupiter Icy Moon Orbiter), which was supposed to closely study the three largest moons of Jupiter (the Galilean moons) from the ice caps, and possibly even to land a research probe on the surface of the moon Europa. The mission was canceled due to budget problems.

Life in other solar systemsLiquid water can also exist on the surface of planets orbiting distant stars (suns), if they are within the limits of their sitting region. There are over a hundred billion stars in our galaxy. Today we know that at least some of them have planets, because over two hundred planets have been discovered in the last decade around the stars close to us.
The accepted estimate today is that at least 10% of all stars have planets, but even if only a much smaller fraction of the stars in the galaxy have planets with conditions similar to those on Earth, it is likely that life in some form has evolved on many planets.

The search for lifeSpace travel to other stars along the lines of "interstellar travel" is beyond our reach (with the spaceships that exist today, a journey even to the nearest star to us, Proxima Centauri, which is about 4 light years away, will take tens of thousands of years!).
Although, plans were proposed to build future spaceships with advanced propulsion methods, such as the "Daedalus" project - a spaceship with a nuclear fusion engine, which was designed to reach a speed of 40,000 kilometers per second (12% of the speed of light), but these plans did not leave the drawing board, And even if they are realized in the future, space travel, even to the closest stars to the Earth, will last decades and hundreds of years.
Within the limitations of current technology, there are two main methods of searching for life in other solar systems: trying to discover intelligent signals (radio transmissions) from outside the Earth (SETI, a method that will be described later), and searching for planets outside the solar system. With this method, astronomers try to discover planets around distant stars (suns), and after they are discovered, look for indirect evidence of the existence of life on their surface - for example, spectrum lines of gases that are created in biological systems (for example, oxygen that may indicate the existence of photosynthesis, or methane that is also emitted in biological processes ).

Extrasolar planets
Extra-solar planets orbit stars that are light years away from our sun. It is very difficult to detect them, both due to the distance and because they do not emit light by themselves, but only reflect the light that falls on them from the parent star, so that they are weak light sources in close proximity to a much stronger light source (see: Tal Inbar, "Planets-like A country outside the solar system", "Galileo" 109). This is equivalent to trying to distinguish a coin that is a meter away from a lighthouse 100 km away.
Since with current technology it is impossible to discover these planets by direct observation, astronomers use indirect methods; The main ones are the radial velocity method (Doppler effect) and the transit method (transit) described below. Future projects that will use space-based interferometry (such as SIM and TPF, described below) are planned to give a direct image of planets.
The radial velocity method is based on the gravitational effect of the planets on the parent star. In fact, the planet and its sun revolve around their center of mass, which is very close to the center of the parent star (which is much more massive). The result is a cyclic movement of the mother star, which causes the Doppler effect - a change in the wavelength of the light coming from it, as it is seen by the observer.
This effect can be detected by sensitive measuring devices, provided the effect is large enough (today the measurement sensitivity reaches velocity changes of meters per second). As of this writing, about 230 such planets are known, but almost all of them are giant planets that resemble Jupiter rather than Earth, and are very close to their parent star (the Sun).
The reason for this lies in the method of discovering these planets, which was described above. Until recently, the sensitivity of the measuring instruments was too low to discover planets with data similar to Earth. Since the strength of gravity is characteristic of the mass of the planet and decreases as the square of the distance from the parent star, until recently only planets much larger than Earth and very close to the parent star were discovered. This proximity is usually expressed in a temperature much higher than the boiling point of water, so the planets found so far are not suitable for the development of biological life as we know it.
To find extrasolar planets, and in particular Earth-like planets, observations were made with large telescopes on the surface of the Earth. More sensitive measuring devices were also built and mounted on telescopes on the surface of the Earth, such as the HARPS spectrometer (High Accuracy Radial Velocity Planet Searcher), which was mounted on a 3.6 meter diameter telescope in Chile. Through it was discovered - as mentioned at the beginning of the article - in April 2007 an earth-like planet located inside the sitting region of its sun, and it is possible, therefore, that there is water on its surface in a liquid state.
This planet, Gliese581c, whose diameter is only 50% larger than the diameter of the Earth, is very close to its parent star, at a distance 14 times smaller than the distance of the Earth from the Sun - so close that a complete revolution, equivalent to a year on the Earth, takes only 13 day. Nevertheless, the estimated temperature on its surface is relatively low (about 40 degrees Celsius), since the parent star is a red dwarf type star, whose radiation intensity is fifty times lower than that of our Sun.

The transition method
One method for discovering extrasolar planets is the transit method. This method, in which a small number of such planets have been discovered so far, is based on the decrease in the light intensity of the parent star when the planet passes by it and hides part of the light coming from it.
It is understood that this method is only applicable to planetary systems that we see exactly "from the side", that is, where the orbit of the planet passes exactly between the mother planet and the Earth. Since the planets are much smaller than the parent star (for example, the diameter of Jupiter is about ten times smaller than the diameter of the Sun, and the diameter of the Earth is a hundred times smaller), the decrease in the amount of light (typical of the area ratio) is extremely small (about a percent in the case of Jupiter, and a hundred percent for Earth). But today there are instruments capable of detecting such a small change, and from the size of the change, as well as from the shape of the beginning and end of the eclipse, it is even possible to calculate the size of the planet.
Several projects have been planned that use the transition method to discover extrasolar planets using dedicated telescopes in space - such as the French-European-Brazilian Corot telescope (COROT), which was launched in December 2006 (and discovered its first extrasolar planet in May 2007), and the Kepler telescope (Kepler) of NASA, planned for launch in February 2009. Kepler (see photo) is expected to discover hundreds of Earth-like planets in the region of the galaxy near the solar system (Orion arm), at a distance of up to 3,000 light years from Earth.
In the longer term, NASA is planning two additional missions based on interferometry (combining the beams of several telescopes that are far from each other, and see: Yoram Ored, "Seeing the Faces of the Suns", "Galileo" 107). One mission is SIM (Space Interferometry Mission), which is expected to discover about a hundred planets with a mass several times the mass of the Earth. The other mission is a more advanced version - TPF (Terrestrial Planet Finder), an array of telescopes in space that will use interferometry and other optical methods - dusting and resetting - to obtain a direct image of Earth-like planets, something that will make it possible to obtain data on the composition of their atmosphere.

Searching for signs of life
The condition that a planet is within the accretion zone does not guarantee the existence of life on its surface, as evidenced by the examples of Venus and Mars in the solar system (and conversely, liquid water may also exist outside the accretion zone, as in Europe). In other words, even if the temperature is in the right range, life may not have evolved for reasons unknown to us, or it may have existed for a while and disappeared. How can we discover "signs of life" in planets with the potential to develop life, such as Gliese581c mentioned earlier?
If we look for organic life similar to life on Earth, we can look for the "signature of life" - such as changes in the composition of the atmosphere typical of biological systems. On Earth, the biosphere had a great influence on the composition of the atmosphere in the process of photosynthesis, which caused a high percentage of free oxygen to appear.
In the primitive atmosphere of the Earth, as well as in the atmospheres of other planets in the solar system, there is almost no free oxygen, because the oxygen is very chemically active, and if the oxygen supply is not regularly renewed in the process of photosynthesis, it reacts with substances found in the ground. Therefore, large amounts of molecular oxygen (as well as ozone, O3, which is formed from oxidation by sunlight) in the atmosphere of a planet, which can be detected by examining the radiation spectrum reflected from it, can indicate the existence of biological life (small amounts of oxygen can be formed in photo processes -chemical, as in Mars and Venus).
So is the presence of methane gas, which is also created in organic decomposition and decay processes by microorganisms. For this reason, some scientists believe that the tiny amount of methane recently found in the Martian atmosphere indicates the existence of microorganisms below the surface.

Dr. Omri Vandel is an astrophysicist at the Rakah Institute for Physics at the Hebrew University, Jerusalem. Creator and lecturer of the course "Astrophysics and Life in the Universe", which has been studied by over a thousand students since 1999. Visiting professor at the University of Los Angeles (UCLA), where he taught, among other things, the above-mentioned course. His main research topics are black holes, quasars and active galaxies. In his spare time he is engaged in sports navigation, photography and the Esperanto language. Originally published in "Galileo" magazine

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