Distance, water and gas: how life began on exoplanets

Parent star and distance to it

Life on other planets may not be like anything on Earth -

it could be a life we ​​don't know about.But it makes sense, at least at first, to look for something more familiar, it is much easier to find it. It is worth starting with the habitable zone or the area around the star, where the temperature of the planet's surface can allow water to accumulate. This means that it must warm the planet, but not incinerate all life on it.

The habitable zone, or the habitable zone, isa conditional area in space, determined from the calculation that the conditions on the surface of the planets in it will be close to the conditions on Earth and will ensure the existence of water in the liquid phase.

Earth-sized planets in wide orbitsaround stars like the Sun are much more difficult to spot than gas giants around red dwarfs. Even if there is a rocky world near them, such stars have a habit dangerous for any life, especially at a young age. The fact is that powerful flares erupt on their surface. They can simply sterilize closely orbiting planets where life has just begun to emerge.

Since our Sun has fed life on Earth almost4 billion years old, conventional wisdom suggests that such stars will be prime candidates for other potentially habitable worlds. However, yellow G-type stars like our Sun are rare in our galaxy.

Stars that are a little colder and less brightthan our Sun - orange dwarfs (or K-dwarfs), are considered by some scientists to be potentially better for advanced life. They can burn continuously for tens of billions of years. This opens up a vast time frame for biological evolution, allowing an infinite number of experiments to produce sustainable life forms. And for every star like our Sun, there are three times as many orange dwarfs in the Milky Way.

K-dwarfs are notable for the fact that they haveintermediate properties of rarer, brighter, but short-lived stars of the solar type (G-class) and numerous red dwarfs (M-class). K stars are the best candidates for a host star for a habitable planet.

However, given that there are not so many of them, scientistsplan to explore worlds around M-dwarfs. The first opportunity to use observations to characterize small exoplanetary environments and possibly search for biosignatures is expected with the launch of the James Webb Space Telescope (JWST). He will just observe small planets orbiting red dwarfs. They are smaller, colder, redder, but more numerous than stars like our Sun.

Water is the source of life

Water plays an extremely important role inthe global circulation of matter and energy, the emergence and maintenance of life on Earth, in the chemical structure of living organisms, in the formation of climate and weather. It is the most important substance for all living things on our planet. Therefore, scientists are sure that if life is somewhere in space, then it needs water.

Planet K2-18b orbits its dim red host star in the illustration. This exoplanet is the first of its kind to contain water vapor in its atmosphere.

By the way, NASA's new Perseverance mission willlook for signs of ancient microbial life, study the geology and climate of the planet, collect samples of rocks and sediments. It is worth noting that the rover landed in Jezero Crater. It is an impact crater on Mars, in the west of the Isis Plain, near the eastern edge of Greater Sirte, with a diameter of about 49 km. Scientists are sure - once the crater was filled with water; dry river channels flow into it, one of which forms a pronounced delta. In addition, the crater contains clay deposits formed by the action of water.

Land, 71% of which is covered with wateroceans, is currently the only known planet in the solar system that contains water in a liquid state. There is scientific evidence that on some satellites of the giant planets (Jupiter, Saturn, Uranus and Neptune), water may be under a thick crust of ice that covers a celestial body. However, at the moment there is no unequivocal evidence of the presence of liquid water in the solar system, except on Earth. Oceans and water can be found in other star systems and / or on their planets and other celestial bodies in their orbit. For example, water vapor was discovered in 2007 in a 1 AU protoplanetary disk. e. from the young star MWC 480.

In 2019, astronomers using dataHubble Space Telescope, announced the detection of water vapor in the atmosphere of a planet the size of Earth. Although this exoplanet orbits a star smaller than our Sun, it falls into the star's habitable zone.

The discovery was the result of many years of observationbehind the exoplanet K2-18b, a super-Earth, which is about 111 light-years from our solar system. It was discovered in 2015 by NASA's Kepler spacecraft.

Biosignatures and among them a new gas

Today, the vast majorityexoplanets discovered and confirmed by indirect methods. Only in rare cases have astronomers been able to obtain real spectra of radiation, which make it possible to accurately determine their chemical composition. However, that will change once next-generation instruments such as the James Webb Space Telescope (JWST) or the Nancy Grace Observatory (100 times more powerful than its predecessor Hubble) are sent into space.

In simple terms, these are uniquedevices will be able to observe our Universe at longer wavelengths, in the near and middle infrared range, and with significantly higher sensitivity than current devices. Biosignatures include chemical characteristics associated with life and biological processes, and also speak of favorable conditions for this. Previously, such markers were considered oxygen and carbon dioxide, which are produced by living organisms on Earth, water and methane released during the decomposition of organic substances, as well as some by-products (hydrogen sulfide, sulfur dioxide, carbon monoxide, hydrogen gas, and so on). However, all this could have accumulated on a lifeless planet. But isoprene is an almost unique, rare compound. And it produces a huge number of diverse life forms (from bacteria to plants and animals), evolutionarily far from each other.

According to the calculations of the researchers, the "primordial planet"(on which life begins to emerge) must necessarily have a large amount of isoprene in its atmosphere. So it was on Earth between four and 2.5 billion years ago, when single-celled organisms were the only form of life, and photosynthetic cyanobacteria slowly created an oxygen atmosphere around the Earth. So now the search will focus on this particular compound, according to Universe Today.

Of course, research will face a whole host ofcomplexities. It is not even known whether this will lead to the discovery of extraterrestrial life during the 21st century. But one thing is clear. In the coming years, astronomers will study in detail the atmospheres of thousands of exoplanets and will have an exhaustive list of planets with the most accurate biosignatures, which they can use to guide them in their search for specific traces of life throughout the galaxy.

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