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In recent decades, thousands of extra-solar planets have been discovered in our galaxy. As of July 28, 2018, a total of 3,374 extra-solar planets have been confirmed in 2,814 planetary systems. Although the majority of these planets were gaseous giants, a growing number of them were terrestrial (ie, rocky) in nature and were found in orbit around the respective habitable areas. of their stars (HZ)
. Solar system shows, HZ does not necessarily mean a planet can support life. Even though Mars and Venus are on the inner and outer edge of the Sun's ZS (respectively), none is able to support life on its surface. And with more potentially habitable planets being discovered all the time, a new study suggests that it might be time to refine our definition of livable areas.
The study, titled "A More Complete Living Area to Find Life on Other Planets", recently appeared online. The study was conducted by Dr. Ramses M. Ramirez, a research scientist at the Earth-Life Science Institute of the Tokyo Institute of Technology. For years, Dr. Ramirez has been involved in the study of potentially habitable worlds and has built climate models to evaluate the processes that make planets habitable.
A diagram illustrating the boundaries of habitable areas and how these boundaries are affected by type of star. Credit: Wikipedia Commons / Chester Harman
As Dr. Ramirez said in his study, the most generic definition of a livable area is the circular region around a star where temperatures surface on one orbit would be sufficient to maintain the water in a liquid state. However, this does not mean that a planet is habitable and other considerations must be taken into account to determine if life could really exist there. As Dr. Ramirez told Universe Today by e-mail:
"The most popular incarnation of the HZ is the clbadic HZ.This clbadic definition badumes that greenhouse gases the most important in potentially habitable planets are carbon dioxide and water vapor, and it also badumes that the habitability of these planets is supported by the carbonate-silicate cycle, as it is the case for the Earth On our planet, the carbonate-silicate cycle is fed by plate tectonics
"The carbonate-silicate cycle regulates the transfer of carbon dioxide between the atmosphere, the surface and the interior of the Earth. It acts as a planetary thermostat over long periods and ensures that there is not too much CO2 in the atmosphere (the planet gets too hot) or too little (the planet gets too cold) . The clbadical HZ also (generally) badumes that habitable planets have total water inventories (eg total water in the oceans and seas) similar in size to that of the Earth. "
This is what we can call the" hanging fruit approach ", where scientists have been looking for signs of habitability on the basis of what we know best as & # 39; Since the only example of habitability is planet Earth, studies of exoplanets have focused on the search for "terrestrial" (ie, Rocky) planets. orbitals and size
The comparison is in the sun.Credit: IAC
However, in recent years, this definition has been questioned by new studies.As the exoplanet research went from simple detection and confirmation from the existence of bodies around other stars and to characterization, new formulations of ZS appeared that attempted to capture the diversity of potentially habitable worlds.
These more recent formulations have supplemented the notions traditionnell es de ZS considering that habitable planets may have different atmospheric compositions:
"For example, they consider the influence of additional greenhouse gases, such as CH4 and H2, early conditions on Earth and Mars. The addition of these gases makes the liveable area wider than would be predicted by the conventional HZ definition. It's great, because the planets that we thought were outside the ZS, like TRAPPIST-1h, can now be part of it. It has also been argued that planets with dense atmospheres of CO2-CH4 near the outer edge of the ZS of warmer stars may be inhabited because it is difficult to maintain such atmospheres without the presence of life. "
Dr. Ramirez and Lisa Kaltenegger, Associate Professor at the Carl Sagan Institute of Cornell University .After a paper they produced in 2017, which appeared in the Astrophysical Journal Letters, the exoplanet hunters could find planets that would someday become habitable due to the presence of volcanic activity – which would be discernible by the presence of hydrogen gas (H²) in their atmospheres
Stellar temperature as a function of the distance from the star to the Earth for the clbadical habitable zone (shaded blue) and the extension of the zone Credit: R. Ramirez, Carl Sagan Institute, Cornell
This theory is a natural extension of the search for "Earth-like" conditions, which considers that the Earth's atmosphere was not always what it is today Fundamentally, planetary scientists theorize that billions of years ago, the Earth 's atmosphere had a plentiful supply of hydrogen gas (H²) due to volcanic degbading and air pollution. 39, interaction between the molecules of hydrogen and nitrogen in this atmosphere.
In the case of the Earth, this hydrogen eventually escaped into space, which is badumed to be the case for all terrestrial planets. However, on a planet where levels of volcanic activity are sufficient, the presence of gaseous hydrogen in the atmosphere could be maintained, thus allowing a greenhouse effect that would keep their surfaces warm. In this regard, the presence of gaseous hydrogen in the atmosphere of a planet could extend the HZ of a star
According to Ramirez, there is also the time factor, which is n Is not usually taken into account in the evaluation of the ZS. In short, stars evolve over time and emit varying levels of radiation depending on their age. This has the effect of changing the area of influence of a star, which may not encompbad a planet being studied. As Ramirez explained:
"[I] It has been shown that the M-dwarfs (really cool stars) are so bright and warm when they form that they can dry out young planets that are later determined to be in the clbadic ZH.This emphasizes the fact that, simply because a planet is currently located in the habitable zone, this does not mean that it is actually habitable (even less inhabited). We should be able to pay attention to these cases.
Finally, there is the question of what types of astronomers are observing in the search for & nbsp; Exoplanets While many studies have examined the yellow dwarf star of type G (which is our Sun), many researches have recently focused on type M (red dwarf) stars because their longevity and the fact that they "although most of the students previous ones have focused on simple star systems, recent work suggests that habitable planets can be found in binary star systems or even giant red or white dwarf systems. Potentially habitable planets can also take the form of desert worlds or even much more humid oceanic worlds than the Earth, "says Ramirez. "Such formulations not only dramatically expand the parameter space of potentially habitable planets to look for, but they allow us to filter out the most (and least) likely worlds to accommodate life."
In the end, this study shows that conventional HZ is not the only tool that can be used to evaluate the possibility of extraterrestrial life. As such, Ramirez recommends that in the future, astronomers and exoplanets hunters should complement the conventional HZ with the additional considerations raised by these new formulations. In doing so, they may be able to maximize their chances of finding life one day.
"I recommend that scientists pay special attention to the early stages of planetary systems, as this helps to determine the probability that a planet is currently present today, the livable area is worthwhile. 39, be further studied for more evidence of life, "he said." I also recommend that the different HZ definitions be used together so that we can best determine which planets are most likely to be used. to house life. In this way, we can clbadify these planets and determine those on which we will devote the most time and energy. Along the way, we will also test the validity of the HZ concept, including the determination of the universality of the carbonate-silicate cycle on a cosmic scale. "
Additional Readings: arXiv
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Matt Williams is the curator of the Universe Today's Guide to Space .He is also a freelance writer , a science fiction writer and Taekwon-Do instructor, lives with his family on the island of Vancouver in beautiful British Columbia.
Earth-like planets, extrasolar planets, Featured, Habitable area, potentially habitable
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