Post by Andrei Tchentchik on Sept 3, 2020 15:21:21 GMT 2
(.#506).- Is ET’s life around orange dwarf stars? The opinion of Franck Selsis.
Should we be looking for extraterrestrial life around these stars? Response from Franck Selsis
Posted on Feb. 23 2020
Laurent Sacco, Journalist
Astrophysicist Franck Selsis, member of the CNRS and the Bordeaux Astrophysics Laboratory (LAB), allowed Futura to resume the article he devoted to the question of whether to favor certain types of stars in the search for potentially habitable planets.
News once a day. Currently, in this month of February 2020, more than 4,180 exoplanets are known to humanity and recorded by the famous site of the Encyclopedia of extrasolar planets founded in 1995 by astronomer Jean Schneider of the Paris observatory. The 2019 Nobel Prize in Physics even rewarded the pioneers of the discovery of exoplanets around the stars on the main sequence, the Swiss Michel Mayor and Didier Queloz.
The quest for the Grail of exobiology by the noosphere of geochemist Vladimir Vernadsky and geologist and paleontologist Pierre Teilhard de Chardin, the collective spirit of Humanity, in a way, continues: an exo-planet of terrestrial type with, not only liquid water on its surface, but also credible bio-signatures in its atmosphere.
We have already started to analyze the composition of the atmospheres of certain exo-planets but we are still in their infancy in this area and this currently only concerns stars which are not potential exo-lands. We need new tools to go further and they are under development. The targets of these new tools can only be planetary systems close to the Solar System like those observed with the successor of the Kepler satellite: Tess.
On the occasion of the discovery of the worlds of Trappist 1, only about 40 light years from the Sun, astrophysicist Franck Selsis explained to us that the determination of credible bio-signatures was not really obvious and that, in the best of all, a lot of work and critical hindsight would be required. He wrote a new article concerning the debate in the scientific community between those who think that we must first seek bio-signatures with exo-planets orbiting solar-type stars, yellow dwarfs of the type G, or even a little less luminous stars, orange dwarfs of type K and of the other, those which think that one must initially concentrate on the case of red dwarfs of type M.
Astrophysicist Franck Selsis studies planetary atmospheres and exobiology. © Benjamin Pavone
Franck Selsis: A study - which, we should point out at the outset, does not present any new results - recently found a lot of media coverage because it suggests that a certain type of star - the K dwarfs - would represent the hosts the most favorable for the search for planets which can shelter life.
So let's go back to the link between the host star and the "habitability" of a planet. But let's get away from these terms, which are a little heavy to carry with habitability and habitable zones. We are interested here in what we will call "sexy" planets, that is to say planets of size / mass and of designation (equivalent of insolation but for any star) terrestrial, and we will be enough generous in this definition. We will not be difficult. We will be able to be more attentive in the future when telescopes give us the possibility of studying more closely the composition of these planets and their atmosphere.
The host star critically influences both the evolution of its planets and the efficiency of our methods of observation. For example, the stars more massive than 2.5 times the mass of the Sun (Msol) live less than 1 billion years and as their luminosity increases strongly during this period, their possible temperate planets remain it only one duration much more weak yet. And these large, very bright stars also make it very difficult for us to detect planets, even giant ones. But stars more massive than 2.5 MSol only represent 1% of the stars in the Galaxy.
Red, orange and yellow dwarfs
We distribute the stars of mass less than or equal to that of the Sun in three categories, the stars M (soil), K (0.5-0.8 Msol) and G (0.8-1.15 Msol, which therefore includes the sun). M stars represent approximately 80% of stars (approximately 10% for K and approximately 7% for G). M or dwarf red stars largely dominate the stellar population, live from tens to hundreds of billions of years, have a very low luminosity, an emission dominated by red and infrared. It is also around the M stars that our methods of detecting exoplanets make it possible to find the “most easily” our famous sexy planets.
Almost all of the sexy exoplanets we know thus belong to an M star. We cannot yet observe the atmosphere of these worlds but if they do have one, we can try to observe them with telescopes to come and in particular the next James-Webb space telescope. But if, and only if these planets orbit around red dwarfs. And again, for this we need small M stars (The category of M indeed covering a vast range of masses ranging from 7% to 50% of the mass of the Sun), as close as possible to the Sun, and including the planet or the planets targeted have the good idea to pass in front of the disc of their star seen from Earth, that is to say that they transit. In short, the very small dwarf next door that winks at you.
Are there any? Yes, there are, because these red dwarfs have the good idea of almost always having sexy planets!
This infographic compares the characteristics of three classes of stars in our Galaxy: Sun-like stars are classified as G stars or yellow dwarfs; the stars less massive and colder than our Sun are K dwarfs or orange dwarfs; and weaker and cooler stars are the red M dwarfs. The graph compares the stars based on several important variables. The longevity of red dwarf M stars can exceed 100 billion years. That of orange dwarfs can range from 15 to 45 billion years. That of our Sun is about 10 billion years old. The flow of stellar radiation capable of heating and eroding the upper atmosphere can be 80 to 500 times more intense in the habitable zone of dwarfs M than in that of the Sun. But only 5 to 25 times more intense in that of the dwarfs K. © Nasa, ESA and Z. Levy (STScI), Franck Selsis
The scientific community is divided into two schools. One, of which I am a member, thanks nature for putting sexy planets within reach of our still in its infancy, and is enthusiastic about the possibility of studying these exotic worlds. Because yes, it must be said, the sexy planets of red dwarfs are necessarily different from Earth in many aspects. For a science fiction lover like me it only makes him sexier but another part of the community does not find them at all sexy, even frankly ugly, and would like to point the telescopes towards more solar stars. But why such discrimination? Well ! there are several arguments.
The first could be stated as follows: “We know that life exists on ONE sexy planet, ours, around a star G, our Sun; we must therefore focus our research on these stars ”. The underlying idea is that if ever the conditions for the emergence of life were very specific and restrictive, they would have little chance of being encountered around stars very different from ours. This argument comes up against a major problem: the Earth owes its characteristics to many other factors than the type of its star.
If we are looking for the specifics of the Solar System that have necessarily influenced the precise nature of our Planet, we must also add our good big Jupiter, and its little eccentric orbit, which immediately places us in a much smaller fraction of the stars , about one in a thousand.
Each planetary system has its particularities, within an incredible diversity. If we start looking for Earths with a big Moon around a G2V star, with a Jupiter-Saturn couple, such initial abundance of Aluminum 26 and Iron 60 (which played a role in the heating and therefore the composition of the first solids), and so on, we set off to find the needle in a galaxy of hay.
But which of these characteristics played in favor of the emergence and the maintenance of life and which made our life more difficult?
Habitable exoplanets despite synchronous rotation
The idea is to say that we are here to talk about it and that it is therefore, by definition, thanks to the specificities of our system. A kind of weak anthropic principle. It seems to me, however, that these heliogeocentric speculations must in any case be confronted with observations of planets belonging to stars and architectures of non-solar planetary systems. Since analogues to the Sun-Earth couple are rarer and harder to observe, why not start with the most accessible targets?
Artist's impression of a cold, locked planet in synchronous rotation around its host star. Ice covers a large part of the planet's surface, but the point directly in front of the planet's host star remains free of ice. © Nasa JPL-Caltech
Another argument has long prevailed: to be of Earth temperature, the planets must be very close to their red dwarf because its brightness is low. This proximity means that these planets are deformed by tidal interactions. This deformation is not a problem in itself but it results in rapid synchronization of the rotation period with the period of revolution. The sexy planets therefore always present the same face to their red dwarf. The risk is that the water (if there is water) is irreversibly trapped as ice in the permanent night hemisphere and at the poles. However, climate models have shown that this scenario is avoided for sufficiently dense atmospheres and / or for sufficient quantities of water. This configuration even presents peculiarities favorable to maintain liquid water and which one does not find in around stars of the solar type. This argument has thus lost a lot of force.
Habitable exoplanets despite magnetic anger?
The great threat posed by low mass stars on their sexy planets comes from their "magnetic activity". The rotation of the stars, coupled with their magnetic field, generates an activity which is manifested by an emission of energetic radiation (X, extreme UV) as well as of a plasma (ions and electrons) which is called stellar wind. The faster the stars turn, the more active they are. At the start of their lives, all stars rotate quickly and are very active. They then emit a thousandth of their luminosity in the form of X-rays, which is our main indicator of activity. This important X emission is accompanied by an intense stellar wind, bursts of luminosity (flares, in English) and coronal ejections. The stars eventually slow down because their stellar wind takes away from the angular momentum, and their activity decreases with this slowdown. But solar-type stars slow down much faster than red dwarfs. The Sun today emits a millionth of its luminosity in the form of X-rays.
The violent explosions of bubbling plasma of young red dwarf stars can make conditions uninhabitable on nascent planets. In this artist's rendering, an active young red dwarf (right) strips the atmosphere of an orbiting planet (left). Scientists have discovered that the flares of the youngest red dwarfs they studied - about 40 million years old - are 100 to 1,000 times more energetic than when the stars are older. They also detected one of the most intense stellar eruptions ever seen in ultraviolet light - more energetic than the most powerful eruption ever recorded for our Sun. © Nasa, ESA and D. Player (STScI)
Our neighbor, the red Proxima dwarf who is about the same age as the Sun, emits less X-radiation than the Sun because it is a small star but it represents a larger fraction of its total brightness (more than one ten -thousandth). The sexy planet Proxima b orbiting this star receives only 60% of the light flux that the Earth from the Sun receives, but it receives 100 times more X-radiation than Earth and is subjected to a much stronger stellar wind.
It was also the case of the Earth in its youth but for only a few hundred million years, this is in any case what we deduce from the observation of young stars similar to the Sun. This activity-related radiation represents only a small part of the energy that the star deposits on the planet, but it is absorbed in the upper atmosphere. The positive point is that the surface is protected from this harmful radiation for organic molecules, but the consequence is that the upper atmosphere receives a very important energy contribution compared to its low density, which can potentially result in an erosion of the atmosphere which escapes towards space.
VIDEO :
Atmosphères et habitabilité des exoplanètes
The study of exoplanets has revealed an incredible diversity of planetary system architectures, but also of types of planets, in terms of mass, radius, temperature and composition. Observation methods now make it possible to probe the structure and composition of their atmosphere, thus opening up a considerable field of research for comparative planetology. Here, in 2016, is a conference by Franck Selsis organized by the Bureau des longitudes (Academy of Sciences) and the geosciences department of the ENS. © École normale supérieure – PSL
Can an active star undress its planets from their atmosphere?
It's an open question. Current models cannot describe the very complex physics of an upper atmosphere subject to these conditions and we can only give upper limits on the rate of atmospheric loss. These upper limits show that the threat is serious and that we now need to develop robust models to quantify it.
This problem is compounded by the early development of red dwarfs. If the luminosity of a red dwarf is incredibly stable during most of its very long life, it goes through a long birth, the "main pre-sequence" phase, during which its luminosity decreases towards its stable value. For a star like the Sun, this phase is very short and ends at the start of planetary formation when the protoplanets are cool, buried in a disk of gas and dust. For red dwarfs this phase extends beyond the training phase. A few tens of millions of years for the most massive red dwarfs, up to 1 billion years for the smallest.
This means that the planets we find sexy today were actually much warmer in their youth. Above a certain insolation (or designation), water can no longer exist in the liquid state on the surface of a planet. The upper atmosphere is then rich in water vapor, which is dissociated into hydrogen and oxygen by UV radiation, and the hydrogen, very light, escapes into space.
It is believed that this is the fate that Venus suffered. The little water that remains in the atmosphere of Venus is very rich in deuterium (heavier than hydrogen), which is interpreted as the result of this loss of hydrogen. The residual oxygen would have reacted with the crust. If today we lowered the brightness of the Sun so as to reduce the insolation of Venus and make it a sexy planet, it would certainly be very different from Earth, very poor in water (unless its coat has not retained any many). The sexy planets of very low mass stars experience a similar scenario. If they spend much less than 4.5 billion years in this extreme "Venusian" climate regime, no one really knows if their potential to shelter water and life has not left feathers ...
An artist's view of Venus a few billion years ago in the context of climate models studied by researchers at the GISS (Goddard Institute for Space Studies). © Nasa
However, we ignore one key parameter: the "typical" gas content of the planets. Is the Earth a sexy planet rich or poor in water, nitrogen, carbon? This is currently one of the big questions in planetology. The terrestrial oceans represent 0.06% of the Earth's mass. In theory, we could form planets containing a lot more: 1%, 10%, 50%. In short, some planets can probably afford to lose a large amount of water. But this erosion of the water tank poses other questions, wouldn't the accumulation of oxygen linked to the loss of hydrogen in the water be harmful to a prebiotic chemistry that can lead to life?
In short, the red dwarfs may seem very inhospitable to us, children of a yellow Sun. What pushes certain astrophysicists to propose the K stars like privileged targets for the research and the study of sexy planets and their atmosphere and why not in a few decades, signs of life. According to them, these orange dwarfs would be the ideal compromise: targets that are less difficult than our sun standard and less exotic and active than the red dwarfs that scare them so much.
My point of view is that we are in a very primordial phase of exploration. We base all our knowledge of terrestrial planets and their atmosphere on the study of Venus, Earth and Mars (come on, I add Titan to you). A meager sample born from the same star and which only shows us a face of 4.5 billion years. There is so much to learn from studying the planets around ALL types of stars. Let's start with the most accessible, the planets around M stars, while developing the instruments to go to K and G stars. If ever the planets around red dwarfs turn out not to be sexy at all, we need to know because they are by far the most numerous and they form you want these are telluric planets.
One of them, almost the smallest and most red as a star, presents us with a procession of seven terrestrial planets. Trappist 1 and its seven worlds will be unparalleled targets for the future James-Webb space telescope. Let us not turn away from them because they do not have the radiance and calm of our mother star.
An artist's impression of the possible surface of Trappist 1f. © Nasa JPL-Caltech T. Pyle IPAC
FI N.
Should we be looking for extraterrestrial life around these stars? Response from Franck Selsis
Posted on Feb. 23 2020
Laurent Sacco, Journalist
Astrophysicist Franck Selsis, member of the CNRS and the Bordeaux Astrophysics Laboratory (LAB), allowed Futura to resume the article he devoted to the question of whether to favor certain types of stars in the search for potentially habitable planets.
News once a day. Currently, in this month of February 2020, more than 4,180 exoplanets are known to humanity and recorded by the famous site of the Encyclopedia of extrasolar planets founded in 1995 by astronomer Jean Schneider of the Paris observatory. The 2019 Nobel Prize in Physics even rewarded the pioneers of the discovery of exoplanets around the stars on the main sequence, the Swiss Michel Mayor and Didier Queloz.
The quest for the Grail of exobiology by the noosphere of geochemist Vladimir Vernadsky and geologist and paleontologist Pierre Teilhard de Chardin, the collective spirit of Humanity, in a way, continues: an exo-planet of terrestrial type with, not only liquid water on its surface, but also credible bio-signatures in its atmosphere.
We have already started to analyze the composition of the atmospheres of certain exo-planets but we are still in their infancy in this area and this currently only concerns stars which are not potential exo-lands. We need new tools to go further and they are under development. The targets of these new tools can only be planetary systems close to the Solar System like those observed with the successor of the Kepler satellite: Tess.
On the occasion of the discovery of the worlds of Trappist 1, only about 40 light years from the Sun, astrophysicist Franck Selsis explained to us that the determination of credible bio-signatures was not really obvious and that, in the best of all, a lot of work and critical hindsight would be required. He wrote a new article concerning the debate in the scientific community between those who think that we must first seek bio-signatures with exo-planets orbiting solar-type stars, yellow dwarfs of the type G, or even a little less luminous stars, orange dwarfs of type K and of the other, those which think that one must initially concentrate on the case of red dwarfs of type M.
Astrophysicist Franck Selsis studies planetary atmospheres and exobiology. © Benjamin Pavone
Franck Selsis: A study - which, we should point out at the outset, does not present any new results - recently found a lot of media coverage because it suggests that a certain type of star - the K dwarfs - would represent the hosts the most favorable for the search for planets which can shelter life.
So let's go back to the link between the host star and the "habitability" of a planet. But let's get away from these terms, which are a little heavy to carry with habitability and habitable zones. We are interested here in what we will call "sexy" planets, that is to say planets of size / mass and of designation (equivalent of insolation but for any star) terrestrial, and we will be enough generous in this definition. We will not be difficult. We will be able to be more attentive in the future when telescopes give us the possibility of studying more closely the composition of these planets and their atmosphere.
The host star critically influences both the evolution of its planets and the efficiency of our methods of observation. For example, the stars more massive than 2.5 times the mass of the Sun (Msol) live less than 1 billion years and as their luminosity increases strongly during this period, their possible temperate planets remain it only one duration much more weak yet. And these large, very bright stars also make it very difficult for us to detect planets, even giant ones. But stars more massive than 2.5 MSol only represent 1% of the stars in the Galaxy.
Red, orange and yellow dwarfs
We distribute the stars of mass less than or equal to that of the Sun in three categories, the stars M (soil), K (0.5-0.8 Msol) and G (0.8-1.15 Msol, which therefore includes the sun). M stars represent approximately 80% of stars (approximately 10% for K and approximately 7% for G). M or dwarf red stars largely dominate the stellar population, live from tens to hundreds of billions of years, have a very low luminosity, an emission dominated by red and infrared. It is also around the M stars that our methods of detecting exoplanets make it possible to find the “most easily” our famous sexy planets.
Almost all of the sexy exoplanets we know thus belong to an M star. We cannot yet observe the atmosphere of these worlds but if they do have one, we can try to observe them with telescopes to come and in particular the next James-Webb space telescope. But if, and only if these planets orbit around red dwarfs. And again, for this we need small M stars (The category of M indeed covering a vast range of masses ranging from 7% to 50% of the mass of the Sun), as close as possible to the Sun, and including the planet or the planets targeted have the good idea to pass in front of the disc of their star seen from Earth, that is to say that they transit. In short, the very small dwarf next door that winks at you.
Are there any? Yes, there are, because these red dwarfs have the good idea of almost always having sexy planets!
This infographic compares the characteristics of three classes of stars in our Galaxy: Sun-like stars are classified as G stars or yellow dwarfs; the stars less massive and colder than our Sun are K dwarfs or orange dwarfs; and weaker and cooler stars are the red M dwarfs. The graph compares the stars based on several important variables. The longevity of red dwarf M stars can exceed 100 billion years. That of orange dwarfs can range from 15 to 45 billion years. That of our Sun is about 10 billion years old. The flow of stellar radiation capable of heating and eroding the upper atmosphere can be 80 to 500 times more intense in the habitable zone of dwarfs M than in that of the Sun. But only 5 to 25 times more intense in that of the dwarfs K. © Nasa, ESA and Z. Levy (STScI), Franck Selsis
The scientific community is divided into two schools. One, of which I am a member, thanks nature for putting sexy planets within reach of our still in its infancy, and is enthusiastic about the possibility of studying these exotic worlds. Because yes, it must be said, the sexy planets of red dwarfs are necessarily different from Earth in many aspects. For a science fiction lover like me it only makes him sexier but another part of the community does not find them at all sexy, even frankly ugly, and would like to point the telescopes towards more solar stars. But why such discrimination? Well ! there are several arguments.
The first could be stated as follows: “We know that life exists on ONE sexy planet, ours, around a star G, our Sun; we must therefore focus our research on these stars ”. The underlying idea is that if ever the conditions for the emergence of life were very specific and restrictive, they would have little chance of being encountered around stars very different from ours. This argument comes up against a major problem: the Earth owes its characteristics to many other factors than the type of its star.
If we are looking for the specifics of the Solar System that have necessarily influenced the precise nature of our Planet, we must also add our good big Jupiter, and its little eccentric orbit, which immediately places us in a much smaller fraction of the stars , about one in a thousand.
Each planetary system has its particularities, within an incredible diversity. If we start looking for Earths with a big Moon around a G2V star, with a Jupiter-Saturn couple, such initial abundance of Aluminum 26 and Iron 60 (which played a role in the heating and therefore the composition of the first solids), and so on, we set off to find the needle in a galaxy of hay.
But which of these characteristics played in favor of the emergence and the maintenance of life and which made our life more difficult?
Habitable exoplanets despite synchronous rotation
The idea is to say that we are here to talk about it and that it is therefore, by definition, thanks to the specificities of our system. A kind of weak anthropic principle. It seems to me, however, that these heliogeocentric speculations must in any case be confronted with observations of planets belonging to stars and architectures of non-solar planetary systems. Since analogues to the Sun-Earth couple are rarer and harder to observe, why not start with the most accessible targets?
Artist's impression of a cold, locked planet in synchronous rotation around its host star. Ice covers a large part of the planet's surface, but the point directly in front of the planet's host star remains free of ice. © Nasa JPL-Caltech
Another argument has long prevailed: to be of Earth temperature, the planets must be very close to their red dwarf because its brightness is low. This proximity means that these planets are deformed by tidal interactions. This deformation is not a problem in itself but it results in rapid synchronization of the rotation period with the period of revolution. The sexy planets therefore always present the same face to their red dwarf. The risk is that the water (if there is water) is irreversibly trapped as ice in the permanent night hemisphere and at the poles. However, climate models have shown that this scenario is avoided for sufficiently dense atmospheres and / or for sufficient quantities of water. This configuration even presents peculiarities favorable to maintain liquid water and which one does not find in around stars of the solar type. This argument has thus lost a lot of force.
Habitable exoplanets despite magnetic anger?
The great threat posed by low mass stars on their sexy planets comes from their "magnetic activity". The rotation of the stars, coupled with their magnetic field, generates an activity which is manifested by an emission of energetic radiation (X, extreme UV) as well as of a plasma (ions and electrons) which is called stellar wind. The faster the stars turn, the more active they are. At the start of their lives, all stars rotate quickly and are very active. They then emit a thousandth of their luminosity in the form of X-rays, which is our main indicator of activity. This important X emission is accompanied by an intense stellar wind, bursts of luminosity (flares, in English) and coronal ejections. The stars eventually slow down because their stellar wind takes away from the angular momentum, and their activity decreases with this slowdown. But solar-type stars slow down much faster than red dwarfs. The Sun today emits a millionth of its luminosity in the form of X-rays.
The violent explosions of bubbling plasma of young red dwarf stars can make conditions uninhabitable on nascent planets. In this artist's rendering, an active young red dwarf (right) strips the atmosphere of an orbiting planet (left). Scientists have discovered that the flares of the youngest red dwarfs they studied - about 40 million years old - are 100 to 1,000 times more energetic than when the stars are older. They also detected one of the most intense stellar eruptions ever seen in ultraviolet light - more energetic than the most powerful eruption ever recorded for our Sun. © Nasa, ESA and D. Player (STScI)
Our neighbor, the red Proxima dwarf who is about the same age as the Sun, emits less X-radiation than the Sun because it is a small star but it represents a larger fraction of its total brightness (more than one ten -thousandth). The sexy planet Proxima b orbiting this star receives only 60% of the light flux that the Earth from the Sun receives, but it receives 100 times more X-radiation than Earth and is subjected to a much stronger stellar wind.
It was also the case of the Earth in its youth but for only a few hundred million years, this is in any case what we deduce from the observation of young stars similar to the Sun. This activity-related radiation represents only a small part of the energy that the star deposits on the planet, but it is absorbed in the upper atmosphere. The positive point is that the surface is protected from this harmful radiation for organic molecules, but the consequence is that the upper atmosphere receives a very important energy contribution compared to its low density, which can potentially result in an erosion of the atmosphere which escapes towards space.
VIDEO :
Atmosphères et habitabilité des exoplanètes
The study of exoplanets has revealed an incredible diversity of planetary system architectures, but also of types of planets, in terms of mass, radius, temperature and composition. Observation methods now make it possible to probe the structure and composition of their atmosphere, thus opening up a considerable field of research for comparative planetology. Here, in 2016, is a conference by Franck Selsis organized by the Bureau des longitudes (Academy of Sciences) and the geosciences department of the ENS. © École normale supérieure – PSL
Can an active star undress its planets from their atmosphere?
It's an open question. Current models cannot describe the very complex physics of an upper atmosphere subject to these conditions and we can only give upper limits on the rate of atmospheric loss. These upper limits show that the threat is serious and that we now need to develop robust models to quantify it.
This problem is compounded by the early development of red dwarfs. If the luminosity of a red dwarf is incredibly stable during most of its very long life, it goes through a long birth, the "main pre-sequence" phase, during which its luminosity decreases towards its stable value. For a star like the Sun, this phase is very short and ends at the start of planetary formation when the protoplanets are cool, buried in a disk of gas and dust. For red dwarfs this phase extends beyond the training phase. A few tens of millions of years for the most massive red dwarfs, up to 1 billion years for the smallest.
This means that the planets we find sexy today were actually much warmer in their youth. Above a certain insolation (or designation), water can no longer exist in the liquid state on the surface of a planet. The upper atmosphere is then rich in water vapor, which is dissociated into hydrogen and oxygen by UV radiation, and the hydrogen, very light, escapes into space.
It is believed that this is the fate that Venus suffered. The little water that remains in the atmosphere of Venus is very rich in deuterium (heavier than hydrogen), which is interpreted as the result of this loss of hydrogen. The residual oxygen would have reacted with the crust. If today we lowered the brightness of the Sun so as to reduce the insolation of Venus and make it a sexy planet, it would certainly be very different from Earth, very poor in water (unless its coat has not retained any many). The sexy planets of very low mass stars experience a similar scenario. If they spend much less than 4.5 billion years in this extreme "Venusian" climate regime, no one really knows if their potential to shelter water and life has not left feathers ...
An artist's view of Venus a few billion years ago in the context of climate models studied by researchers at the GISS (Goddard Institute for Space Studies). © Nasa
However, we ignore one key parameter: the "typical" gas content of the planets. Is the Earth a sexy planet rich or poor in water, nitrogen, carbon? This is currently one of the big questions in planetology. The terrestrial oceans represent 0.06% of the Earth's mass. In theory, we could form planets containing a lot more: 1%, 10%, 50%. In short, some planets can probably afford to lose a large amount of water. But this erosion of the water tank poses other questions, wouldn't the accumulation of oxygen linked to the loss of hydrogen in the water be harmful to a prebiotic chemistry that can lead to life?
In short, the red dwarfs may seem very inhospitable to us, children of a yellow Sun. What pushes certain astrophysicists to propose the K stars like privileged targets for the research and the study of sexy planets and their atmosphere and why not in a few decades, signs of life. According to them, these orange dwarfs would be the ideal compromise: targets that are less difficult than our sun standard and less exotic and active than the red dwarfs that scare them so much.
My point of view is that we are in a very primordial phase of exploration. We base all our knowledge of terrestrial planets and their atmosphere on the study of Venus, Earth and Mars (come on, I add Titan to you). A meager sample born from the same star and which only shows us a face of 4.5 billion years. There is so much to learn from studying the planets around ALL types of stars. Let's start with the most accessible, the planets around M stars, while developing the instruments to go to K and G stars. If ever the planets around red dwarfs turn out not to be sexy at all, we need to know because they are by far the most numerous and they form you want these are telluric planets.
One of them, almost the smallest and most red as a star, presents us with a procession of seven terrestrial planets. Trappist 1 and its seven worlds will be unparalleled targets for the future James-Webb space telescope. Let us not turn away from them because they do not have the radiance and calm of our mother star.
An artist's impression of the possible surface of Trappist 1f. © Nasa JPL-Caltech T. Pyle IPAC
FI N.
