2023-09-12 15:29:27
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[EN VIDÉO] Understanding the mission of the James Webb Space Telescope in one minute The James Webb Space Telescope, the new flagship of space observation, will be launched on December 18…
As for the James-Webb Space Telescope, it is mainly expected to take over from Hubble to study what happened from a few hundred million years ago to a billion years ago. years following the Big Bang at the level of galaxiesgalaxies on the one hand, and on the other hand to allow a very significant leap in the study of the composition of the atmospheres of exoplanets in the suburbs close to the SunSun in the Milky WayMilky Way.
On this last aspect, the dream is that we will be able to find in the not too distant future a definition of what a particularly convincing biosignature would be and ultimately, to find it in the atmosphere of a potential exo-Earth, this which would allow us to say that life really exists elsewhere than in the Solar SystemSolar System. This would be a major scientific and philosophical revolution.
ESO presents the discovery of K2-18b. To obtain a fairly accurate French translation, click on the white rectangle at the bottom right. English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Automatically translate”. Choose “French”. © ESA-Hubble, M. Kornmesser, Nasa’s Goddard Space Flight Center Conceptual Image Lab
K2-18 b, de Hubble au James-Webb
The Hubble telescope had already been mobilized a few years ago to study the mini-Neptune K2-18 b which is located in what we might call the naive habitable zone around its host star, i.e. one where one might expect to find liquid water due to the average temperature assumed to exist. We know that this notion is not simple, already because the existence or not of an atmosphere can change everything. Venus and Earth have almost the same mass and probably the same chemical composition and yet Venus is hell.
The exoplanet K2-18 b is approximately 110 light-years from the Solar System, so it is a target of choice for the James-Webb, especially since with Hubble we had found traces of vapor d water in its atmosphere.
A press release from NasaNasa indicates that new analyzes carried out with the James-Webb instruments now reveal the presence of methane (CH4) and carbon dioxide in the atmosphere of K2-18 b but with no ammonia ammonia (NH3) detectable.
A biosignature that remains elusive
Some researchers, such as Nikku Madhusudhan, an astronomer at the University of Cambridge, conclude that this strengthens the theory that the exoplanet is an ocean planet with an atmosphere essentially composed of hydrogen. This led them to introduce a new concept of potentially habitable planet, that of a hycean planet or hyceanic planet from the words “hydrogen” and “ocean(ic)”.
But not everyone agrees with this hypothesis, which Madhusudhan and his colleagues mention in the article devoted to the JWST observations concerning K2-18 b and a version of which can be found on arXiv.
Researchers are also asking the question of the credibility of the signature of a dimethyl sulfide molecule (DMSDMS) which we know in the case of the Earth that it betrays the presence of phytoplanktonphytoplankton. There remains work to confirm, or not, the presence of this molecule and even if it is there, it is too early to deduce that it would be proof of the existence of biotic processes and that it does not exist abioticabiotic processes to produce it. We have the same problem with Venus phosphine.
What should we think of all this?
It seems relevant to recall what astrophysicist astrophysicist Franck Selsis, member of the CNRS and the Bordeaux Astrophysics Astrophysics Laboratory (LAB) explained to us four years ago when he graciously allowed Futura to reprint the article that he had devoted to the discovery of water vapor in the atmosphere of the exoplanet K2-18b, and which was published on the LAB website. He set the record straight regarding this discovery, the notion of a habitable zone and its link with the search for life elsewhere, as well as the possibility that K2-18b is an ocean planet.
Here is his text!
“The detection of water vapor in the exoplanet K2-18b had a great media impact. This observation constitutes a very important step for the study of exoplanets but its link with the search for life elsewhere where habitability has been greatly exaggerated by certain media. So let’s come back to this fascinating system, to this famous observation and its implications.
The star K2-18 is 110 light years away, making it a relatively close system. To give an idea, there are regarding 15,000 stars closer than that. It is a star of a very common type, a red dwarf that is regarding a third the mass of the Sun and whose luminosity is regarding 3% that of the Sun.
Its planet b orbits in 32 days around the star at a distance of 0.14 times the Earth-Sun distance, which gives it an insolation insolation almost identical to that of the Earth in energetic terms (but for a much redder radiation ). The planet is between 6 and 10 Earth masses and its radius is 2.3 times that of our Planet. The density of K2-18b is therefore far too low for it to be a rocky planetrocky planet: with this radius, a terrestrial composition would imply a mass greater than 20 times that of the Earth. The planet must therefore be composed largely of so-called volatile constituents, forming a fluid envelope, the main candidates of which are molecular hydrogen, heliumhelium, and water. This type of planet is generally called a mini-Neptune, for lack of better terminology. It is an intermediate world between, for example, Earth (or Venus) and UranusUranus (or NeptuneNeptune).
This planet has manifested itself to us through its transitstransits, that is to say that we, observers, are in the plane of its orbit and that, from our point of view, it passes every 32 days in front of the disk of its star by reducing its apparent brightness. This is how it was discovered with the Kepler space telescope, in its K2 observation mode, hence the name of the system.
The recently published observation by two groups (Tsiaras et al., Benneke et al.) was carried out with the Hubble Space TelescopeHubble Space Telescope during the transit of this planet. By comparing the spectrum of the star during transit and outside of transit with a spectrometer, they analyzed how the limbos of the atmosphere filter the star’s light. These observations revealed the spectroscopic signature of water vapor. This is not a first, water vapor has been highlighted on several exoplanets since 2007. Until now, these were larger and hotter planets, which made observation easier. .
The presence of an atmosphere and water vapor on a planet of this density is expected, it is its absence which would be a surprise. But this observation is technically very difficult and might have been made impossible by the presence of clouds, the activity of the star and many other factors. Furthermore, if the planet did not have a hydrogen envelope but only heavier constituents: water vapor, carbon dioxide, carbon dioxide, methane, etc., the atmosphere would undoubtedly not have been able to be detected. because the limbs of the atmosphere would have been too thin, like a thin eggshell around the planet. Hydrogen being a very light gas, the atmosphere is very extensive and makes it possible to detect the minor constituents it carries, including water vapor.
The planet K2-18b is presented in both studies as a habitable-zone planetin other words a planet located in the zone habitable. This means that its insolation does not prohibit the presence of liquid water on its surface provided, however, that numerous factors are favorable: the water content of the planet, the composition of the atmosphere and the rotation of the planet, the existence of a surface… This expression may have been misunderstood by the media who sometimes spoke of ” habitable planet », which, for astrophysicists, would mean that the planet does indeed have liquid water on its surface. We will see that this seems unlikely.
The study of exoplanets has revealed an incredible diversity in the architectures of planetary systems, but also in the types of planets, with regard to masses, radii, temperatures and compositions. Observation methods now make it possible to probe the structure and composition of their atmosphere, thus opening a considerable field of research to 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
The habitability of K2-18b
First of all, given the insolation of the planet, we do not know if it is located in this ” zone habitable “. Although its insolation is the same as that of the earth in terms of energy, the spectrum of its star is very different, much more effective in heating an atmosphere. If we placed the Earth in the place of K2-18b, its oceans would be vaporized because, instead of reflecting 30% of the incident energy, it would reflect three times less. Some studies, however, propose that if such a planet has a rotation synchronous with its orbit – that is, in the case of K2-18b, if it rotates on its axis in 32 days and thus always shows the same facing its star -, the thick clouds forming on the daytime hemisphere would allow excess light to be reflected and to have a habitable climateclimate. It is very possible, due to the stellar tides exerted on the planet, that it is indeed in synchronous rotation, but the effect that this would have on the climate of an Earth-like atmosphere is currently much debated.
Furthermore, observation tells us that we are dealing with an atmosphere rich in hydrogen and not “ land “. Hydrogen is a very efficient greenhouse gas and water vapor is a minor constituent, it is very unlikely that water is present in a liquid state, neither on the planet’s surface nor even as droplets in clouds. One of the two studies concludes that liquid water is likely present, but it is clear to the community that the argument put forward to demonstrate this is erroneous..
The planet may well be very rich in water. Much more than Earth. Neptune and Uranus are and it is very likely that K2-18b is as well. But our models – which can still be improved – tell us that this water is never liquid. It is in the gaseous state in the atmosphere, certainly also in the form of ice particles in the upper atmosphere. At depth and depending on the composition, we might find water in the form of a supercritical fluid, then water ice but in high pressure and high temperature states, different from the usual ice we know. It is very possible that this planet does not have a surface and that we continuously pass from the external atmosphere to a supercritical envelope as is the case in Jupiter, SaturnSaturn, Uranus and Neptune.
These observations are fascinating and very important for comparative planetology. We have no such planets in the Solar System and have a lot to learn regarding them. K2-18b and similar planets will be prime targets for future James-Webb and Ariel space telescopes.”
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