As for the James Webb Space Telescope, it is expected that it will primarily replace the Hubble Telescope to study what happened a few hundred million to a billion years ago. Years after the Big Bang at the level of galaxiesgalaxies on the one hand, and on the other hand, allowing a very significant leap in the study of the composition of the atmospheres of exoplanets in the suburbs near the SunSun in the Milky Way.
As for this last aspect, the dream is that in the not too distant future we may find a definition of what would be a particularly compelling biosignature, and ultimately find it in the atmosphere of a potential exo-Earth. Allow us to say, that life actually exists somewhere other than the solar system. This would be a major scientific and philosophical revolution.
ESO presents the discovery of K2-18b. To get a reasonably 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 click on “Subtitles” and finally “Auto-translate”. Select “French”. © ESA-Hubble, M. Kornmesser, NASA Goddard Space Flight Center Conceptual Image Lab
K2-18 b, from Hubble to 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 the so-called naive habitable zone around its parent star, that is, in a zone where liquid water could be expected assumed average temperature. We know that this idea is not easy, simply because the existence or non-existence of an atmosphere can change everything. VenusVenus and Earth have almost the same mass and probably the same chemical composition and yet Venus is hell.
The exoplanet Exoplanet K2-18 b is about 110 light-years light-years away from the solar system and is therefore a target of choice for James-Webb, especially since we used Hubble to find traces of water vapor in its atmosphere.
A NASA press release says new analyzes conducted with the James Webb instruments now reveal the presence of methane (CH4) and carbon dioxide in the atmosphere of K2-18 b, but without the presence of ammonia-camonia ( NH3) is 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 oceanic planet with an atmosphere consisting essentially of oxygen. This led them to introduce a new concept of a potentially habitable planet, that of a Hycean planet or Hycean planet from the words “hydrogenhydrogen” and “ocean(ic)”.
But not everyone agrees with this hypothesis, which Madhusudhan and his colleagues mention in the article about the JWST observations on K2-18 b, a version of which can be found on arXiv.
Researchers are also questioning the credibility of the signature of a dimethyl sulfide molecule (DMSDMS), which in the case of Earth we know reveals the presence of phytoplankton. Work still remains to confirm the presence or not of this molecule, and even if it is present, it is still too early to conclude that this would be evidence of the existence of a biotic process and that it is not an abiotic- There are abiotic processes for production. We have the same problem with Venus phosphine.
What should we make of all this?
It seems important to remember what the astrophysicist Franck Selsis, member of the CNRS and the Bordeaux Astrophysics Astrophysics Laboratory (LAB), explained to us four years ago when he generously allowed Futura to publish the article he dedicated to the discovery of water vapor had to reprint the atmosphere of the exoplanet K2-18b and was published on the LAB website. He set the record straight about this discovery, the idea of a habitable zone and its connection to the search for life elsewhere, and the possibility that K2-18b is an oceanic planet.
Here is his text!
“The detection of water vapor in the exoplanet K2-18b had a major media impact. This observation represents a very important step in the study of exoplanets, but is related to the search for life elsewhere, where habitability has been greatly overstated by certain media. So let’s get back to this fascinating system, 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 about 15,000 stars nearby. It is a very common type of star, a red dwarf, which has about a third of the mass of the Sun and whose luminosity is about 3% of that of the Sun.
Its planet b orbits the star in 32 days at a distance equal to 0.14 times the Earth-Sun distance, giving it solar radiation almost identical in energy to Earth’s (but with a much redder radiation ). The planet has 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 planet: at this radius, a terrestrial composition would mean a mass more than 20 times that of Earth. The planet must therefore consist largely of so-called volatile components, which form a liquid shell, the main candidates of which are molecular hydrogen, helium and water. This type of planet is commonly referred to as mini-Neptune, for lack of better terminology. It is an intermediate world, for example between Earth (or Venus) and UranusUranus (or NeptuneNeptune).
This planet has manifested itself to us through its transits, which means that we observers are in the plane of its orbit and that, from our perspective, it passes the disk of its star every 32 days, reducing its apparent brightness. This is how it was discovered using the Kepler space telescope in K2 observation mode, hence the name of the system.
The recently published observation by two groups (Tsiaras et al., Benneke et al.) was made with the Hubble Space Telescope during the transit of this planet. By comparing the star’s spectrum during and outside the transit with a spectrometer, they analyzed how the atmosphere’s limbus filters the star’s light. These observations revealed the spectroscopic signature of water vapor. This is not the first time that water vapor has been detected on multiple exoplanets since 2007. So far these have been larger and hotter planets, which made observation easier. .
The presence of an atmosphere and water vapor on a planet of this density is expected, but their absence would be a surprise. However, this observation is technically very difficult and could have been made impossible by the presence of clouds, the activity of the star and many other factors. In addition, if the planet did not have a hydrogen shell, but only heavier components: water vapor, carbon dioxide, carbon dioxide, methane, etc., the atmosphere would undoubtedly not have been detectable, since the members of the atmosphere would have been too thin, round like a thin eggshell around the planet. Since hydrogen is a very light gas, the atmosphere is very extensive and allows detection of the minor components contained therein, including water vapor.
The planet K2-18b is shown in both studies as a planet of the habitable zone, i.e. as a planet that is in the habitable zone. This means that its solar radiation 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, which sometimes spoke of a “habitable planet,” which to astrophysicists would mean that the planet actually has liquid water on its surface. We will see that this seems unlikely.
The study of exoplanets has revealed incredible diversity in the architecture of planetary systems, but also in planet types in terms of masses, radii, temperatures and compositions. Observation methods now make it possible to research the structure and composition of their atmosphere, thereby opening up a considerable field of research for comparative planetary science. A conference by Franck Selsis will take place here in 2016, organized by the Bureau des longitudes (Academy of Sciences) and the Department of Geosciences of the ENS. © École Normale Supérieure – PSL
The habitability of K2-18b
First, given the planet’s solar radiation, we don’t know whether it is in this “habitable zone.” Although its solar radiation is the same energetically as Earth’s, its star’s spectrum is quite different and much more effective at warming an atmosphere. If we placed Earth in the location of K2-18b, its oceans would evaporate because they would reflect three times less of the incoming energy instead of 30%. However, some studies suggest that if such a planet has a rotation that is synchronous with its orbit – that is, in the case of K2-18b, if it rotates on its axis in 32 days and thus always faces the same way towards its star –, The dense clouds that form in the daytime hemisphere would allow excess light to be reflected and create a habitable climate. Due to the tides exerted on the planet, it is entirely possible that it is actually in a synchronous rotation, but the impact this would have on the climate of an Earth-like atmosphere is currently much debated.
Furthermore, observation shows us that we are dealing with a hydrogen-rich and not “terrestrial” atmosphere. Hydrogen is a very efficient greenhouse gas and water vapor is a minor component. It is very unlikely that water exists in a liquid state, either on the planet’s surface or 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 purporting to prove this is false..
The planet could very well be very rich in water. Much more than Earth. Neptune and Uranus are, and it’s very likely that K2-18b is too. But our models – which can still be improved – tell us that this water is never liquid. It exists in a 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 could 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 has no surface and we are continually transitioning from the outer atmosphere into a supercritical envelope, as is the case with Jupiter, Saturn, Saturn, Uranus and Neptune.
These observations are fascinating and very important for comparative planetology. We have no such planets in the solar system and still have a lot to learn about them. K2-18b and similar planets will be prime targets for future James Webb and Ariel space telescopes.”