Un expert en astrobiologie fait sensation en affirmant avoir découvert des preuves de vie extraterrestre. Il ne parle pas de petits hommes verts, mais plutôt de minuscules organismes microscopiques qui pourraient exister au-delà de la Terre. Ses recherches ont suscité beaucoup d'enthousiasme et de débat au sein de la communauté scientifique, certains pensant qu'il pourrait avoir trouvé quelque chose d'important. Bien que les découvertes soient encore en cours d'examen, l'idée que nous pourrions ne pas être seuls dans l'univers est vraiment intrigante. Qu'il ait raison ou non, cela pousse définitivement tout le monde à regarder les étoiles avec un peu plus de curiosité. Animation créée par Sympa.
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FunTranscript
00:00Hello dear earthlings, if you think you are this special because you live on the only habitable planet in the universe, you are wrong.
00:08What makes a planet conducive to life? A whole set of factors, like the planet itself, its neighbors and the star around which it gravitates.
00:17A habitable planet is a planet that can welcome life, especially thanks to access to water, energy sources and nutrients.
00:25Earth, for example, is in the habitable zone, so it can have liquid water on its surface.
00:31But being in this zone does not automatically mean that a planet is really habitable.
00:37Excessive radiation levels, for example, can make a planet uninhabitable, even if it has the right temperature.
00:45In short, we need water, a good orbit and a set of nutrients of chemical origin that can be found in the periodic table of the elements of Mendeleev or in your favorite snacks.
00:58Look at this exoplanet, K2-18b. The term exoplanet does not mean that it is exotic, but simply that it is located outside the solar system.
01:09Most planets must be orbiting something, unless they are vagabond planets, i.e. objects the size of a planet floating in space, but do not orbit a star or a brown dwarf.
01:23K2-18b is orbiting a red dwarf called K2-18. It is not an ordinary star visible to the naked eye in the night sky.
01:33If this star is difficult to observe, it is because of its low luminosity.
01:38K2-18b is indeed a very, very distant planet. It is located 38 parsecs from Earth. A parsec corresponds to about 31,000 billion kilometers.
01:49Make the calculation. A clue, there are many zeros. K2-18b is a sub-Neptune, which means that its radius is less than that of Neptune.
01:59However, it is much larger than our humble home. Its radius is about 2.6 times larger than that of Earth. As for its weight, K2-18b is much more voluminous than Earth.
02:10It is about 8 times more massive. This planet takes 33 days to orbit its star and receives about as much stellar light as Earth receives from the Sun.
02:20The luminous intensity of this planet is 1.28 times higher than that of Earth, and its equilibrium temperature is less than 2 degrees Celsius.
02:29No, this does not mean that K2-18b looks like New York in January. It only gives us an idea of the theoretical temperature that reigns there.
02:39The planetary equilibrium temperature is in fact the temperature that a planet would have if it were perfectly balanced in terms of radiation.
02:47That is, when the amount of heat that leaves a planet is the same as the one that enters it.
02:54The billions of kilometers that separate us are not a problem for the powerful instruments we have, such as the Kepler Space Telescope, which discovered this planet, and the James Webb Space Telescope, which studies its atmosphere.
03:07In 2019, scientists announced that they had found water vapor in the atmosphere of the exoplanet K2-18b, which aroused considerable interest in this unusual world.
03:18Four years later, the James Webb Space Telescope detected carbon dioxide and methane swirling in the air of this planet. Carbon dioxide is our best friend on Earth.
03:29Humans need it to breathe properly and maintain the pH of their blood, and plants love it too, because it helps them produce oxygen thanks to photosynthesis.
03:39It is obvious that we could not survive without it, and without methane either.
03:43However, this little gem has aroused even more enthusiasm among some scientists, who have discovered in its atmosphere rather bizarre dimethyl sulfide gas molecules.
03:55In 2023, scientists from the University of Cambridge detected molecules containing carbon, methane and carbon dioxide on K2-18b.
04:05This could mean that this exoplanet has an atmosphere rich in hydrogen and water at its surface.
04:12It is also possible that dimethyl sulfide is found there, more simply known as DMS.
04:19On Earth, this substance is generally linked to life, because it is usually produced by the phytoplankton of our oceans.
04:26The fact that we have detected traces of DMS on K2-18b is comparable to the discovery of a winning lottery ticket in a cosmic chocolate bar.
04:35It gives you shivers, doesn't it?
04:39Why is DMS so important?
04:42It plays a crucial role in the sulfur cycle, and even influences our climate by contributing to the formation of clouds.
04:49The presence of DMS on other planets could be a sign of a biological presence similar to ours.
04:57It would seem that this planet is a good place, with its not-too-rigorous temperatures and all the nutrients necessary to keep us alive.
05:05What do you think of its general appearance?
05:08It probably took a few million years for the planet to form.
05:12Even if things heat up in the depths, it shouldn't make a big difference on the surface.
05:18Some experts think that it could shelter an ocean of water or lava in fusion with an atmosphere rich in hydrogen,
05:24a bit like on gas giants such as Uranus or Neptune.
05:28The latter does not really correspond to the standards of our Earth, since our atmosphere is mainly composed of nitrogen.
05:36If there is an ocean, it is probably under a thick layer of ice and rocks, which could disrupt the climate of the planet.
05:43When the temperature reaches the peaks, the border between the ocean and the sky erodes.
05:49We are not quite sure of the existence of a liquid ocean on K2-18b.
05:54It is difficult to see it from so far away.
05:56The size and weight of the planet is not enough to enlighten us on this subject.
06:02So we wonder if there is a liquid ocean on this planet.
06:06At first, we thought that water in a supercritical state was more likely.
06:11But new observations indicate that it is rather a liquid ocean.
06:15Suspicious gases, hydrocarbons and ammonia could pass from air to water if it is an ocean,
06:22which could mean that there is no clear border between the sky and the sea.
06:26But some experts believe that a fused rock ocean could play the same role.
06:31Others believe that a gas-rich mini-Neptune could also explain the situation.
06:35The Hubble Space Telescope has done some research and discovered that K2-18b has an atmosphere filled with hydrogen.
06:42We think there may also be water vapor, but it is still a bit mysterious.
06:46Apparently, the James Webb telescope took a look and detected less than 0.1% water vapor,
06:54perhaps because the planet has a dry stratosphere.
06:59As for ammonia, it is almost non-existent in this distant world.
07:03Methane and carbon dioxide seem to dominate the atmosphere with a proportion of about 1%.
07:09But don't expect to see other carbon oxides mingling at the party.
07:14Their concentration is only an assumption at this stage.
07:18The atmosphere of K2-18b represents only a small fraction of its total mass, about 6.2%,
07:25and it is probably similar to that of Uranus and Neptune.
07:29From the point of view of the mist, there is not much, and the water clouds remain a mystery.
07:34These are probably frozen clouds, but it is not excluded that there are also liquid water clouds.
07:41In addition to water, the atmosphere could also contain clouds of ammonium chloride, sodium sulfide, potassium chloride,
07:49and zinc sulfide, depending on the nature of the planet.
07:53If this is the case, then things could become a little more complicated in the atmosphere of this planet,
07:58with a temperature inversion and some problems in the stratosphere, one day like the others in space.
08:05So far, scientists have proposed several climatic models for K2-18b.
08:11For example, some think that the temperature is the same on the whole planet, unless it turns very fast,
08:16and then there would be a difference between the poles and the equator.
08:20But it would hardly be noticeable, but for the moment it is only speculation.
08:26The amount of radiation that K2-18b receives from its star is similar to that of the Earth.
08:32The temperature of the planet could therefore be very cold or relatively warm.
08:36It is the atmosphere and the clouds that determine whether the planet can shelter life.
08:40Scientists think that terrestrial microorganisms could survive there, even with all this hydrogen.
08:46But it is difficult to say if there is life on K2-18b,
08:50because the gases that we usually look for could not be present in its atmosphere.
08:55Scientists think that the James Webb Space Telescope could be able to detect different life markers on this planet,
09:02if we give it time.