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.
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