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Astronomers have made a very interesting discovery: for the first time, water vapor was detected in the atmosphere of an exoplanet located in the habitable zone for its star.
The detection seems very good, and it has potentially exciting implications, but start with:
NO, THIS DOES NOT MEAN THAT THE PLANET IS EARTH OR LIVING.
Sufficiently clear? The quickest is that this planet looks more like Neptune than the Earth, a gigantic world with a thick atmosphere. When this news was announced yesterday, I saw a lot of lot) misinformation, mainly confusion or errors in the understanding of science, associated with breathtaking titles designed to get clicks.
Here is the real deal.
The star K2-18 is a red dwarf, smaller, colder and much weaker than the Sun. It is about 120 light-years away, which is close as the stars disappear (which is not so surprising, it is difficult to see red dwarves much further than that because they are so dark) .
There is at least one planet in orbit around it, called K2-18b (some observations strongly indicate that there could also be a second planet orbiting the star). Using the transit method, K2-18b was found to be 2.7 times the diameter of the Earth and using the reflex velocity method, a mass of 8.6 times the mass of the Earth. This makes it bigger than the Earth but smaller than Neptune; most of the exoplanets found are in this size range.
Planets whose diameter is about twice the diameter of the Earth are called super-lands, but when they become larger, they tend to have a thick atmosphere, which calls us gaseous giants. That's why we call these mini-Neptunes (to learn more about this distinction, my friend communicator Emily Lakdawalla posted a fantastic thread on Twitter about it). If one does the math, the density of the planet is about 2.5 grams per cubic centimeter, just between the density of the Earth (5.5) and that of Neptune (1.6). So, it is likely that there is a rocky core / metal and a thick atmosphere around it.
What makes this planet more interesting than average is its location: 21 million kilometers from its star, it receives about the same solar radiation as its star host (which, remember, is a little more darker and colder than the Sun) than the Earth. Is. This means that it is in the habitable zone of the star, where liquid water could exist.
The stress "could". Notice, technically, Venus and Mars are both in the Sun's habitable zone, and coming out on the surface of one or the other would kill you pretty quickly. From where my previous announcement.
What becomes so interesting with a planet like this is that its atmosphere is thick and he passes his star (that is, he passes directly between us and the star, creating a mini-eclipse). As it travels, starlight travels through the upper layers of its atmosphere to Earth. This is a great opportunity for astronomers because different atoms and molecules in the air absorb very specific light colors (wavelengths). If we observe the planet in transit, these substances will block these colors, making them detectable!
That's what a team of astronomers does. With the help of Hubble, they observed that K2-18b was passing 8 times on the star, looking for changes in the spectrum due to gases. Unsurprisingly, they discovered that the atmosphere was mainly composed of hydrogen, which is consistent with the fact that it looks more like Neptune than the Earth. But, moreover, they saw something else that intrigued: they found traces of water vapor in the atmosphere of the planet.
It's very cool! Note that the presence of water vapor has been observed in other exoplanets (like this one), but this is the first time that we see it in an exoplanet located in the habitable zone of its star. Given the temperature at the top of the clouds, the authors find that it is possible that water could condense into droplets and rain down. Whether on one surface or another is an entirely different problem (even if at a surface; the gas giants do not do it).
But the most important aspect of all this is that these observations show that find water vapor in the atmospheres of temperate planets is possible. This planet is large and has a thick atmosphere, which makes things easier, but in principle, planets of the same size and temperature as the Earth could also be analyzed in this way. We love hearing about water vapor because life on Earth depends on it – and to be honest, it's a great way to develop life. Therefore, if we are looking for signatures of life on other planets, having water in its atmosphere is a decent place to start.
It will also become easier with the enlargement of our telescopes, as with the James Webb Space Telescope to be launched later. We are actually pretty good at finding planets, and it's clear that we are improving at characterizing their.
That's what I find exciting. We are still at the beginning of all this exoplanet activity and we still have a lot of future.
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