2023-11-23 20:03:11
In our atmosphere, we would tend, in these times of anthropogenic global warming, to want to see methane disappear. A powerful greenhouse gas. In the atmosphere of exoplanets, on the contrary, astronomers are excited to find them, because it opens up many perspectives for them.
In the constellation Eagle, more than 160 light years from our Earth, there is an exoplanet that astronomers call WASP-80b. A hot Jupiter, they say. Because it is roughly the size and mass of our Jupiter. But there is a temperature more than four times higher there. And this hot Jupiter has just been the subject of a rare discovery. Thanks to the lynx eye of the James Webb space telescope, NASA astronomers have indeed found methane (CH4) in its atmosphere. They detail their discovery in the journal Nature.
Methane in the atmosphere of an exoplanet!
There is methane in our Earth’s atmosphere. It is a greenhouse gas. But there are also many in the atmospheres of Jupiter, Saturn, Uranus and Neptune, the giant planets of our Solar System. But researchers are struggling to detect the molecule by spectroscopy in the atmospheres of transiting exoplanets.
Transit and eclipse reveal exoplanet’s atmosphere
Let us specify that the transit method consists of taking advantage of a moment when the planet passes in front of its star – in relation to its terrestrial observer – to study the composition of its atmosphere. Because a thin ring of this atmosphere is then illuminated by the star. The molecules found there will absorb part of the wavelengths and make, for these colors in particular, the atmosphere more opaque.
To complete these data, astronomers used the eclipse method. They observed the planet pass this time behind its star. Because all objects emit thermal radiation – infrared radiation, therefore – whose intensity and color depend on the heat emitted by the object. So when the eclipse occurs, the brightness changes. And astronomers can then measure the infrared light emitted by the exoplanet. The molecules present in its atmosphere leave traces of absorption which generally appear as drops in light intensity.
The certainty of having detected methane
To transform the raw data they obtained into useful spectra, NASA astronomers chose to use two different approaches. They then interpreted these results using two types of models. Both come to one and the same conclusion: the atmosphere of WASP-80b contains methane. According to the researchers, there would be no more than a chance in almost a billion that they were on the wrong track. So what ?
Thanks to methane, better understand the exoplanet
The researchers first explain that the detection of methane in the atmosphere of WASP-80b will help them understand where the exoplanet was born and how it evolved. Indeed, a measurement of the quantity of methane and water on the planet also gives the ratio between carbon atoms and oxygen atoms. A ratio known to change depending on where and when planets form within their system. So examining this carbon-to-oxygen ratio may provide clues as to whether WASP-80 b formed close to its star or further away before gradually moving inward. Today, it is so close to its star that it goes around it in just three days!
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Astronomers are also excited that the method will now allow them to compare giant exoplanets to the giant planets of our Solar System, in order to identify similarities and differences.
And scientists are already dreaming of other discoveries that they might make thanks to the James-Webb space telescope. For example, they are waiting to probe the atmosphere of WASP-80b at other wavelengths. With the hope of discovering other carbon-rich molecules such as carbon monoxide (CO) or carbon dioxide (CO2).
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