Spitzer’s swan song

At the present time, such a study would not have been possible without the NASA space telescope Spitzer. It offers several capabilities necessary for such a precise observation. “On a planetary scale, these temperature variations are indeed very great”, concedes the researcher “And yet, they only correspond to a miniscule variation in the brightness of the system, in the order of 200 ppm, or 0.02%!” Why has Spitzer succeeded in doing what would seem to be impossible for other telescopes?” Firstly, all the instruments present on Earth are disqualified due to their unsuitability. The atmosphere limits their precision by around 0.1%. Therefore, a telescope like Spitzer that can work in infrared was required. At those wavelengthswavelengths there is a better contrast between the thermal emission of a planet and its star. As the star is much hotter, it essentially emits shorter wavelengths. Thirdly, Spitzer was able to continuously record large fractions of the planet’s orbit. No Earth-based satellite is capable of this because our planet occults their field of vision half of the time. Spitzer follows the Earth on its heliocentric orbit, though it is progressively moving further away from it.  Today, it is at a distance of one astronomical unit from our planet and is therefore very far away from the light of our parasitic light. But Spitzer is getting old. Two of its three instruments are out of use and the third is only usable to 50% of its original capacity, and its distance makes the transfer of data more and more difficult. It should however continue to be usable during the interim period leading up to the point when the JWST space telescope becomes operational. “Its mirror will be 6.5 meters in diameter as opposed to 85 centimeters for Spitzer, says a delighted Michaël Gillon. “And its instruments will record the emissions of photons at several wavelengths. Each wavelength teaches us something different about a planet, combining the different wavelengths informs us about the vertical structure of the atmosphere, the composition of the surface etc. We got an intriguing first glimpse from Spitzer but an entire series of dynamic phenomena still remains unknown”.   

Towards “habitable Earths” 

Very much in demand, the JWST telescope will be capable of pointing its mirror at some hand-picked candidates. The challenge is to detect these planets with the aid of other instruments and to discern those with the most attractive profiles. With the aptly named SPECULOOS project and with the help of TRAPPIST, Michaël Gillon and his colleagues are at the cutting edge of technology in the area of astrophysics. They are seeking to detect even smaller planets, with geological and atmospheric conditions similar to those of the Earth and which orbit their star in the habitable zone. “We want to discover Earth-like exoplanets that are suitable for detailed study by the most modern telescopes such as the JWST, particularly the search for traces of life in the composition of their atmospheres.  This will only be possible for planets which transit the smallest and coldest stars in the immediate neighborhood of the Sun. Known as “ultracool dwarfs”, these stars are very frequent in the galaxy, much more so than sun-like stars. They are similar in size to the planet Jupiter, and have a temperature which is more than two times smaller than that of the Sun. With such a small and dim host star, the signal of a planet the size of the Earth is not totally drowned out by the flux of the star. By way of comparison, the Earth, around the sun is tiny and only emits a very small amount of light. A similar object orbiting a star that is not very bright and that is around the same size as Jupiter, will provide a significant part of the overall thermal emission. It will therefore be easier to study”. 

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