2023-10-17 19:09:50
This is the first time that quartz has been identified in the atmosphere of an exoplanet and it is the James-Webb telescope which made this discovery possible, which is also the first of a specific cloud species identified in an exoplanet at the occasion of transits.
The festival of discoveries with the James Webb Space Telescope (JWST) does not disappoint, even if unfortunately Hubert Reeves is no longer with us to comment on them. This time and as explained in a press release from NASA, the successor to Hubble has turned its gaze towards WASP-17 b which is one of the three exoplanets targeted by the Dreams research program (Deep Reconnaissance of Exoplanet Atmospheres) which are designed to bring together a comprehensive set of observations of a representative of each key class of exoplanets: a hot Jupiter, a hot Neptune, and a temperate rocky planet.
WASP-17 b is a hot Jupiter, located regarding 1,300 light years from Earth, and it is in the mid-infrared with the Miri instrument (Webb’s Mid-Infrared Instrument) that David Grant, a researcher at the University of Bristol in the United Kingdom, made the discovery with his colleagues. Discovery published in an article byAstrophysical Journal Letters and which can be read freely on arXiv.
« We were delighted! We knew from Hubble observations that there must be aerosols – tiny particles that make up clouds or haze – in WASP-17 b’s atmosphere, but we didn’t expect them to be quartz », Explains the astrophysicist in the NASA press release.
« We fully expected to see magnesium silicates. But what we see instead are likely the building blocks of these particles, the tiny “seed” particles needed to form the larger silicate grains we detect in cooler exoplanets and brown dwarfs. », Explains in turn his colleague and co-author of the article, Hannah Wakeford, also from the University of Bristol.
Clouds of polymerized silicates
Clearly, astrophysicists have identified the spectral signature of quartz nanocrystals in the high altitude clouds of WASP-17 b. Remember that quartz, even if it has the chemical formula SiO2, is part of tectosilicates which are minerals formed by association of tetrahedral elementary patterns [SiO4]4− by all their vertices. These are therefore sort of large polymerized molecules forming a crystal lattice.
This is not the first time that silicates, that is to say minerals rich in silicon and oxygen, have been detected in exoplanet atmospheres. But these were minerals also rich in magnesium such as olivine and pyroxene. We can even cite the case of olivine crystals observed with the defunct Spitzer telescope in exocomets. We can even say that observations carried out over the years show that silicates are very common in the Milky Way and that therefore rocky planets like the Earth, also containing a lot of silicates, must not be a rarity in the observable cosmos.
But this is the first time that we have found the spectral signature of quartz crystals, therefore a silicate largely described by the formula SiO2. This is not the case with olivine crystals such as forsterite, with the formula Mg2SiO4, or even fayalite with the formula Fe2SiO4. It is even worse with pyroxenes, of which the example of augite is enough to show it as (Ca,Na)(Mg,Fe,Al)[(Si,Al)O3]2.
Quartz clouds discovered during planetary transits
Analysis of hot Jupiter’s atmosphere was made easier by the fact that the exoplanet has a volume more than seven times that of Jupiter despite having a mass less than half that. This is due to the fact that WASP-17 b is one of the largest and most inflated exoplanets known due to its proximity to its star which gives it a temperature capable of melting certain metals. The highly dilated atmosphere therefore makes it the ideal planet for transmission spectroscopy: a technique which consists of measuring the filtering and scattering effects of a planet’s atmosphere on starlight, in this case the sun. of WASP-17 b. Webb observed the WASP-17 system for nearly 10 hours, collecting more than 1,275 measurements of mid-infrared light brightness from 5 to 12 microns.
Methods for detecting exoplanets have greatly diversified since the 1990s. They can be classified into two main categories, direct methods and indirect methods. The three main methods are the direct imaging method, the indirect transit method, and the indirect radial velocity method. Discover exoplanets through our nine-episode web series. A video to be found every week on our YouTube channel. A playlist proposed by the CEA and Paris-Saclay University as part of the H2020 Exoplanets-A European research project. © CEA Research
David Grant also explains that with regard to quartz nanocrystals because “ WASP-17 b is extremely hot – around 1,500 degrees Celsius – and the pressure at which they form in the atmosphere is only regarding a thousandth of that we experience on Earth’s surface, these solid crystals can form directly from gas, without going through a liquid phase first ».
The researcher adds that with regard to the clouds where these crystals are present, the exact quantity of which is difficult to determine, they “ are likely present along the day/night transition (the terminator), which is the region our observations are probing. Winds might move these tiny mineral particles at thousands of kilometers per hour ».
We can also think that the nanocrystal clouds circulate around the planet, but vaporize when they reach the warmer day side.
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