The EXOPLANET WASP-121B is an ultra-hewn gas giant, which circles its mother star very closely. New data from the James webb telescope are now providing new information about how and where this hot planet was once created. As the astronomers determined, the proportions of some elements in the gas cover of WASP-121B, including carbon, silicon and oxygen, are significantly above the corresponding values of his mother star. This suggests that the planet was not formed near the star, but much further outside, beyond the snow limits for water, but still within the zone, from which methane also freezes. Only after the gas planet had already accumulated most of its atmosphere did he continue to move into its current position in his system.
The planet WASP-121B, which is around 850 light years away, is a hot Jupiter-a gas giant that circles its mother star very closely. He only takes around 30.5 hours for a circulation and always returns the same side to his star. As a result, his night side is “only” a good 1,200 degrees, that’s still enough to melt most metals and form clouds and rain from melted metal droplets. The WASP-121B day side, on the other hand, is heated up to temperatures of more than 2,700 degrees-enough to tear molecules and to evaporate solids such as silicon or metals. “This offers WASP-121B an opportunity to narrow down the frequencies and parts of its basic components through atmospheric observations,” explains Thomas Evans-Soma from the Max Planck Institute for Astronomy (MPIA) in Heidelberg and his colleagues. Because in the extreme heat of the day side, even the chemical elements and minerals are gaseous, from which the solid core of the gas planet consists.
First detection of silicon oxide in an exoplanet atmosphere
Evans-Soma and his team used the high-resolution near infrared spectrometer NIRSPEC of the James-Webb-WirraumTelescop. With him they watched the planet for almost 38 hours and thus during a whole area of his mother star. During this time, WASP-121b returned to the telescope of different areas of its surface, so that astronomers could measure the conditions and the chemical composition of the day and night side of the planet. With the help of the spectrald data and a coupled atmospheric model, they determined, among other things, the amount of water, CO2, carbon monoxide (CO), silicon hydride (SiH), silicon oxide (SIO) and ionized hydrogen (h–). The results confirmed that the heat of the day side itself tears molecules such as water and molecular hydrogen apart.
In addition, Evans-Soma and his team detect a weak spectral signature of gaseous silicon oxide. This connection had already been suspected due to previous observations in the UV area, but the new data of the WebB telescope now show the presence of silicon oxide in the gas cover of WASP-121b. “The detection of silicon oxide in the atmosphere of WASP-121B is groundbreaking-it is the first clear identification of this molecule in a planetary atmosphere,” says co-author Anjali Piette from the University of Birmingham. This is particularly exciting because the proportion of silicon and other evaporated solids provides information about the composition of the planet and thus also through its educational mechanism and place of origin. The evaluation of the spectral element signatures showed an increased proportion of silicon, oxygen and carbon compared to the mother star of the planet.
Planet was created far outside instead of close to the star
This new data provides information on where the Gas giant Wasp-121b was once formed. “In particular, the carbon/oxygen and carbon/hydrogen conditions indicate that WASP-121b has accumulated most of its gas envelope beyond the water ice limit, but on this side of the methane ice limit,” write Evans-Soma and his team. In this region of the star protoplanetar, it was cold enough to freeze water, but not yet so cold that the gaseous methane also becomes ice cream. Wasp-121b would therefore have been transferred to our solar system far outside between the Jupiter and Uranus lanes. This confirms existing assumptions for the formation of hot gas giants such as WASP-121b. Because astronomers have long suspected that these planets must have mainly acquired their mighty gas cover in the outer areas of their planetary systems. Only after they had already reached a certain size did they continue to hike inwards and got into the extreme closeness to their star today.
This scenario also explains why the gas cover from WASP-121B has an increased proportion of carbon to oxygen: on its way into the inner planetary system, the planet attracted tons of stone and ice-rich crumbs. These “pebbles” were spirally moving through the growing gas cover of the planet and finally crashed on his surface. The composition of the chunks and the distance of the planet from the star determined whether they evaporated or not on their way through the gas cover on their way. With WASP-121B, the increased carbon content of the atmosphere reveals that methane-containing pebbles evaporated, but chunks of water have remained intact, such as Evans-Soma and his colleagues explain. This indicates that the gas giant completed most of its growth in the zone between water ice and methane ice borders.
Source: Thomas Evans-Soma (Max Planck Institute for Astronomy, Heidelberg) et al., Nature Astronomy, DOI: 10.1038/S41550-02513-X)
