The universe is so immense that it should not surprise us that it is full of exceptions. But even so, there are still findings so puzzling that they obsess astronomers. This is, for example, the case of TOI-5205 b, an exoplanet that attracts attention due to its size, too large for its star. That alone would be truly exceptional, but a new study has found that, if that were not enough, it also has a very unusual atmosphere.
Too big for a red dwarf. TOI-5205 b is a gas giant, slightly larger than Jupiter. But only a little. While Jupiter orbits the Sun, this exoplanet orbits a red dwarf. That is, a relatively cold and very small star, with a mass ranging from 7.5% to 50% of the mass of our Sun.
Typically, stars are MUCH larger than the planets that orbit them. However, the radius of this red dwarf is only four times that of TOI-5205 b. To continue with the comparisons, our Sun has a radius approximately 10 times larger than that of Jupiter. And it’s not just a radio issue. The mass of this exoplanet is also striking, as it is equivalent to 0.3% of the mass of the red dwarf. Jupiter’s mass is approximately 0.095% of the Sun’s mass. All this tells us that TOI-5205 b is too big for its star.
An even more disconcerting clue. Recently, a team of scientists from NASA and the Carnegie Science Institute decided to study the composition of TOI-5205 b’s atmosphere, looking for clues about its origin that would explain why it is so large. However, what they discovered was even more disconcerting. They carried out the analysis of the atmosphere studying the transit of the planet. That is, analyzing the changes in the light of its star when the planet passes in front of it.
When light interacts with the planet’s atmosphere, it interacts with the molecules found in it. Each element reflects light in different bands of the electromagnetic spectrum, so, with the help of a spectrograph, you can know which elements the light has interacted with and, consequently, what the composition of the atmosphere is.
Astronomical metals. For astronomers, any element other than hydrogen or helium is considered a metal. Just for them, chemists don’t like this. The point is that the concept of metallicity arises from this decision. It refers to the proportion of metals that a planet or star has in its atmosphere.
When a star forms, it is assumed to take most of the hydrogen and helium present in the stellar nursery with it. Therefore, when a planet later forms around it, it is normal for its atmosphere to have a higher proportion of metals. For this reason, it is said that the metallicity of the planets is higher than that of their stars. But with TOI-5205 b that does not happen. According to analyzes of its transit, its metallicity is lower than that of the red dwarf.
hidden metals. To verify what causes this phenomenon, the authors of the recently published study carried out a series of mathematical models. With them, they wanted to check how the atmosphere of this exoplanet could have evolved under different scenarios. This allowed them to verify that the current situation is consistent with their metals having been buried inside the planet.
It is true that when it was formed absorbed a greater amount of metals, since the star had taken more helium and hydrogen. However, these metals did not remain in the atmosphere, but were they saved inside TOI-5205 b. In the atmosphere, however, there is some helium and hydrogen, but also other compounds, such as methane and hydrogen sulfide.
What this exception teaches us. As one of the authors of the study, Anjali Piette, explained in a statement, “these findings have implications for our understanding of the process of giant planet formation that occurs early in the life of a star.” Sometimes, it is the exception to the rule that can provide us with the most data. There’s nothing like thinking outside the box.
Imagen | Katherine Cain (Carnegie Science)
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