Madrid, 23 years old (Europe Press)
One of the first displays of Webb’s scientific prowess came last summer, when he turned his attention to a transit of a Jupiter-sized, Saturn-sized exoplanet called HAT-P-18b.
The team, led by Guanggui Fu (Johns Hopkins University), detected several molecules in the planet’s upper atmosphere using the Near Infrared Imaging and Slitting Spectrometer (NIRISS) instrument, but what they didn’t find was even more surprising, AAS Nova reports.
The first of these surprises was a signature of helium uptake, but not orbiting the planet: instead, their results suggest that HAT-P-18b trails a weak, runaway helium tail. Similar features have been observed behind other planets, but this feature was so subtle that ground-based observatories missed it.
The second surprise concerns a molecule that hasn’t been displaced from the planet, but is probably completely absent. One of the main drivers for focusing specifically on HAT-P-18b is its location in the corner of the parameter space which is exceptionally useful for modelers working on the methane puzzle.
Hot planets with surface temperatures over 1,000 K are not expected to have methane in their atmospheres, as thermodynamics in these extreme conditions favors other species. However, simple models suggest that any world cooler than this should show signs of absorption caused by methane molecules in the upper atmosphere that intercept photons of a specific wavelength.
However, oddly enough, this expectation was not fulfilled in previous studies. In searching for several planets that must have methane, none were found. This tension called for further analysis: Were the assumptions of the models wrong, or was there something strange about the first worlds studied? With an equilibrium temperature of 800 K, the HAT-P-18b was the perfect target to help move the needle one way or the other.
Fu and his colleagues, whose study is published in The Astrophysical Journal Letters, did not conclusively detect methane, leading to further delving into the puzzle of ill-fitting models. Models assuming the atmosphere is in chemical equilibrium fail to reproduce the mixture of non-methane and yes-water observed in the data, suggesting more mechanisms for expected gas removal. Even more surprising, other models that did not assume an equilibrium also did not prefer to include methane in the final composition rather than omit it altogether.
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