They detect an exoplanet that gives off a helium tail

The James Webb Space Telescope spotted a Jupiter-sized exoplanet, depicted with a tail produced by a helium leak, called HAT-P-18b.

The team, led by Guangwei Fu _of Johns Hopkins University_, discovered several molecules in the upper atmosphere of the planet using the Near Infrared Imager and Slitless Spectrograph (NIRISS) instrument, but what they didn’t find was more surprising, reports AAS Nova.

The first of these surprises was a helium absorption signature, but not surrounding the planet. Instead, their results indicate that HAT-P-18b trails a weak escaping helium tail. Researchers have already observed similar features behind other planets, but this one was so subtle that ground-based observatories had missed it, he details. Europe Press.

The second surprise concerned a molecule not displaced from the planet, but possibly absent altogether. One of the main motivations for specifically focusing on HAT-P-18b is its position in a exceptionally useful corner of parameter space for modelers working on the methane mystery.

Hot planets with surface temperatures above 1,000 K are not expected to have methane in their atmospheres, as thermodynamics in these extreme conditions prefer other species. However, simple models suggest that any world colder than this should show signs of absorption caused by methane molecules in the upper atmosphere intercepting photons with a specific wavelength.

However, strangely, this prediction has not been fulfilled in previous studies. In the search for several planets that should contain methane, none were found. This tension called for further analysis: were the models’ assumptions wrong, or was there something strange about the first worlds studied? With an equilibrium temperature of 800 K, HAT-P-18b was the perfect target to help move the needle one way or the other.

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Fu and his colleagues, who have published their study in The Astrophysical Journal Lettersdid not conclusively detect methane, further delving into the puzzle of the misfit of the models.

Until 2022, 5,000 exoplanets had been detected. (YO)