James Webb discovers an exoplanet giving off a helium tail

The James Webb Space Telescope  has found the tiny tail of a nearby exoplanet that is losing its atmosphere.


Observations with the James Webb Space Telescope have revealed a strange world, which subverts expectations of atmospheric chemistry and also presents a tail produced by a helium leak .

One of the first displays of James Webb ‘s scientific power came last summer, when it turned its attention to the transit of a Jupiter-sized, Saturn-mass exoplanet called HAT-P-18b.

astronomical surprises

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

The first of these surprises was a helium absorption signature , but not circling the planet: instead, their results indicate that HAT-P-18b is trailing a faint escaping helium tail. Similar features have been observed behind other planets, but this one was so subtle that ground-based observatories had missed it.

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 corner of parameter space that is exceptionally useful for modelers working on the methane mystery.

paradigm shifts

Hot planets with surface temperatures above 726°C are not expected to leech methane into 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 526°C, HAT-P-18b was the perfect target to help move the needle one way or the other.

Fu and his collaborators, who have published their study in The Astrophysical Journal Letters , did not conclusively detect methane, further delving into the puzzle of the misfit of the models. Models assuming the atmosphere is in chemical equilibrium failed to reproduce the non-methane and yes-water combination observed in the data, suggesting more mechanisms to remove the expected gas. And even more surprisingly, other models that did not assume an equilibrium also did not prefer to include methane in the final fit rather than omit it entirely. (Europe Press)