TOI-421b, a mini-Neptune with a very special atmosphere

If there's one type of exoplanet that astronomers didn't expect to observe thirty years ago, it's mini-Neptunes. And yet, with the accumulation of observations, they've had to face the facts: these mini-Neptunes are everywhere. Of the 7,464 exoplanets listed in the Exoplanet.eu database as of April 2, 2025, 1,568 (or 21%) are mini-Neptunes. This proportion rises to 26.5% if we exclude unclassifiable planets, making them the most common type of planet observed to date. An "oddity" that astronomers, who live in a solar system that has none, don't always fully understand.

To answer their questions, scientists have been relying on the James Webb Space Telescope (JWST) and its unprecedented infrared and near-infrared observation capabilities since 2021. This is what an American team did, publishing on May 5 in The Astrophysical Journal Letters , the study of the atmosphere of a mini-Neptune, TOI-421b .

To their surprise, the results differ markedly from those obtained for other mini-Neptunes observed by JWST so far. These small gas planets have a radius between 1.9 and 3 times that of Earth (Neptune, by comparison, is 3.9 times larger than our planet).

While the results should be viewed with caution, as the signal is weak and the "noise" difficult to reduce, the researchers found traces of carbon monoxide and sulfur dioxide, but, more importantly, found no signs of carbon dioxide or methane. This data suggests that TOI-421b is a methane-poor mini-Neptune, unlike the six other mini-Neptunes studied by JWST.

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