Astronomers were able to study the atmosphere of a mini-Neptune in detail for the first time. This was possible thanks to the power of the James Webb Telescope.
Mini-Neptunes are a class of planets that occupy an intermediate position between rocky super-Earths and gas giants like Uranus and Neptune. They are thought to have rocky cores surrounded by thick shells composed of substances such as water and ammonia, and hydrogen-helium atmospheres.
Until recently, however, this was only speculation. There are no planets of this class in the Solar System. And, although mini-Neptunes are several times larger than Earth, they are still much smaller and usually colder than the giant planets, making them much more difficult to observe. Another problem is that they are usually surrounded by haze, making it very difficult to study the atmosphere.
This has now changed thanks to JWST. Astronomers have engaged it to study the mini-Neptune TOI-421 b, located 244 light-years away from Earth. It was not chosen by chance. According to the researchers’ estimates, its temperature is about 720°C. At such high values, the atmosphere of TOI-421 b should not undergo photochemical reactions leading to haze formation.
The researchers’ calculations were justified. JWST obtained a spectrum of TOI-421 b, giving us the first clue as to the mystery of what mini-Neptunes are. The telescope detected water vapor in the exoplanet’s atmosphere, as well as traces of carbon monoxide and sulfur dioxide. At the same time, it failed to find signs of methane and carbon dioxide in it.
The big surprise for scientists was that the atmosphere of TOI-421 b appeared to contain large amounts of hydrogen. The fact is that data collected by JWST during observations of other mini-Neptunes indicated that their atmospheres should be dominated by heavy molecules. However, the results of the TOI-421 b study refute this assumption.
The researchers concede that TOI-421 b may have formed and evolved differently than cooler mini-Neptunes observed previously. The hydrogen-dominated atmosphere is also interesting because it mimics the composition of the host star. Perhaps the difference is due to the fact that most previously observed mini-Neptunes orbit around red dwarfs, while TOI-421 b orbits a sun-like star.
In the future, scientists are going to continue studying mini-Neptunes to see if TOI-421b is an anomaly, the norm, or whether planets of this class are very diverse and don’t have any single pattern to which they can be reduced.
We previously reported on how James Webb confirmed the existence of the coldest exoplanet.
According to NASA
