The Solar System is constantly passing through a very diffuse gas from the space between the stars.
James Webb has just directly detected helium in a large portion of its observations. This signal, often considered simple background noise, can now be used to study our nearby galactic environment.
The Sun’s magnetosphere and heliosphere
This helium belongs to the interstellar medium, the gas found between the stars of the Milky Way. The Sun moves through this material with its planets. Some helium atoms can penetrate deep into the Solar System before being altered by solar radiation.
The discovery comes from earlier observations made with James Webb for other research. Upon reexamining them, astronomers found the signature of helium in about 80% of the data suitable for their study. The gas was therefore already present in many observations, even though it had not been directly sought.
The signal nevertheless changes considerably over time. In the same direction, its intensity can vary greatly in just a few hours or days. These rapid changes surprised the researchers and remain poorly understood.
James Webb’s position also plays a role. The Sun’s gravity slightly concentrates helium atoms passing nearby. A region where this gas is more abundant thus forms behind the Sun. When the telescope passes through this zone, the signal becomes significantly stronger.
Solar activity also appears to alter this flow. When the Sun is more active, its radiation can remove an electron from helium atoms. They then become harder to detect using the method employed in this study.
This discovery is significant in two ways. Astronomers will need to take this signal into account in some James Webb observations. But they also have a new tool for tracking interstellar gas over time and better understanding why its presence varies so rapidly around the Sun.