An international scientific team led by J. Callingham (University of Leiden) has discovered polarized coherent radio emissions, also called "aurorals", from 19 nearby dwarf stars located less than 170 light-years away. Dwarf stars are the most common and least hot stars in our Galaxy.
If most of these radio emissions are probably related to the magnetic and coronal activity of these stars, some others could be the result of magnetic interactions between these stars and their exoplanets.
These signals have been detected in the survey of the northern hemisphere sky carried out in recent years by the Low Frequency Array Radio Telescope (LOFAR) which is the most powerful in the world at wavelengths of 1 to 3 meters.
More precisely, these are signals from 19 red dwarf stars, four of which could be explained by the existence of planets orbiting them. These detections complement recent results obtained with LOFAR in 2020 on the detection of a signal from the red dwarf GJ1151, and the Tau-Boötes.
We have long known that the planets of our own Solar System emit powerful radio waves when their magnetic fields interact with the solar wind. This same process is responsible for the beautiful auroras we see at the Earth’s poles. However, only LOFAR today provides the sensitivity needed to detect auroral emissions outside our Solar System. It is a very powerful tool to help find planets outside our Solar System and determine their magnetic fields.
A new way to detect exoplanets
Philippe Zarka, astrophysicist at the Observatoire de Paris - PSL at the Laboratoire d’études spatiales et d’instrumentation en astrophysique and co-author of the article, predicted as early as 2001 the existence of magnetic connections between stars and their exoplanets in tight orbits, "giant" analogues of the interaction that we already know between Jupiter and its moon Io, generating powerful radio waves.
In this new study, the model proposed to explain the radio emissions from some of the dwarf stars detected (those that do not show flares in visible light) is an even more powerful version of the Jupiter - Io pair : here the exoplanet is enveloped in the magnetic field of its star, feeding accelerated particles into vast currents that give rise, in the same way, to the radio auroras near the star.
The scientific team is now trying, with the help of optical telescopes, to directly highlight the presence of planets around the star and is looking for a periodicity in the new data set collected by LOFAR.
To this quest, the teams of Paris Observatory - PSL are now also contributing in an original way by using the NenuFAR radio telescope, located at the Nançay radio astronomy station, for the same purpose, but at even lower frequencies.
The discoveries made with LOFAR are only a beginning, but the telescope only has the capacity to detect relatively close stars, located up to about 200 light-years away. With the next-generation Square Kilometre Array radio telescope coming online in 2029, the team predicts it will be able to detect hundreds of stars at much greater distances.
This work shows that radio astronomy is poised to revolutionize our understanding of the electrical and magnetic environment of planets outside our Solar System.
Reférence
The radio detections were published in the journal Nature Astronomy, in a paper entitled "The population of M dwarfs observed at low radio frequencies", Callingham et al.
DOI : 10.1038/s41550-021-01483-0
URL : https://www.nature.com/articles/s41550-021-01483-0 )