University of Helsinki
University of Leicester astronomers confirmed the existence of infrared aurora on the second last planet of the Solar System – Uranus. This achievement lets the scientists discover more about magnetic fields and atmospheres of far-away planets. Uranus’ peculiar magnetic field (by peculiar I mean that it’s off-centre, not aligned with its rotation axis by a whole 59 degrees, and changes direction almost every Uranian day!) allows astronomers to study how exactly magnetospheres are created on different types of planets. The discovery gives us more tools to rate exoplanets on how habitable they are and if they’re suitable for future colonisation.
In the article published in Nature Astronomy, the researchers conducted an analysis of long-speculated infrared aurora borealis (or northern lights as it’s commonly known). Auroras are created due to interaction between highly energetic charged particles emitted during solar flares which slip through the planet’s magnetic field near magnetic poles and the planet’s atmosphere. Those collisions cause the atmosphere’s particles to get rid of the excess energy in the form of light. Earth’s atmosphere, which is made up of mostly nitrogen, emits the energy in the visible light spectrum. That is not the case on Uranus since its atmosphere is mostly hydrogen in helium, which would emit less energised electromagnetic radiation that falls in the infrared spectrum.
The analysis of auroral data from Uranus took almost 30 years. Thanks to data collected by the Keck II telescope in Hawaii they were able to conduct a spectral analysis. In spectral analysis, scientists analyse the light obtained from the object they were observing. Each element emits (or absorbs) unique wavelengths of light that act as its identifier. In this case, the brightness of the line created by the wavelength emitted by a particle H3+ (a positive ion of a molecule composed of three hydrogen atoms) depends on the temperature of the particle, making it possible to measure the temperature of Uranus’ atmosphere. Researchers found higher amounts of H3+ in places where an infrared aurora was present, but the temperature of those particles didn’t change.
These results will help scientists in future analysis of exoplanets, as most of them are ice giants like Uranus, to determine if they are suitable for human life. The results also put into question previous results obtained on Neptune, which has a similar magnetic field as Uranus and auroras should behave similarly. It also may explain how ice giants like Uranus and Neptune are much hotter than models predict if their only heat source was the Sun. Infrared auroras may deliver heat that is later moved towards the magnetic equator and effectively warming up the planet. Finally, Uranus is a great opportunity to study the effects that changing magnetic field may have on a planet and to discover different mechanisms of creating a planetary magnetosphere compared to what we know about this process on Earth. That research would have to be conducted on both Uranus and Earth to get the most accurate results. Faraway planets still hide many mysteries from us and unveiling these mysteries will help us not only in deep space travel but to also tackle issues in our own backyard.
Thomas, E. M., Melin, H., Stallard, T. S., Chowdhury, M. N., Wang, R., Knowles, K., & Miller, S. (2023). Detection of the Infrared Aurora at Uranus with Keck-NIRSPEC. Nature Astronomy. https://doi.org/10.1038/s41550-023-02096-5