X-ray astronomy usually deals with far-away, gigantic and violent cosmic objects such as black holes, galaxy clusters and the deaths of massive stars. But many things in our Solar System emit X-rays too - from the Sun, to planets, comets and moons. I look into how the Jupiter and Uranus produce X-rays and what that tells us about the planets.​

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The shimmering, dancing curtains of light from our planet’s aurorae have mesmerised people for generations. However, it wasn’t until 1979 that the first extra-terrestrial aurora was detected during Voyager 1’s visit to Jupiter. The gas giant planet’s auroral emissions span several parts of the electromagnetic spectrum, including X-rays. Studies show that Jupiter’s powerful magnetic field accelerates ions (positively charged atoms) into the planet’s atmosphere above the polar regions. These ions seem to come from inside Jupiter's magnetosphere (the magnetic bubble produced by the planet's magnetic field) and originally from the volcanic moon Io instead of the solar wind. The ions lose most of its outer electrons before they charge exchange with neutral atoms and molecules in Jupiter's atmosphere and produce soft (low energy) X-rays. These emissions also often pulse with periods of tens of minutes. A ring of high energy auroral X-rays arising from electron bremsstrahlung usually surrounds the soft X-ray emissions. Jupiter’s X-ray aurora are fixed on the planet’s frame so that as Jupiter spins on its axis, the aurorae rotate in and out of view, rather like how a pulsar’s beam of radiation sweeps across the sky. We are fortunate that the Juno spacecraft is currently orbiting Jupiter, as connections between the in-situ measurements that it takes, and X-ray signatures detected remotely by XMM-Newton can be made to achieve this aim.

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X-rays from Uranus was first discovered in 2021 after reanalysing data taken by the Chandra X-ray Observatory in 2002. It was long predicted that the ice giant would emit X-rays for decades as its atmosphere was thought to scatter and reflect X-rays from the Sun in the same way as Jupiter and Saturn. But the X-ray flux that Chandra measured was too high. Does the Uranian atmosphere reflect more solar X-rays than its gas giant cousins, or does Uranus produce X-rays in other ways too? My observations of Uranus from late 2022/early 2023 were the first by XMM-Newton. With its higher sensitivity and ability to measure the energies of the X-rays, what can we find out about this enigmatic planet?