Astronomers first detected X-rays from Uranus using NASA’s Chandra space telescope (launched back in 1999 by the Columbia shuttle). This result could help scientists learn more about the mysterious giant ice planet in our solar system.

Superimposed X-ray image of Uranus, 2002 (NASA | CXO | University College London | W. Dunn et al | WM Keck Observatory)

Uranus is the seventh planet farthest from the Sun and has two sets of rings around its equator. The axis of rotation of this celestial body, the diameter of which is four times larger than the Earth, lies, as it were, “on its side” relative to the plane of rotation of this planet around the Sun – this makes it different from all other planets of the solar system. Voyager 2 was the only spacecraft ever to fly relatively close to Uranus. Therefore, astronomers, wanting to learn about this distant and cold planet, consisting mainly of hydrogen and helium, currently have to rely on telescopes located much closer to Earth – for example, Chandra or Hubble.

In the new study, the scientists used Chandra’s observations made in 2002 and 2017. They found clear x-ray fixation in the first observation, only recently analyzed, and a suspected x-ray burst in images that were taken fifteen years later. The above image shows an X-ray image of Uranus from “Chandra” 2002 (purple), superimposed on an optical image from the Earth telescope “Kek I”, obtained in 2004. The latter shows the planet in roughly the same orientation as when observed by Chandra in 2002.

What could have caused Uranus to emit X-rays? Astronomers believe that the reason is in the sun. They observed that both Jupiter and Saturn scattered X-rays emitted by the Sun, just as the Earth’s atmosphere scatters Sunlight. While the authors of the new study on Uranus initially expected most of the detected X-rays to be the result of scattering, there is reason to believe that there is at least one other source of X-rays. If further observations confirm the theory, it could change some ideas about Uranus.

One of the possible reasons is that the rings of Uranus themselves produce X-rays, as happens with the rings of Saturn. Uranus in its nearby space environment is surrounded by charged particles such as electrons and protons. When these particles collide with rings, they can cause the latter to glow in the X-ray spectrum. Another reason could be that at least some of the X-rays come from the auroras on Uranus, a phenomenon previously observed on this planet at other wavelengths.

Superimposed X-ray image of Uranus, 2017 (NASA | CXO | University College London | W. Dunn et al | WM Keck Observatory)

Uranus is an interesting X-ray target due to the unusual orientation of its rotational axis and magnetic field. While the axes of rotation and magnetic fields of other planets in the solar system are almost perpendicular to the plane of their orbit, the axis of Uranus is almost parallel to the trajectory of rotation around the sun. In addition, while Uranus is “tilted on its side,” its magnetic field is tilted differently and offset from the center of the planet. This can make auroras unusually complex and volatile. Identifying the sources of X-rays from Uranus could help astronomers better understand how more exotic space objects in space, such as black holes and neutron stars, emit X-rays.