Astronomers first detected X-rays of Uranus, using NASA’s Chandra X-ray Observatory. This result could help scientists learn more about this enigmatic giant ice planet in our solar system.
Uranus is the seventh planet from the Sun and has two sets of rings around its equator. The planet, which is four times the diameter of Earth, spins on its side, making it different from all other planets in the solar system. Since Voyager 2 was the only spacecraft to be flown by Uranus, astronomers currently rely on telescopes much closer to Earth, like Chandra and the Hubble Space Telescope, to learn more about this distant, cold planet that is almost entirely composed of hydrogen and helium.
In the new study, the researchers used Chandra’s observations taken at Uranus in 2002 and again in 2017. They saw clear x-ray detection of the first sighting, just scanned recently, and a possible x-ray flare in those obtained fifteen years later. The main graphic shows a Chandra x-ray image of Uranus from 2002 (in pink) superimposed on an optical image from the Keck-I telescope obtained in a separate study in 2004. The latter shows the planet in roughly the same orientation as ‘she was during Chandra’s sightings in 2002.
What could cause Uranus to emit X-rays? The answer: mainly the sun. Astronomers have observed that Jupiter and Saturn scatter X-ray light emitted by the Sun, in the same way that the Earth’s atmosphere scatters light from the Sun. While the authors of the new Uranus study initially expected that most of the x-rays detected would also come from scattering, there are tantalizing clues that at least one other source of x-rays is present. If further observations confirm this, it could have intriguing implications for the understanding of Uranus.
One possibility is that the rings of Uranus themselves produce X-rays, which is the case with the rings of Saturn. Uranus is surrounded by charged particles such as electrons and protons in its near space environment. If these energetic particles collide with the rings, they could cause the rings to glow in the x-rays. Another possibility is that at least some of the x-rays come from auroras on Uranus, a phenomenon that has already been observed on this Uranus. planet at other wavelengths.
On Earth, we can see colorful light shows called the Northern Lights in the sky, which occurs when high-energy particles interact with the atmosphere. X-rays are emitted in the Earth’s auroras, produced by energetic electrons after traveling the magnetic field lines of the planet to its poles, and are slowed down by the atmosphere. Jupiter also has the Northern Lights. Aurora x-rays on Jupiter come from two sources: electrons traveling down magnetic field lines, as on Earth, and positively charged atoms and molecules raining down in Jupiter’s polar regions. However, scientists are less certain of what causes the auroras on Uranus. Chandra’s observations can help to understand this mystery.
Uranus is a particularly attractive target for X-ray observations due to the unusual orientations of its spin axis and magnetic field. While the axes of rotation and magnetic field of the other planets in the solar system are almost perpendicular to the plane of their orbit, the axis of rotation of Uranus is almost parallel to its path around the Sun. Also, while Uranus is tilted on its side, its magnetic field is tilted by a different amount and shifted from the center of the planet. This can make its aurorae unusually complex and variable. Determining the sources of Uranus’ x-rays could help astronomers better understand how more exotic objects in space, such as growing black holes and neutron stars, emit x-rays.
An article describing these results appears in the latest issue of the Geophysical Research Journal. The authors are William Dunn (University College London, United Kingdom), Jan-Uwe Ness (University of Marseille, France), Laurent Lamy (Paris Observatory, France), Grant Tremblay (Center d’astrophysique | Harvard & Smithsonian), Graziella Branduardi- Raymont (University College London), Bradford Snios (CfA), Ralph Kraft (CfA), Z. Yao (Chinese Academy of Sciences, Beijing), Affelia Wibisono (University College London).
NASA’s Marshall Space Flight Center manages the Chandra program. The Chandra X-ray Center at the Smithsonian Astrophysical Observatory controls Cambridge Massachusetts science and Burlington, Massachusetts flight operations.