Only one spacecraft has been to Uranus. NASA’s Voyager 2 gathered much of its critical information about the mysterious planet, including its rings and moons, in just six hours after traveling more than 1.8 billion miles (3 billion kilometers) in nine years.
Astronomers used NASA’s Chandra X-ray Observatory to detect X-rays from Uranus for the first time. This finding could help scientists learn more about our solar system’s enigmatic ice giant planet.
Uranus is the seventh planet from the Sun, with two rings circling its equator. The planet, four times the size of Earth, rotates on its side, distinguishing it from all other planets in the solar system. Because Voyager 2 was the only spacecraft to ever fly by Uranus, astronomers are now relying on telescopes much closer to Earth, such as Chandra and the Hubble Space Telescope, to learn more about this distant and cold planet made almost entirely of hydrogen and helium.
The new study made use of Chandra observations taken in Uranus in 2002 and again in 2017. They discovered a clear detection of X-rays in the first observation, which was only recently analyzed, and a possible flare of X-rays in those obtained fifteen years later. The main image shows a 2002 Chandra X-ray image of Uranus (in pink) superimposed on an optical image obtained in a separate study in 2004. The latter depicts the planet in roughly the same orientation as it was during the Chandra observations in 2002.
The X-rays emitted by Jupiter’s auroras are produced by two processes: electrons traveling down magnetic field lines, as they do on Earth, and positively charged atoms and molecules raining down at Jupiter’s polar regions.
What might be causing Uranus to emit X-rays? The answer is primarily the Sun. Astronomers have discovered that Jupiter and Saturn scatter the Sun’s X-ray light in the same way that Earth’s atmosphere scatters the Sun’s light. While the authors of the new Uranus study initially assumed that the majority of the X-rays detected were due to scattering, there are tantalizing hints that at least one other source of X-rays is present. If this is confirmed by additional observations, it could have intriguing implications for understanding Uranus.
One possibility is that the rings of Uranus are producing X-rays themselves, which is the case for Saturn’s rings. Uranus is surrounded by charged particles such as electrons and protons in its nearby space environment. If these energetic particles collide with the rings, they could cause the rings to glow in X-rays. Another possibility is that at least some of the X-rays come from auroras on Uranus, a phenomenon that has previously been observed on this planet at other wavelengths.
On Earth, we can see auroras, which are colorful light shows in the sky caused by high-energy particles interacting with the atmosphere. X-rays are emitted by energetic electrons that travel down the planet’s magnetic field lines to its poles and are slowed down by the atmosphere. Jupiter, too, has auroras.
The X-rays emitted by Jupiter’s auroras are produced by two processes: electrons traveling down magnetic field lines, as they do on Earth, and positively charged atoms and molecules raining down at Jupiter’s polar regions. Scientists are less certain, however, about what causes auroras on Uranus. Chandra’s observations could aid in solving this mystery.
Uranus is an especially interesting target for X-ray observations due to the unusual orientations of its spin axis and magnetic field. While the rotation and magnetic field axes of the other planets in the solar system are nearly perpendicular to the plane of their orbits, Uranus’ rotation axis is nearly parallel to its path around the Sun.
Furthermore, while Uranus is tilted on its side, its magnetic field is tilted by a different amount and offset from the planet’s center. This may cause its auroras to be 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.
NASA’s Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory’s Chandra X-ray Center controls science from Cambridge Massachusetts and flight operations from Burlington, Massachusetts.