The team at the John Hopkins Applied Physics Laboratory (APL) in Maryland (USA), who assess results from NASA’s Chandra X-ray observatory, has learned that Pluto is interacting with the solar wind in an unexpected and energetic fashion. Each time Chandra pointed at Pluto - four times in all during 2014/15 - it detected X-rays being emitted from the planet. The latest report (Sept 2016) details a surprising result given that Pluto - thought to be cold, rocky and without a magnetic field - should have no natural mechanism for emitting X-rays.
New Horizons discovered Pluto's atmosphere was much more stable than previously thought. Scientists know that the interaction between the gases surrounding such planetary bodies and the solar wind - the constant streams of charged particles from the Sun that speed throughout the solar system - can create X-rays. However New Horizons found that Pluto's interaction with the solar wind is much more like the interaction of the solar wind with Mars, rather than being comet-like, as they had expected. The immediate mystery is that Chandra's readings on the brightness of the X-rays are much higher than anticipated from the interaction of the solar wind with Pluto's atmosphere. Although Pluto is technically releasing enough gas from its atmosphere to make the observed X-rays, in simple models for the intensity of the solar wind at Pluto’s distance from the Sun, there isn't enough solar wind flowing directly at Pluto to make them out there.[i]
X-Rays mainly have their source in stars - which in our case means the Sun. Or, of course, they can be created artificially, as we do on Earth in our X-ray machines. Scientists are now considering whether perhaps the low density of the solar wind in the outer reaches of the solar system could allow for the formation of a doughnut, or torus, of neutral gas centred around Pluto's orbit.[ii] It will take deeper and higher resolution images of X-rays from Pluto's environment than we currently have from Chandra to distinguish between these possibilities.
[i] NASA/Chandra "The puzzling detection of x-rays from Pluto by Chandra" - C.M. Lisse et al. - Icarus - 2016 - http://dx.doi.org/10.1016/j.icarus.2016.07.008