Pluto is emitting x-rays, and we don’t know why.
X-rays aren’t something we’d expect from Pluto, since the planet has no clear way of generating them. It’s a small, cold world with little magnetic field. Some solar x-rays might scatter off Pluto in our direction, but the level of x-rays is higher than what could be produced by scattering. So what gives?
The most likely explanation is that the x-rays are produced through an interaction between the solar wind and Pluto’s atmosphere. As the New Horizon’s flyby found, Pluto’s atmosphere is actually quite stable, so interactions with the solar wind could produce x-rays. Similar interactions between the solar wind and the comas of comets have been seen to produce x-rays. But the level of x-rays from Pluto is even higher than we’d expect from such an interaction, so that isn’t the whole story.
While these x-rays are a mystery, it’s important to keep in mind that the amount of data is actually quite small. There’s enough data from the Chandra spacecraft to know that it’s not a random fluke, but it’s difficult to get much detail from such a small sample. What this study does show is that Pluto does produce x-rays, and perhaps we should give it a closer look.
Paper: C.M. Lisse, et al. The puzzling detection of x-rays from Pluto by Chandra. Icarus. (2016) DOI: 10.1016/j.icarus.2016.07.008