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stormwyrm writes:

In a paper (paywalled) published in the journal Icarus, a team of scientists led by Carey Lisse and Ralph McNutt of the Johns Hopkins University Applied Physics Laboratory (that designed, built, and operates the New Horizons spacecraft) have made, using the Chandra X-Ray Telescope, the puzzling detection of X-ray emissions from Pluto. Being a cold, icy world with no magnetic field, Pluto has no obvious mechanism for producing X-rays, but it is known that the interaction of gases surrounding such bodies and the solar wind can produce X-rays, though the intensity of the emissions is still higher than would be expected given the measurements of the dwarf planet's tenuous atmosphere and its great distance from the sun. From the JHUAPL press release:

While NASA's New Horizons spacecraft was speeding toward and beyond Pluto, Chandra was aimed several times on the dwarf planet and its moons, gathering data on Pluto that the missions could compare after the flyby. Each time Chandra pointed at Pluto — four times in all, from February 2014 through August 2015 — it detected low-energy X-rays from the small planet.

[...] "We've just detected, for the first time, X-rays coming from an object in our Kuiper Belt, and learned that Pluto is interacting with the solar wind in an unexpected and energetic fashion," said Carey Lisse, an astrophysicist at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland, who led the Chandra observation team with APL colleague and New Horizons Co-Investigator Ralph McNutt. "We can expect other large Kuiper Belt objects to be doing the same."

[...] The immediate mystery is that Chandra's readings on the brightness of the X-rays are much higher than expected from the solar wind interacting with Pluto's atmosphere.

[...] Lisse and his colleagues [...] suggest several possibilities for the enhanced X-ray emission from Pluto. These include a much wider and longer tail of gases trailing Pluto than New Horizons detected using its SWAP instrument. Other possibilities are that interplanetary magnetic fields are focusing more particles than expected from the solar wind into the region around Pluto, or the low density of the solar wind in the outer solar system at the distance of Pluto could allow for the formation of a doughnut, or torus, of neutral gas centered around Pluto's orbit.

Other coverage from Starts With A Bang and Gizmodo.

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