Recent observations of Pluto with NASA’s Chandra observatory have revealed something surprising. The planet is surrounded by a curious X-ray glow, which makes it the furthest object in the Solar System with this peculiar feature.

Pluto is too cold, small, and rocky to emit X-rays, so the researchers believe that it’s the interaction between the solar wind and the gas escaping the dwarf planet that produces the detected emission. This incredible discovery, published in the journal Icarus, could lead to a completely different understanding of the Kuiper Belt, the region of the Solar System beyond Neptune.

"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 team leader Carey Lisse, from John Hopkins University, in a statement.

"We can expect other large Kuiper Belt objects to be doing the same."

X-rays emission is not a common occurrence in the universe. It is usually associated with very hot gas (in the millions of degrees) or very energetic events, so the presence of such a glow poses some interesting problems.

The interaction between solar wind and the escaped gas from Pluto fits the bill, but according to data from New Horizons, there’s not enough solar wind there to explain the glow.

Lisse and his team proposed different explanations. Maybe the gas from Pluto’s atmosphere forms a longer tail behind the dwarf planet than what New Horizons measured. Or maybe the interplanetary magnetic field is deflecting extra particles onto the region around Pluto.

The discrepancy between Chandra and New Horizons might seem a big problem, but it actually helps to cast a light on the Kuiper Belt.

"When you have a chance at a once in a lifetime flyby like New Horizons at Pluto, you want to point every piece of glass – every telescope on and around Earth – at the target," added co-author Ralph McNutt, also from John Hopkins University.

"The measurements come together and give you a much more complete picture you couldn't get at any other time, from anywhere else."

Although it has left Pluto behind, New Horizons can still provide data to solve this mystery. The spacecraft is en route to another Kuiper Belt object, and it will continue to measure the solar wind properties at the edge of the Solar System.