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Unexpected, mysterious x-rays found emanating from Pluto

The image above may look like a blue blob, but it’s actually the dwarf planet Pluto. This is Pluto as seen by the Chandra x-ray telescope, and it’s very odd that this instrument can see Pluto at all because we usually associate x-ray emissions with very hot objects. That’s one thing Pluto definitely is not. If this is confirmed, Pluto would be the most distant x-ray source in the solar system. So, what’s the deal?

Pluto is already a much more interesting object than we expected. Prior to the arrival of NASA’s New Horizons probe last year, astronomers thought Pluto would be a uniform lump of rock. But now we know that it has a thin atmosphere, interesting geological formations, and even a giant heart-shaped plain that may provide hints to Pluto’s surface conditions. Knowing that Pluto had an atmosphere, Carey Lisse of Johns Hopkins Applied Physics Laboratory and Scott Wolk from the Harvard-Smithsonian Center for Astrophysics started to search for signs that Pluto might be visible in the x-ray spectrum.

pluto x-ray optical

They expected it to be tough to spot Pluto, but it was a snap. Chandra was pointed at Pluto four times between February 2014 and August 2015, and the team detected seven x-ray photons from the dwarf planet. There are three possible explanations for Pluto’s x-ray activity, one of which was probably ruled out by New Horizons. If Pluto had an active magnetic field, it might be produce some x-ray energy. However, New Horizons didn’t spot any magnetic field activity as it flew past. Alternatively, solar x-rays might be scattered by Pluto’s thin atmosphere, but the photons observed by Chandra aren’t the right temperature for solar x-rays. That leaves the strong possibility that charged particles from the sun are hitting Pluto’s atmosphere, causing it to emit x-rays. This would tell scientists some important things about Pluto’s present and future atmosphere.

Most of Pluto’s atmosphere is close to the surface as it intermittently freezes and sublimates back to gas. However, some of it is much higher, where the the solar wind could collide with these atoms of nitrogen, carbon, and oxygen. And indeed, that would match the temperature of the x-rays detected by Chandra. That also means that Pluto’s already thin atmosphere is being stripped away by the solar wind.

pluto wind

The fact that so little atmosphere is producing detectable x-rays is important to other celestial objects too. There’s hope that dwarf planets farther out in the Kuiper belt could be visible in the x-ray spectrum as well. The team behind the Pluto analysis is already planning another survey with Chandra to look for more x-rays from the former ninth planet.

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