Why is Pluto Emitting X-Rays?

Recently a group of astronomers have found a new X-Ray source in the universe. This is nothing unusual, the universe is full of X-Ray emitting stars, galaxies, and the like. But in this case the source is unusually weird. They're coming from the planet Pluto.

Now, Pluto presenting us with something puzzling is nothing new. Ever since the New Horizons probe passed by and explored Pluto last year the planet has tossed one enigma after another at us ranging from the planet being geologically active to the planet having an escaping atmosphere that is partly depositing itself on its main moon.

But these new developments may top even those mysteries. Researchers Carey Lisse and Ralph McNutt and their colleagues using the Chandra X-Ray Observatory have found that Pluto, a frozen Kuiper Belt object, is emitting X-Rays. This is unique for the outer solar system, the furthest planet known to be emitting such things is the gas giant Saturn. Everything else is quiet, even the ice giant planets. Very mysterious indeed, but we do have some ideas on how this can be happening.

Planets, even outer minor planets, are bombarded by the solar wind that streams off the sun. What's interesting however is that Pluto lacks a magnetic field and has no apparent way to produce X-Rays on its own. But we do have a clue. Comets produce X-Rays when the gaseous tail they emit interacts with the solar wind. Pluto seems to be doing the same thing somehow, but there are problems.

Pluto is very cold. But it's still exposed to the sun and heat causes gases to evaporate and create an atmosphere, though this atmosphere seems to freeze out depending on where Pluto is in its orbit. As the sun heats those gases with the ionized particles of the solar wind, they react and an X-Ray is emitted. Seems straightforward. But here's the mystery, the X-Ray emissions at Pluto are significantly more powerful than they should be and are at odds with the findings of New Horizons. The spacecraft's measurements indicated that the solar wind is 40 times too low to create the amount of X-Rays that Chandra detected.

That would suggest that Pluto not only has an atmosphere, but also a substantial tail somewhat like a comet. Just such a thing was seemingly detected by New Horizons, but again seems significantly weaker than it should be to produce all those X-Rays, though there are other indications it could be. Another possibility is that interplanetary magnetic fields might be the culprit and are focusing the solar wind around Pluto. Still further, a torus of gas centered on Pluto's orbit might do the trick.

The theories abound and no doubt will multiply, and this is especially interesting since this observation using Chandra was originally met with much opposition from the scientific community and almost didn't happen. Few thought Pluto could emit X-Rays. Why devote telescope time to a wild goose chase? In this case, the goose was easily caught it seems.

But we won't know what's going on here for sure until more powerful and accurate observations can be made of the X-Ray emissions. But chalk yet another one up for Pluto, it's mystified us yet again which it's done repeatedly for the last year. Personally, I suspect it's just taking revenge for having its planet status revoked and downgraded to minor planet.