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Puzzling Out Pluto’s X-Ray Emissions

The latest news from the Chandra X-Ray Observatory is that the spacecraft, 100 times more sensitive to X-ray sources than any previous X-ray telescope, has found that Pluto is emitting X-rays. This marks the first time we’ve detected X-rays from a Kuiper Belt object. In fact, until now, the previous most distant Solar System body with detected X-rays was Saturn. But four Chandra observing runs from early 2014 through the summer of 2015 have detected X-rays, in work on Pluto done in coordination with the the New Horizons effort.

Carey Lisse (JHU/APL) led the Chandra observing runs, working with New Horizons co-investigator Ralph McNutt (also at JHU/APL). Says Lisse:

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

The New Horizons pass by Pluto/Charon in July of last year is a useful background to what Chandra has found. At first glance, Pluto would not seem to be a likely X-ray emitter. The dwarf planet lacks a magnetic field and has no evident mechanism for producing X-rays. But we’ve learned through studies of comets that the gases associated with Solar System objects can interact with the solar wind, the stream of charged particles flowing from the Sun, to produce X-rays.

Thus a comet, or a planet, can produce the material needed for the solar wind interaction to take place,. We know the mechanism is potent, because back in 1996, comet Hyakutake was shown to be producing X-rays in data from the German ROSAT observatory. It was a natural thought that Pluto, which was known to be venting atmosphere in some ways similar to a comet, could produce the same. At the Chandra site, Lisse and McNutt have written a useful essay on the recent detections. Here’s their take on interactions with the solar wind:

The way comets emit X-rays is not because they are hot and highly energetic, like all the other X-ray sources we know of in the sky like stars, black holes, neutron stars, and colliding shock waves. Instead comets are very cold, but they boil off gas from their ices when they approach the Sun — and that’s half of what is needed to make X-rays. The other half is the contribution of the biggest and hottest thing in our solar system: the Sun.

The Sun is a potent source indeed, and we’ve often looked at its solar wind from the perspective of potential spacecraft in various ‘magsail’ configurations. In terms of X-rays, the solar wind creates complicated effects.

The Sun not only emits its own X-rays as it boils and seethes and twists magnetic fields on its incendiary surface, it also blows out a stream of high energy ionized plasma – a gas composed of free electrons and free atomic nuclei – from the million degree tenuous corona (or atmosphere) surrounding its surface. This stream, called the solar wind, contains highly charged ions of hydrogen, helium, carbon, nitrogen, oxygen, iron, magnesium, neon, and sulfur, to name the most abundant species. If one of these ions ever gets near another atom with all its electrons, it will rip one or two of them off, emitting X-ray photons in the process.

Thus possible interactions were on the minds of New Horizons mission planners from the beginning, which is why the craft carried its Solar Wind Around Pluto (SWAP) instrument, as well as PEPPSI (the Pluto Energetic Particle Spectrometer Investigation), designed to pick up such activity. The ALICE UV spectrometer was also aboard to examine the rate of atmospheric loss into space, which would give a read on the material available for such interactions.


Image: The main panel in this graphic is an optical image taken from New Horizons on its approach to Pluto, while the inset shows an image of Pluto in X-rays from Chandra. There is a significant difference in scale between the optical and X-ray images. New Horizons made a close flyby of Pluto but Chandra is located near the Earth, so the level of detail visible in the two images is very different. The Chandra image is 290,000 kilometers across at the distance of Pluto, but the planet is only 2400 kilometers across. Pluto is detected in the X-ray image as a point source, showing the sharpest level of detail available for Chandra or any other X-ray observatory. This means that details over scales that are smaller than the X-ray source cannot be seen here. Credit: JHU/APL; Chandra X-Ray Observatory.

The surprise in this work is not so much that we have found X-rays in the first place, but that our models for solar wind intensity at Pluto’s distance don’t work out as neatly as they might. Pluto, New Horizons discovered, is releasing enough atmosphere to produce the X-rays, but they shouldn’t be emitted at this level of intensity given the amount of solar wind that actually reaches the dwarf planet. In fact, the New Horizons measurements peg the solar wind flux at too low by a factor of 40 to produce interactions as energetic as those the X-ray observations reveal.

We are left with an assortment of possibilities to explain why the X-ray intensity is as high as it is. Pluto may, for example, produce a larger gas ‘tail’ than what New Horizons detected with SWAP, a tail that would not necessarily be detected by Chandra at X-ray wavelengths. This is the prime candidate, and the conclusion that Lisse and McNutt favor, describing it thus:

It likely means that Pluto significantly perturbed the solar wind, and causes it to wrap around itself and focus into a long downstream tail, where it mixes with Pluto’s escaping atmosphere and makes X-rays. In fact, the New Horizons SWAP instrument has said just that, that they have detected a tail leading away from Pluto that contains ~1024 CH4 (methane) molecules per second getting ionized, which is consistent with what we need to create the X-rays detected by Chandra. They also figure that this tail extends at least 100 times the radius of the planet.

But there are other possibilities, as this Chandra news release makes clear. Interplanetary magnetic fields may be concentrating more solar wind particles than expected into the regions near Pluto. Also in the mix is the possibility that a torus of neutral gas could form, centered on Pluto’s orbit, thanks to the low density of the solar wind in the outer system. Chandra can’t make the distinction between these alternatives, which points to the need for higher resolution images of Pluto’s X-ray emissions to settle the matter.

The paper is Lisse et al., “The puzzling detection of x-rays from Pluto by Chandra,” in press at Icarus (abstract). Also pertinent is Bagenal et al., “Pluto’s interaction with its space environment: Solar Wind, Energetic Particles & Dust,” Science Vol. 351, Issue 6279 (abstract).


Comments on this entry are closed.

  • David OHara September 19, 2016, 22:11

    I’ve often wondered if solar sails might be optimized for absorption of solar X-rays as the sun emits a lot of X-rays. If the sun is a black body radiator then it will have emission in the several KeV X-Ray region that could fluoresce X-rays from Pluto accounting for some of them.

    • David September 20, 2016, 20:43

      David OHara,

      The Sun is only a rough black body emitter at UV to IR wavelengths. Non-thermal emission
      greatly dominates thermal emission at wavelengths longer and shorter. To absorb X-rays would require a much thicker sail than one optimized for absorbing visible light. A thicker sail means more mass and there is also the separate issue of there being sufficient X-Ray

      • David September 20, 2016, 22:44

        Sorry, let me rephrase “To absorb X-rays would require a much thicker sail than one optimized for absorbing visible light” Since Sails need to be thin, X-rays are more likely to pass through them then visible light and so little momentum is transferred to the sail.

  • Michael September 20, 2016, 12:21

    I wonder if this is coming from ionied particles near Pluto been hit by solar wind particles. If they are ionised they will expose the inner electrons which get hit by the solar ions and then when the electron falls back they will emit x rays.

  • Harry R Ray September 20, 2016, 16:55

    I have been desparately searching for a RECENT posting on this website where it wouldn’t be TOO OT for the following comment: The internet is GOING CRAZY over the POSSIBLE DETECTION(and maybe even IMAGING)of guysers on Europa! I chose THIS ONE because of Pluto’s POTENTIAL cryovolcanoes and HYPOTHETICAL global ocean. NASA is holding a press conference next Monday. Then, we will FINALLY KNOW FOR SURE! Keep your fingers crossed!!

    • ljk September 21, 2016, 8:28


      Sept. 20, 2016

      MEDIA ADVISORY M16-111

      NASA to Hold Media Call on Evidence of Surprising Activity on Europa

      NASA will host a teleconference at 2 p.m. EDT Monday, Sept. 26, to present new findings from images captured by the agency’s Hubble Space Telescope of Jupiter’s icy moon, Europa.

      Astronomers will present results from a unique Europa observing campaign that resulted in surprising evidence of activity that may be related to the presence of a subsurface ocean on Europa. Participants in the teleconference will be:

      Paul Hertz, director of the Astrophysics Division at NASA Headquarters in Washington

      William Sparks, astronomer with the Space Telescope Science Institute in Baltimore

      Britney Schmidt, assistant professor at the School of Earth and Atmospheric Sciences at Georgia Institute of Technology in Atlanta

      Jennifer Wiseman, senior Hubble project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland

      To participate by phone, media must contact Dwayne Brown at 202-358-1726 or dwayne.c.brown@nasa.gov and provide their media affiliation no later than noon Monday.

      Audio of the teleconference will stream live on NASA’s website at:


      For information about NASA’s Hubble Space Telescope, visit:


    • ljk September 21, 2016, 8:36

      FYI, this is the latest article about Europa on Centauri Dreams:


      I wonder – if the upcoming news event is about geysers, will this be the mechanism that puts some Europan ocean life on the moon’s icy surface?

      Perhaps we won’t have to drill through the ice crust and use a submarine for the job of finding native organisms, though of course that still needs to happen at some point for proper science exploration whether Europa has aquatic life or not. But it might make the next few probe missions cheaper and therefore become reality.

      And could life survive IN the ice itself? Ice does make a good radiation shield.

      • Harry R Ray September 21, 2016, 13:07

        The same can be said for Enceladus, and NOW, Ceres(for SURE) and Pluto(maybe, though I think it is UNLIKELY, it is POSSIBLE if the two cryovolcano CANDIDATES are VERIFIED. Anyway, I had to get IN here to be on topic). Speaking of TWO cryovolacnoes(one small and ACTIVE, the other HUGE and extinct, any brine-loving EXTREMOPHILES extracted from the ICE(Ahuna Mons) or SALT(Occator Crater Dome)and bruoght back to Earth from a surface sample return mission is MOST LIKELY GOING TO BE the FIRST life we discover OFF EARTH(unless we get VERY LUCKY on Mars).

  • Geoffrey Hillend September 20, 2016, 16:59

    Michael The emission of x-rays is probably caused by charge exchange. The methane is being ionized by the solar wind by charge exchange; The methane looses an electron since it was a normal or balanced atom (unionized). It looses the electron which is captured by a collision with the ionized solar wind atom. The electron then falls down to a lower energy level and the solar wind atom emits an x ray. The methane could emit an x ray only if it gets hit twice by a solar wind particle which first ionized it and then it would have to run into another atom to capture an electron which would then fall to a lower level and emit an x ray

  • Robert September 21, 2016, 15:26

    There are a lot of marginally explained EUV and X-ray sources in space. When the science behind the hydrino discovery is firmly established, hopefully by next year or so, it will be a unifying principle for many things including dark matter, white dwarfs, solar corona and many other phenomonon. Most of the matter in the universe is hydrogen and most of that is most likely in hydrino form.

  • ljk October 19, 2016, 9:01
  • ljk October 19, 2016, 10:34

    Rapidly Rotating Regular Satellites and Tides

    Posted by Darin Ragozzine

    2016/10/17 19:29 UTC

    As we begin the 2016 Division for Planetary Sciences (DPS) meeting, I’d like to provide a hypothesis to explain a surprising result from the 2015 DPS meeting. We were all excited and awed to see the new results from NASA’s New Horizons mission and its Pluto flyby. As an orbital dynamicist, perhaps the most striking piece of news was the unusual rotation rates and states of Pluto’s small moons.

    I agreed with Emily Lakdawalla’s assessment that the revelation of rapid rotation at right angles was the most interesting result. Quietly, in a later session, my student Danielle Hastings (then at Florida Tech, now a graduate student at UCLA) presented results that Hi’iaka, the outer satellite of Haumea – another dwarf planet – was also rapidly rotating. Is there a connection?

    Full article here: