Pluto Has Tail, X-Rays

Did you ever notice that Pluto doesn’t have much of a tail? No, not that Pluto! This Pluto:


This has been known for a while. NASA noted this last year:

New Horizons has discovered a region of cold, dense ionized gas tens of thousands of miles beyond Pluto — the planet’s atmosphere being stripped away by the solar wind and lost to space. Beginning an hour and half after closest approach, the Solar Wind Around Pluto (SWAP) instrument observed a cavity in the solar wind — the outflow of electrically charged particles from the Sun — between 48,000 miles (77,000 km) and 68,000 miles (109,000 km) downstream of Pluto. SWAP data revealed this cavity to be populated with nitrogen ions forming a “plasma tail” of undetermined structure and length extending behind the planet.

Not long ago it was not known that Pluto had an atmosphere. But it does, and it is probably made from solid ice that makes up a good portion of the planet. When Pluto is nearer the Sun, this atmosphere burns off and forms an unimpressive tail. (Existentially impressive, but not fireworks impressive.) If Pluto were to come really close to the sun, like a typical comet, it would … well, it would essentially be a a comet. A pretty big one, at first. But then after several passes…

Anyway, more recently, it has been discovered that Pluto also puts out X-rays, and if confirmed, this is interesting. The total number of X-rays that have been detected is very small. The existence of these X-rays is likely linked to the atmosphere. From NASA:

Scientists using NASA’s Chandra X-ray Observatory have made the first detections of X-rays from Pluto. These observations offer new insight into the space environment surrounding the largest and best-known object in the solar system’s outermost regions.

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.

Pluto is the largest object in the Kuiper Belt, a ring or belt containing a vast population of small bodies orbiting the Sun beyond Neptune. The Kuiper belt extends from the orbit of Neptune, at 30 times the distance of Earth from the Sun, to about 50 times the Earth-Sun distance. Pluto’s orbit ranges over the same span as the overall Kupier Belt.

“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 team recently published its findings online in the journal Icarus. The report details what Lisse says was a somewhat surprising detection given that Pluto – being cold, rocky and without a magnetic field – has no natural mechanism for emitting X-rays. But Lisse, having also led the team that made the first X-ray detections from a comet two decades ago, knew the interaction between the gases surrounding such planetary bodies and the solar wind – the constant streams of charged particles from the sun that speed throughout the solar system ­– can create X-rays.

New Horizons scientists were particularly interested in learning more about the interaction between the gases in Pluto’s atmosphere and the solar wind. The spacecraft itself carries an instrument designed to measure that activity up-close – the aptly named Solar Wind Around Pluto (SWAP) – and scientists are using that data to craft a picture of Pluto that contains a very mild, close-in bowshock, where the solar wind first “meets” Pluto (similar to a shock wave that forms ahead of a supersonic aircraft) and a small wake or tail behind the planet.

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.

“Before our observations, scientists thought it was highly unlikely that we’d detect X-rays from Pluto, causing a strong debate as to whether Chandra should observe it at all,” said co-author Scott Wolk, of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. “Prior to Pluto, the most distant solar system body with detected X-ray emission was Saturn’s rings and disk.”

The Chandra detection is especially surprising since New Horizons discovered Pluto’s atmosphere was much more stable than the rapidly escaping, “comet-like” atmosphere that many scientists expected before the spacecraft flew past in July 2015. In fact, New Horizons found that Pluto’s interaction with the solar wind is much more like the interaction of the solar wind with Mars, than with a comet. However, although Pluto is releasing enough gas from its atmosphere to make the observed X-rays, in simple models for the intensity of the solar wind at the distance of Pluto, there isn’t enough solar wind flowing directly at Pluto to make them.

Lisse and his colleagues – who also include SWAP co-investigators David McComas from Princeton University and Heather Elliott from Southwest Research Institute – 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.

That the Chandra measurements don’t quite match up with New Horizons up-close observations is the benefit – and beauty – of an opportunity like the New Horizons flyby. “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,” McNutt says. “The measurements come together and give you a much more complete picture you couldn’t get at any other time, from anywhere else.”

New Horizons has an opportunity to test these findings and shed even more light on this distant region – billions of miles from Earth – as part of its recently approved extended mission to survey the Kuiper Belt and encounter another smaller Kuiper Belt object, 2014 MU69, on Jan. 1, 2019. It is unlikely to be feasible to detect X-rays from MU69, but Chandra might detect X-rays from other larger and closer objects that New Horizons will observe as it flies through the Kuiper Belt towards MU69.

The Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland, designed, built, and operates the New Horizons spacecraft and manages the mission for NASA’s Science Mission Directorate. NASA’s Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA’s Science Mission Directorate. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra’s science and flight operations.



  1. #1 Eric Lund
    September 19, 2016

    That Pluto has a tail is not exactly surprising. Venus has one, too, and the physics would be about the same if Pluto doesn’t have an intrinsic magnetic field (last I heard, that was still an open question).

    The X-ray result is a bigger surprise. You will get some X-rays from the interaction of solar wind electrons (which are mildly relativistic) with an atmosphere, but that apparently isn’t enough to explain the observations. But you can boost the X-ray output if Pluto has an intrinsic magnetic field. There may be other ways to do it, but that’s the explanation that comes to mind.

  2. #2 Brainstorms
    September 19, 2016

    It’s not just solar wind particles; it’s also from cosmic rays. And it’s not just Pluto. Our own moon is bright in gamma rays due to this phenomenon:

  3. #3 Wow
    September 19, 2016

    Are you sure you mean a tail, or are you confusing the stripping of the atmosphere of Venus with a tail created from evanescent material *and particles* in a cometary tail?

    Pluto doesn’t have an atmosphere, and not because the sun’s so close it’s blowing off the atoms.

  4. #4 Brainstorms
    September 19, 2016

    Comets have TWO tails: particles that “boil” off their surfaces, and trail in the direction of their orbits, and ions that are swept off and pushed away from the Sun due to solar wind.

    Pluto DOES have an atmosphere, although rather tenuous, and subject to change (sublimation & freezing on the surface) as its orbit takes it further from the Sun.

  5. #5 Brainstorms
    September 19, 2016

    Note the potential differences in the angles between the two tails… Depends on where they are in their orbits.

  6. #6 Eric Lund
    September 19, 2016

    Are you sure you mean a tail, or are you confusing the stripping of the atmosphere of Venus with a tail created from evanescent material *and particles* in a cometary tail?

    These two things are the same physical process. As long as the material is volatile, it’s a detail whether the picked-up ion was already a gaseous neutral atom/molecule or something that was ablated from a solid surface before it was photoionized. Either way, the particles are confined to a plasma tail by the surrounding solar wind.

  7. #7 Greg Laden
    September 19, 2016

    My use of the word tail comes from expert comments by experts.

  8. #8 Wow
    September 22, 2016

    “These two things are the same physical process. As long as the material is volatile”

    And no, they aren’t being ejected because they are volatile.

    We have an atmosphere, pluto doesn’t. One is the result of sublimation, the other from the force of thermal motion of the gas BEFORE IT LEAVES THE PLANET.

    They aren’t the same thing, eric. They aren’t the same thing.

    The tail pluto shows has a different source from that of venus.

    You can wonder if, in absence of an atmosphere, like mercury, whether it would have a tail, and THAT would be a similar one to that from pluto.

    But the one you’re talking of is from a different mechanism.

  9. #9 Wow
    September 22, 2016

    Indeed, a gas cannot, by definition, be volatile from its state as a gas.

    So if it was a gas before it got hit by the solar wind, it can’t have been volatile.

  10. #10 Wow
    September 22, 2016

    “Pluto DOES have an atmosphere, although rather tenuous”

    No, it’s got one in the same way the moon has one. There are molecules in gas form trapped by the planet, but it’s not one thick enough to have a thermodynamic temperature, because it’s far too rare.

  11. #11 Eric Lund
    September 22, 2016

    You can wonder if, in absence of an atmosphere, like mercury, whether it would have a tail, and THAT would be a similar one to that from pluto.

    We don’t have to wonder whether Mercury has a tail, because such a tail has been observed. Mercury is known to have an intrinsic magnetic field.. This field is only about 1% of the strength of Earth’s magnetic field, so distance and time scales of the solar wind’s interaction with Mercury are considerably shorter than for Earth, but the physics is otherwise the same. Material from the sunward side of Mercury is ablated and photoionized into an ionosphere (as opposed to being dissociated and then photoionized at Earth), and these ions (Na+ being the most abundant, because sodium is relatively common and has a very low first ionization potential) are detectable downtail, in the same way we see O+ from Earth’s ionosphere downtail. The chemistry may be different, but the physics is the same.

    Venus does not have an intrinsic magnetic field, so the physics of how the solar wind interacts with it has important differences. The entire dayside atmosphere is exposed to the solar wind, not just the cusp and polar cap regions like at Earth and Mercury. The interaction is similar to what we see at comets, the main difference being that Venus has a much stronger gravitational field and therefore loses proportionately less material.

    The unknown with Pluto is whether it has an intrinsic magnetic field or not. If yes, then the interaction will be similar to Mercury’s, where an ionosphere that has formed due to ablation from the surface contributes to a magnetically confined tail. If not, then Venus is your model for the interaction. Again, it’s not necessarily the same chemistry, but it is the same physics. The solar wind doesn’t care whether the ion it picks up was already in the atmosphere or freshly ablated from the surface.