Pluto in visible light and X-rays, at different scales. X-ray: NASA/CXC/JHUAPL/R. McNutt et al.; optical: NASA/JHUAPL. Source.
In a result announced in September 2016, astronomers working with NASA’s Chandra X-ray Observatory detected
everyone’s favourite dwarf planet, Pluto, emitting X-rays, and it's the first time an object in the Kuiper Belt has been found to do so.
This strange discovery could help researchers understand more about Pluto’s atmosphere, as well as the atmospheres of other objects at the very edges of our Solar System.
"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 Johns Hopkins University.
"We can expect other large Kuiper Belt objects to be doing the same."
The team was made aware of the X-ray emissions during the New Horizons mission in 2015.
As the craft headed out to the distant planet, which lies at its furthest point 7.5 billion kilometres (4.67 billion miles) away from Earth, Chandra astronomers observed Pluto on four separate occasions, finding evidence of an X-ray glow each time.
This is a surprising discovery, because unlike Earth and other celestial bodies in our Solar System, Pluto has no known intrinsic global magnetic field and is extremely far from the Sun. These conditions made a detectable signal unexpected, not physically impossible.
"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 team member Scott Wolk, from the Harvard-Smithsonian Centre for Astrophysics.
"Prior to Pluto, the most distant Solar System body with detected X-ray emission was Saturn’s rings and disk."
Despite the doubts, Lisse knew from previous research that the gas surrounding planetary bodies can interact with charged solar wind particles to create X-rays - an hypothesis that is now backed up by the Chandra observations.
But the weird thing is that Pluto is emitting far more X-rays than a body with only gas surrounding it should, at such a vast distance from the Sun.
While there's still a lot to figure out here, the team says that the emissions might be caused by interplanetary magnetic fields pushing more solar particles toward Pluto, causing more emission than they would otherwise predict.
Or it could be that there's a longer trail of gases lingering behind the dwarf planet that might have been missed by New Horizons.
Further research will hopefully help us to figure out what's behind Pluto's rather bright X-ray glow, and given that the findings suggest that other Kuiper Belt objects might also emit X-rays, astronomers will probably be looking for proof of that, too.
The team’s work was published in Icarus.
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What Chandra detected
NASA’s Chandra X-ray Observatory observed Pluto on four occasions during the period surrounding New Horizons’ approach and 2015 flyby. The resulting detection was announced in September 2016.
Chandra’s account of the observations explains an important detail about the familiar comparison image. The large, sharply defined Pluto is a visible-light image from New Horizons. The blue inset represents X-ray observations from Chandra, operating near Earth.
They have very different scales and resolutions. The X-ray panel does not reveal a detailed blue surface on Pluto; it shows an unresolved source associated with the dwarf planet’s position.
How a cold world can be associated with X-rays
X-rays do not always require a whole object to be extremely hot. One relevant process involves highly charged particles in the solar wind interacting with neutral gas. When an ion captures an electron, the resulting changes in its energy state can produce X-ray emission.
This process, called charge exchange, had already been studied around comets. It gave researchers a physically plausible way to think about X-rays associated with gas around Pluto.
Pluto’s lack of a known intrinsic global magnetic field therefore does not make the observation impossible. A magnetic field is not the sole route to X-ray production in the Solar System.
Why the amount of emission was puzzling
The difficulty was accounting for the observed signal with the solar-wind particles expected to encounter Pluto’s atmosphere. At Pluto’s enormous distance from the Sun, the available particle flow is much lower than closer in.
New Horizons also found an atmosphere that was escaping less rapidly than some pre-flyby expectations suggested. Researchers had to reconcile the distant X-ray observation with measurements made by a spacecraft passing nearby.
The study, published in Icarus, explored the detection as evidence requiring explanation, rather than a demonstration of a new law of physics.
Two instruments, two kinds of view
A flyby spacecraft can make close measurements along its path. A telescope near Earth can watch radiation from the target over a different observing window. Neither automatically replaces the other.
That is why it helps to ask three questions when looking at a space image:
- What kind of light is shown? Visible light and X-rays reveal different processes.
- Where was the instrument? A nearby spacecraft and an Earth-orbiting telescope have very different viewing geometry.
- What scale does each panel use? A magnified inset is not necessarily a closer photograph of the surface.
The same care improves our understanding of the Chandra and Event Horizon Telescope views of M87, where a combined graphic also brings together different instruments and scales.
Pluto remains more than a distant point
NASA’s Pluto facts page describes a world with mountains, plains, glaciers and a thin atmosphere. New Horizons transformed our view of that landscape, while observations at other wavelengths ask questions about its wider environment.
The solar wind links this faraway research to something more familiar: the auroras photographed above Earth. The details differ, but in both cases particles from the Sun help reveal interactions around a world.
Pluto’s X-rays are a good reason to enjoy an unresolved scientific question. An unexpected signal gives researchers something specific to test—and gives us another way to look at a place once known mostly as a faint dot.
Additional source and image references: New Horizons’ detailed visible-light view of Pluto beside a blue, unresolved Chandra X-ray inset
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