Using NASA’s Chandra X-ray Observatory, astronomers have made the first detections of X-rays from Pluto. This X-ray emission comes from interaction between the dwarf planet’s atmosphere and the solar wind.
The first detection of Pluto in X-rays has been made using NASA’s Chandra X-ray Observatory. 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 180,000 miles across at the distance of Pluto, but the planet is only 1,500 miles 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. Image credit: X-ray – NASA / CXC / JHUAPL / R.McNutt et al; optical – NASA / JHUAPL.
As NASA’s New Horizons spacecraft approached Pluto in 2014 and then flew by the dwarf planet during the summer of 2015, Chandra obtained data during four separate observations. During each observation, Chandra detected low-energy X-rays from Pluto.
“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 Dr. Carey Lisse of the Johns Hopkins University Applied Physics Laboratory.
“We can expect other large Kuiper Belt objects to be doing the same.”
A report published in the journal Icarus details what Dr. Lisse says was a somewhat surprising detection given that Pluto has no natural mechanism for emitting X-rays.
But Dr. Lisse knew the interaction between the gases surrounding such planetary bodies and the solar wind can create X-rays.
Members of the New Horizons science team 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 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 and a small wake or tail behind the dwarf planet.
The immediate mystery is that Chandra’s readings on the brightness of 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 Dr. Scott Wolk, of the Harvard-Smithsonian Center for Astrophysics.
“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.
Dr. Lisse and his colleagues 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 of neutral gas centered around Pluto’s orbit.
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C.M. Lisse et al. The puzzling detection of X-rays from Pluto by Chandra. Icarus, published online July 27, 2016; doi: 10.1016/j.icarus.2016.07.008
This article is based on a press-release issued by NASA.
