The little planet that couldn’t satisfy everyone has suddenly become the celestial body that boldly defies classification. As astronomers sift through the treasure trove of data sent back from the New Horizons spacecraft, one little instrument on the probe has made a big revelation that changes everything we’ve known and assumed about Pluto.
This is a type of interaction we’ve never seen before anywhere in our solar system. The results are astonishing.
That’s David J. McComas, a professor of astrophysical sciences at Princeton University who helped develop the device and monitors it. That device is the Solar Wind Around Pluto (SWAP) instrument and its purpose (gee, do I really have to explain this?) is to measure solar wind around Pluto. This breeze of charged particles blows out from the Sun at over 100 million miles per hour and space objects are forced to feel the charge or deflect it away. Planets with magnetic fields are lucky enough to be able to forcefully block the solar wind. On the other hand, comets get hit by the wind and gently (NASA’s description) slow it down.
Then there’s Pluto.
It's not comet-like, and it's not planet-like. It's in-between.
According to McComas, it was first thought that Pluto’s size meant it would act like a comet against solar wind. However, it also has enough gravity to confine a small magnetic field. On other planets, some of these ions are knocked into space by the solar wind, but Pluto loses very few of them. New Horizons found that Pluto has a long ion tail like Earth. It blocks solar wind, but at a lower elevation than other planets. This combination creates a thin so-called “Plutopause” or tail boundary between the ions and the solar wind.
Can anyone put it simply? Fortunately, McComas does in his report in the Journal of Geophysical Research:
Pluto interaction with the solar wind appears to be a hybrid with the bow shock generated by mass-loading like at a comet, but the obstacle to the solar wind flow – the Plutopause – sustained by atmospheric thermal pressure as at Venus and Mars.
So what to we call Pluto now? Hybrid planet? Comnet? Solar Prius? Giant dwarf? Better Than Venus?