Pluto might be a dwarf planet, but that doesn’t stop it interacting with the solar wind like the big boys. Astronomers thought the solar wind would sweep the Plutonian atmosphere away and make Pluto look like a comet, but data from New Horizons shows that this is not the case.

Earth has a sharply defined magnetosphere (the region of the atmosphere swept back by solar wind), but according to this research Pluto's is much broader – but still smaller, comparatively, than a comet. This puts it in a newly discovered "intermediate" category, where its magnetosphere is not as narrow as a planet's, but also not as wide as a comet's.

The discovery was made by a team led by Princeton Professor David McComas. They described how Pluto has a long tail of heavy ions, similar to Earth's own magnetosphere, stretching for over 120,000 kilometers (73,800 miles). Beyond the tail is the "Plutopause," a region between the Solar System's bow shock (where interplanetary plasma interacts with the Sun) and the edge of the tail. Upwind, the Plutopause is only few thousand kilometers from the surface, and this set up is the reason that makes Pluto unique.  

Pluto is small enough to interact gently with the solar wind, slowly pushing the solar wind on a new trajectory and thus creating a wide tail (just like a comet). But the interaction is quite sharp and more similar to planets like Mars or Venus than to the minor objects of the Solar System.

"This is an intermediate interaction, a completely new type. It's not comet-like, and it's not planet-like. It's in-between," McComas said in a statement. "We've now visited all nine of the classical planets and examined all their solar wind interactions, and we've never seen anything like this."

The uniqueness of Pluto's space environment has been described before, but this latest analysis has provided more details about it as well as the first observations of the ions that escape Pluto’s thin atmosphere. A paper with the results was published in the Journal of Geophysical Research: Space Physics.

The team believes the ions that form the tail are mostly methane molecules moving at about 90 kilometers (56 miles) per second. The measurements of the ions also led to another discovery: the loss rate of methane is only one percent of the expected loss rate from Pluto.

McComas added that New Horizons' SWAP (Solar Wind Around Pluto) data will be reanalyzed "for many years to come as the community collectively grapples with Pluto's unique solar wind interaction — one that is unlike that at any other body in the Solar System.

"The range of interaction with the solar wind is quite diverse, and this gives some comparison to help us better understand the connections in and beyond our Solar System.”