Artist's concept of the interaction of the solar wind with Pluto's predominantly nitrogen atmosphere.
NASA and the team behind its New Horizons spacecraft announced today that Pluto - the dwarf planet - has a giant tail.
It's not a physical tail like a dog's, of course, but rather a frigid cloud of ionized gases trailing an estimated 48,000 to 68,000 miles behind Pluto, according to a NASA press release.
This giant tail is actually part of Pluto's atmosphere. Except that the bits of atmosphere are being stripped away by solar wind, a torrent of electrically-charged particles that constantly pours out of the sun in all directions.
"We see the atmosphere way far out," Randy Gladstone, a New Horizons co-investigator at Southwest Research Institute in San Antonio, said during a NASA press conference on Friday. "We see it from the ground out to 1,000 miles above the surface."
Because Pluto is such a tiny planet - its a fraction of a percent as massive as the Earth - its atmosphere escapes directly into space, Gladstone said.
Gladstone and others discovered the tail after examining data from the Solar Wind Around Pluto (SWAP) instrument on the New Horizons spacecraft. The device found an anomaly in the solar wind around Pluto: a depression composed of nitrogen ions. This depression is the tail, and it extends an unknown length behind the planet.
"We have actually flown through this [tail]," Fran Bagenal, a New Horizons co-investigator from the University of Colorado, Boulder, said during the press conference.
Ionized gas forms when a bunch of energy pummels atmospheric atoms and molecules. This bombardment pops electrons off the atmospheric gas particles, allowing their electrons to freely circulate. The end result is plasma: a fourth state of matter after solids, liquids, and gases.
As far as we know, plasma is the most common state of matter in the universe. There's simply a lot of energy pouring out of stars, and a lot of gas floating in space to form plasma. So, while it might seem surprising, plasma tails like Pluto's aren't new. They even exist behind other planets in the Solar System, including Venus and Mars.
The team still hasn't determined the precise shape of Pluto's newly discovered tail. They also don't know exactly how the was formed.
So, NASA is anxiously waiting for New Horizons to beam back more data. By August the team hopes to calculate how fast Pluto is losing its atmosphere to space - and, likewise, how quickly the icy world is shrinking.