In the ongoing debate over Pluto's status as a planet, dwarf planet, or even a comet, it turns out that the solar system's little celestial outcast may be none of the above.
Astronomers studying the wealth of data that continues to pour in from New Horizons' rendezvous with Pluto last summer have discovered something extremely puzzling about the former ninth planet. An instrument on New Horizons designed to capture readings on how solar wind interacts with Pluto has sent back data that conforms with absolutely nothing else in our solar system.
Pluto isn't just an outcast, it's a downright rebel.
"It's not comet-like, and it's not planet-like. It's in-between," said lead author David J. McComas, who manages the Solar Wind Around Pluto (SWAP) instrument aboard New Horizons. "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."
Solar wind is a plasma of charged particles that our sun regularly blasts into space at speeds of more than 100 million miles per hour. From asteroids to planets, everything in our solar system is regularly bathed in this soup of electrons and protons. Planets, thanks to their magnetic fields, abruptly deflect solar wind. Comets on the other hand generally feature a large region of gentle slowing of the solar wind. Pluto does a bit of each of these things, making it the first hybrid celestial body ever discovered.
Because Pluto is so small (roughly the surface area of Russia) and so far away from the sun (more than 3.7 billion miles) it was assumed that its atmosphere would not be able to hold onto molecules from solar wind. Instead, the astronomers found a dense concentration of atmospheric ions, a characteristic shared by much larger planets.
"Many people were surprised by Pluto's complex geology and atmosphere," Alan Stern, New Horizons principal investigator at the Southwest Research Institute, told PhyOrg. "This paper shows there's even more that's surprising there, including its atmosphere-solar wind interaction."
More surprises are likely yet to come, as New Horizons won't fully complete sending all of its data on Pluto until later this summer.