Pluto has a tail!

The path of New Horizons as it flew past Pluto on July 14 took it directly into the dwarf planet's shadow, and while this provided a fascinating opportunity to see its atmosphere lit up by the Sun, it also allowed the spacecraft to see the effects of solar wind on Pluto.

What New Horizons found is that the atmosphere is being stripped away by the solar wind, creating a huge region of cold and dense ionized gas that extends tens of thousands of miles beyond Pluto. This essentially creates a “hole” or cavity in the surrounding solar wind, which was detected between 77,000 and 109,000 kilometers (48,000 and 68,000 miles) behind Pluto.

It is mostly composed of nitrogen ions, which form a plasma tail, although scientists aren’t yet sure what shape or size this tail is. It resembles the gaseous ion tails of comets, which extend far behind the icy rocks as they travel through the Solar System.

“This is just a first tantalizing look at Pluto’s plasma environment,” said co-investigator Fran Bagenal from the University of Colorado, Boulder, who leads the New Horizons Particles and Plasma team, in a statement.

“We’ll be getting more data in August, which we can combine with the Alice and Rex atmospheric measurements to pin down the rate at which Pluto is losing its atmosphere. Once we know that, we’ll be able to answer outstanding questions about the evolution of Pluto’s atmosphere and surface and determine to what extent Pluto’s solar wind interaction is like that of Mars.”

As mentioned by Bagenal, this tail is interesting because similar plasma tails have been found at Mars and Venus. Nitrogen ions had also been found in front of Pluto before the flyby by the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) on New Horizons, indicating that the atmosphere was being lost in all directions.

Working out the atmospheric loss rate of Pluto could be crucial in understanding how the dwarf planet has evolved over time, and what sort of world it is today. With more data expected next month, we can only wait and see what more surprises this distant world has in store for us.