Thanks, Comet Pluto. Solar System Nomenclature Needs A Major Rethink

Pluto can’t seem to catch a break lately. After being reclassified in 2006 by the International Astronomical Union, it seemed that what had been the 9th planet of the Solar System was now relegated to the status of “dwarf planet” with the likes of Ceres, Eris, Haumea, and Makemake. Then came the recent announcements that the title of “Planet 9” may belong to an object ten times the mass of Earth located 700 AU from our Sun.

And now, new research has been produced that indicates that Pluto may need to be reclassified again. Using data provided by the New Horizons mission, researchers have shown that Pluto’s interaction with the Sun’s solar wind is unlike anything observed in the Solar System thus far. As a result, it would seem that the debate over how to classify Pluto, and indeed all astronomical bodies, is not yet over.

In a study that appeared in the Journal of Geophysical Research, a team of researchers from the Southwest Research Institute – with support from the Johns Hopkins University Applied Physics Laboratory, the Laboratory of Atmospheric and Space Physics at University of Colorado and other institutions – examined data obtained by the New Horizon mission’s Solar Wind Around Pluto (SWAP) instrument.

Basically, solar wind effects every body in the Solar System. Consisting of electrons, hydrogen ions and alpha particles, this stream of plasma flows from our Sun to the edge of the Solar System at speeds of up to 160 million kilometers per hour. When it comes into contact with a comet, there is a discernible region behind the comet where the wind speed slows discernibly.

Meanwhile, where solar wind encounters a planet, the result is an abrupt diversion in its path. The region where this occurs around a planet is known as a “bow shock”, owing to the distinctive shape it forms. The very reason the New Horizons mission was equipped with the SWAP instrument was so that it could gather solar wind data from the edge of the Solar System and allow astronomers to create more accurate models of the environment.

But when the Southwestern Research Institute team examined the SWAP data, which was obtained during the New Horizons’ July 2015 flyby of Pluto, what they found was surprising. Previously, most researchers thought that Pluto was characterized more like a comet, which has a large region of gentle slowing of the solar wind, as opposed to the abrupt diversion solar wind encounters at a planet like Mars or Venus.

What they found instead was that the dwarf planet’s interaction with solar wind was something the fell between that of a comet and a planet. As Dr. David J. McComas – the Assistant Vice President of the Space Science and Engineering Division at the Southwest Research Institute – said during a NASA news release about the study: “This is a type of interaction we’ve never seen before anywhere in our solar system. The results are astonishing.”

Examining both the lighter hydrogen ions that are thrown off by the Sun, and the heavier methane ions that are produced by Pluto, they found that the former showed a 20% rate of deceleration behind Pluto. This, and the bow shock Pluto produces, were both consistent with that of a comet. At the same time, they found that Pluto’s gravity was strong enough that it is able to retain the heavier methane ions, which is consistent with a planet.

Between these two readings, it seems that Pluto is something of an anomaly, behaving as something of a hybrid. Yet another surprise from a celestial body that has been full of them lately. And under the circumstances, it may lead to another round of “classification debates”, as astronomers attempt to find a new class for bodies that behave like both comets and planets.

As Alan Stern of the Southwestern Research Institute, and the principal investigator of the New Horizon’s mission, explained, “These results speak to the power of exploration. Once again we’ve gone to a new kind of place and found ourselves discovering entirely new kinds of expressions in nature.”

Further Reading: Journal of Geophysical Research