A while back I posted about the Pluto Situation, and in that post I mentioned the then forthcoming New Horizons mission. Well, of course, New Horizons has completed its fly-by of the Pluto system, and as I think most of us hoped, we learned lots of amazing things about the once (and possibly future) planet and its satellites. And, of course, even more will be learned as the data from the fly-by continues to beam back to researchers here on Earth from the Little Probe That Could.

Many others have spoken about our new understanding of Pluto in greater depth and with much more eloquence than I’m capable of. But I’d always planned on at least a small follow-up once New Horizons had completed its amazing encounter. Of course I dawdled, and forgot, and fixated on other things, but then in the last few days some intriguing news has come out from the planetary research community: hints of a possible large body somewhere out in the dark reaches beyond Pluto. Another planet? Maybe. Another really big planet? Maybe.

At this point, the evidence is circumstantial. No one has found an actual planet out there. What has been found are alignments of smaller bodies in the Outer Solar System which suggest the presence of a larger body, perhaps a large planet. There are other possible explanations for this, but recent work has strengthened the argument for a new planet. More work and observation needs to be done, but the possibility is certainly intriguing.

What does this have to do with Pluto’s status as a planet/dwarf planet? Well, as part of our increasing knowledge of the outer Solar System, it’s sure to inform our understanding of what makes a planet and planet vs. a dwarf planet vs. an asteroid vs.—well, you get the picture.

Plus, in this fascinating interview about the possible new planet, planetary science Hal Weaver explains why he views Pluto as a planet—which would make this possible new planet number ten. The current IAU definition of planets vs. dwarf planets (the definition that demoted Pluto) is based heavily on orbital dynamics. Weaver argues that a better definition would be based on geophysical processes. He notes that New Horizons showed us “all kinds of incredible things taking place on Pluto and [its moon] Charon that I think are planetary processes.”

In my original post, I hope I was clear that I don’t consider Pluto’s status permanently settled fact, but rather a provisional expression of our current understanding of our Solar System. I’m less interested in what we call Pluto than what we understand about it, but what we do call it needs to be founded on that understanding. In the couple of years since my original post, so much new knowledge has come to light than an update to our definitions may be in order. Not that I get a say in the matter—what do I know? But experts like Hal Weaver do.

I don’t know what will happen in the planet/dwarf planet debate. Maybe nothing. But what I do know is every day we’re learning something new, and that’s damned exciting.