New Horizons Explores the Bones of the Outer Solar System

In just about six weeks, the New Horizons spacecraft will begin its close flyby of Pluto, Charon and three (or more) other moons in this distant system. That mission will change our perspective on the solar system in ways we haven’t even thought of yet.

Why do I say that? Well, let’s look at how we define the objects in our solar system. It’s a moveable feast of planets, moons, rings, asteroids, comets, and now dwarf planets.

The discovery of Pluto was a big thing, with people acting like we’d finally found the “edge” of the solar system. Well, that’s not exactly true. In the years since Clyde Tombaugh found this little world, the existence of Pluto has fostered a lot of thought about how we define worlds and other objects that orbit stars. The 2006 decision to reclassify it as a dwarf planet is just a change of name, nothing more. It doesn’t change what Pluto is, and more importantly, it doesn’t affect what Pluto is telling us (and will tell us) about the solar system.

Pluto is in a distinctly different part of the solar system that contains worlds unlike those found closer to the Sun. This region, called the Kuiper Belt, stretches out from the orbit of Neptune, and is a realm of icy, frigid worlds that are only now (in the past few decades) being discovered. Most (if not all) of those objects are leftovers of solar system formation, and that makes them some of the most valuable real estate in the solar system. Just as planetary scientists study the asteroid belt to understand those ancient rocky leftovers of the birth of the planets, they now look to the Kuiper Belt — or, as it’s also called, “Trans-Neptunian” space — to find objects that existed in the primordial nebula from which the solar system came. In that sense, it’s a treasure trove of information giving insight into what conditions were like for icy bodies in our solar system 4.5 billion years ago.

The Kuiper Belt contains three dwarf planets: Pluto, Haumea, and Makemake. There are two more currently confirmed: Ceres (in the Asteroid Belt), and Eris (with its moon Dysnomia) farther out in the Kuiper Belt. There are likely dozens more dwarf planets out there, somewhere.

The Kuiper Belt is likely the origin of most of the periodic comets we see. In addition, scientists suspect that the moons Triton (at Neptune) and Phoebe (at Saturn) may have formed in this region and were somehow captured into orbits far from their birthplaces. So, the Kuiper Belt is something of a dynamical enigma. It has worlds that didn’t form there, and some of the places that did form there aren’t there anymore. They’re scattered around. That tells us that the newborn solar system was an active place, with worlds forming, migrating, zinging around, and settling into othe orbits.

The planetesimals that gathered in the Kuiper Belt are part of the bone collection of the original solar disk. These are the icy ones, whereas the asteroids are the rocky bones . The migration of the larger planets Jupiter and Saturn, and probably Uranus and Neptune, from their own formation creches closer to the Sun, changed the outer solar system in ways that we are just beginning to suspect. A “jostling” march of the giant planets sent shock waves through the collections of Kuiper Belt objects (KBOs), and scattered them. Today, planetary scientists are trying to understand the dynamics of this scattering, and how it produced the populations of objects that exist beyond Neptune. The story of this region of the solar system is not finished.

And, the orbital dynamics of the KBOs are just one part of the story. The New Horizons mission will give scientists very close-up look at these worlds, and the spacecraft’s instruments will probe as best they can to supply more precise information about the structure of Pluto. What’s inside it? Does it have an ocean? What is its atmosphere doing? Why? What about the structures of its moons? What are the chemical compositions of all those worlds in the Pluto system? Lots of questions that, when answered, are going to revolutionize our understanding of the Kuiper Belt.

So, in a sense, it doesn’t matter as much what we call Pluto. It’s a dwarf planet and this year is the one where we visit two (Ceres and Pluto). To me, “dwarf planet” is a fine moniker. It has an element of science behind it (more so than if we just said “planet”) and it presents planetary scientists with new grist for arguments about how we define planets today. Think of Pluto as the key to the outer solar system. What we find there is very likely to be echoed across many Kuiper Belt worlds. Pluto’s secrets are their secrets. And, we’re only 6 weeks away from learning what those secrets are.