Citizen Science #36 by Trisha Muro

Moons, Dwarf Planets and the Solar System’s ‘Dust Bunnies’

Poor Pluto.

Long considered the ninth planet in Earth’s solar system, Pluto lost that status in 2006 when the International Astronomical Union officially reclassified Pluto as a dwarf planet. There were many reasons for this decision, but in my own mind the rationale I find most compelling is based on Pluto’s orbit. While most planets in the solar system move such that their paths around the Sun are more-or-less constrained within the system’s flat disk, Pluto’s orbit is tilted unusually high. Furthermore, its orbit is so elliptical that for a significant portion of its path around the Sun, Pluto is actually closer to the Sun than Neptune. Each planet has its own unique (wacky?) characteristics, of course, but Pluto is just so… not the same as the others.

So, let’s take a different approach and ponder whether Pluto could be considered a moon instead. What’s a moon, and what’s not a moon?

Earth, of course, has only one moon (uncapitalized) and we humans continue to call it “The Moon” (capitalized). Just as we previously defined a planet as being an object that orbits a star, a moon is a natural object that orbits a planet. The word natural is in there mainly so that we don’t end up counting spacecraft and satellites as moons, but it’s important to note that moons may or may not evolve naturally in their currently observed locations. (Planets can—and do—“capture” moons!) Our Solar System has well over 400 moons orbiting the planets, with another 470 or so orbiting dwarf planets, asteroids and other objects. Only Mercury and Venus have no moons at all, and Saturn wins the crown for the most moons at 293. Some moons have been known for hundreds of years, while others were discovered literally last year.

Moons have been pivotal to helping us understand our place in the cosmos, too. Jupiter’s four primary moons (Io, Europa, Ganymede and Callisto) were discovered by Galileo Galilei in 1609-1610. His sketches of how these four bright spots shifted position around Jupiter night after night were the basis of his argument proving a Sun-centered solar system as opposed to an Earth-centered arrangement. But that’s another story…

The term astronomers landed on, then, for Pluto is that it’s a dwarf planet. And it’s not the only one! You might have heard of Eris or Ceres, but our Solar System also has Makemake and Haumea as officially recognized dwarf planets for a total (so far) of five. In general, dwarf planets are bigger and more rounded than a typical asteroid. They may or may not have moons of their own, but their orbits around the Sun are a bit atypical and there’s often more debris along their orbital paths.

(And if you’re wondering, yes, Pluto has moons. Its main moon, Charon, is almost half the size of Pluto itself!)

But just as dust bunnies in our homes tend to accumulate at the edges of a room, lots of “other stuff” tends to gather at the edges of a star’s system. Around our Sun, we’ve named the Kuiper Belt as the region of space beyond Neptune’s orbit. It’s not quite as flat as the rest of the Solar System (Remember how I mentioned that Pluto’s orbit is tilted quite a bit? Same with many of the objects in the Kuiper Belt. Astronomers therefore think of the Kuiper Belt as being “doughnut-shaped.”), and it is home turf to lots of …stuff. The dwarf planets, yes, but also some comets and perhaps millions of other rocky asteroids and space rocks. This region of space is so far away and (most importantly) so dim and cold that seeing or characterizing these objects is extremely difficult. We simply do not have a census or even much of a tally of objects out here like we do for the Asteroid Belt.

If you’re wondering why not, remember that objects in space have to give off light in order to be seen by telescopes. Planets and even nearby asteroids can reflect enough of the Sun’s light for astronomers to see them, but in addition they also are close enough that their motion is readily detectable.

Space is big, and while it’s mostly empty in the in-between regions, there’s also a lot of stuff out there. There’s so much more to explore!

NASA’s website hosts a multitude of resources for learning more, at https://science.nasa.gov/dwarf-planets/; https://science.nasa.gov/solar-system/moons/; https://science.nasa.gov/dwarf-planets/pluto/; and https://science.nasa.gov/solar-system/kuiper-belt/.

Trisha Muro is a freelance science writer specializing in physics and astronomy and, although she’s never worked for NASA, she’s been a space nerd for as long as she can remember. Her first book, “It’s (Just) Rocket Science,” was published this spring by Johns Hopkins University Press.