Look Up

The Curious Case of the Planet of the asteroids

by Mitchell Tester, College Student

As I discussed last month, there is much more to our solar system than just our planets. We learned the difference between meteors, meteoroids, meteorites, asteroids, and comets.

It is only natural that I discuss the next part of our solar system—there are not many topics as polarizing when it comes to space like our small friend, Pluto. Pluto was once our ninth planet, but why did it get demoted to what we call a “dwarf planet”? It being a name coined by the likes of planetary scientist, Alan Stern.

As someone who agrees that Pluto is a dwarf planet, I find that many will disagree with me. It is true that Pluto is spherical, just as is Mercury or any of the other planets. Not to mention, Pluto is actually roughly half the size of our smallest planet, Mercury. Should size matter, though? Well, in the case of celestial objects, it does. The most concrete reason as to why Pluto is a dwarf planet and not a main planet is that it is not the dominant mass in its orbit. Meaning that Pluto is not dominant because of the large gas planet Neptune, as Pluto is a TNO (Trans-Neptunian Object, meaning any minor planet that resides past Neptune, our most distant planet). As we learned last month, the asteroid belt (the region between Jupiter and Mars) was once thriving planetesimals early in our solar system’s life. Although, due to Jupiter (and a little bit of Mars), that area was stopped by Jupiter’s and Mars’ mass. Jupiter is the dominant mass in its orbit, while another dwarf planet like Pluto, named Ceres, is not, therefore making it a dwarf planet. If Ceres was, let’s say, larger than Jupiter, Ceres would be the planet, not Jupiter. What makes a planet a planet, and a dwarf planet a dwarf planet, is when a planet is the dominant mass in its orbital region, large enough to “clear the neighborhood” of its surrounding orbit. While a dwarf, or minor planet, which is mostly round, is not the dominant mass therefore it doesn’t clear out any smaller celestial masses. What separates a dwarf planet, such as Ceres, from an asteroid is simply the size. Ceres actually accounts for 25 percent of the asteroid belt’s mass, despite that it is still much smaller than Pluto. Nonetheless, the spherical nature of Ceres, which to be spherical means that it is larger (more mass, more gravity inward, forming it into a sphere). A dwarf planet is mostly spherical, orbits the sun similar to a planet, is not a satellite (moon) of any planet, and is not the dominant mass in its orbital path.

Now that we have the definition out of the way, let’s talk about our solar systems’ little planets! There are a total of five dwarf planets (that we know of). Let’s start with our most distant minor planets, our TNOs, reaching even farther past our gas giant, Neptune, a region where comets roam.

The first minor planet I will discuss is, of course, Pluto! Pluto is the largest dwarf planet, with an equatorial diameter of 1,477 miles, which is roughly one-fifth of the Earth’s. Pluto is an icy, frozen world, made up of mostly frozen nitrogen, methane, and carbon monoxide. Something that came as a surprise to me (just as you are, I am always learning new things, too!), Pluto actually has moons (or natural satellites) that orbit it. Charon, Pluto’s largest moon, is made up of mostly water ice on its surface. The rest of the moons of Pluto are much smaller and not so spherical, with the names Charon, Nix, Styx, Kerberos, and Hydra. And remember these moons are 4.6 billion miles away from us!

The next dwarf planet I will be discussing is the second largest, named Eris. Eris is technically smaller than Pluto by a small amount. Although its mass is larger than Pluto—coming in at 27 percent larger—Eris also is a TNO, which lies in the Kuiper belt, just as Pluto and the rest of the dwarf planets do—besides the asteroid belt dwarf planet, Ceres, which I will discuss last. Eris’ composition is made up of methane, nitrogen, and ice water. Its composition is similar to Pluto’s, to say the least. It is speculated that both planets could have a subsurface of water! An ocean beneath the surface. Eris has one small moon, named Dysnomia. These moons around the dwarf planets are a good way for astronomers to calculate the mass of the parent celestial body, by way of a technique called “Kuipers Third Law.” This means you can calculate the mass of the parent body by way of the mathematical relationship between the orbital period of the moon and its distance from the parent body. This is used for other celestial bodies besides just dwarf planets and their moons.

The third largest dwarf planet in our solar system is named Haumea. Named after the Hawaiian goddess of childbirth and fertility, as well as its additional meaning with the element “stone.” Haumea is made up of mostly rock, with a thin layer of ice surrounding it. What is particularly interesting about this dwarf planet TNO, besides its mostly rocky composition, is that it is, to put it simply, shaped strangely. If you look up pictures of Haumea, it is shaped like an American football or an elongated egg. With its elongated shape, it also has a big red spot on it. The spot is believed to be the impact crater from a violent impact many years ago, possibly explaining its strange shape and its very rapid rotation. Haumea has two moons, named Hi’laka and Namaka.

The fourth dwarf planet, Makemake, is the second-brightest object in the Kuiper belt (second to Pluto). It is reddish brown in color, similar to Pluto as well. Astronomers have detected frozen methane and nitrogen. Makemake has a reddish-brown color because of tholins, molecules that form whenever solar ultraviolet light interacts with substances such as methane. The red color of Pluto and other reddish-brown objects is often caused by tholins. Makemake has one moon, nicknamed MK2, of which has a radius of only 50 miles.

The last dwarf planet and the smallest, mentioned earlier, is named Ceres. Ceres resides in the asteroid belt, a rocky region between Mars and Jupiter. Ceres is very rocky, but it could be made up of as much as 25 percent water. If this is true, that means that Ceres has more water than Earth does! It has large deposits of salts, such as sodium carbonate and ammonium chloride. Ceres, unlike its TNO dwarf planet friends, does not have any moons.

There are other objects that exist past Neptune that are not considered dwarf planets by some astronomers because they do not satisfy the IAU’s (International Astronomical Union) definition of a dwarf planet—due to it not having an absolute magnitude brighter than +1. Absolute magnitude is the calculation of how bright an object actually is, not just how bright it appears from Earth. In my opinion (which is not important when it comes to a complicated topic such as this), I believe these distant objects are dwarf planets, just as much as Pluto is. I will save the discussion of these non-dwarf planet objects for another day, though.

Thank you for reading this, and I hope you have learned something new. I look forward to possibly teaching you something new next month. Be sure to get outside at night, despite the cold. I feel that the Moon looks particularly bright these past couple of weeks!