Guest Writer Feature: The Year of Dwarf Planets by Pecier Decierdo

Among other things, the year 2015 will be remembered as the year of dwarf planets.

As of this writing, Dawn, a spacecraft straight out of a science fiction novel, is undergoing a survey of the closest dwarf planet, Ceres. As if this were not enough, a few weeks from now we will also witness the New Horizons spacecraft make its closest approach to the distant dwarf planet Pluto.

During the flyby of NASA's awesome, ion-propelled Dawn spacecraft, it has found mysterious bright spots and an intriguing 5-km high "pyramid" on Ceres. Conspiracy theorists need to relax, however, as all evidence indicates that the bright spots and pyramid are all natural formations. 

Many planetary astronomers are guessing that the bright spots on Ceres are bodies of ice or salt. Because ice and salt can reflect much more sunlight than rock, this might explain why the spots found on Ceres are bright compared to their surroundings.

As Dawn spirals closer to Ceres in the coming months, scientists hope to get a better look at these bright spots and narrow down the possible set of answers to this puzzle.                 
Image credit: Jet Propulsion Laboratory, California Institute of Technology

Dawn is the first spacecraft to visit a dwarf planet. Before visiting Ceres, Dawn also performed a flyby of the asteroid Vesta. 

The second spacecraft to visit a dwarf planet will be the NASA's New Horizons, which is already on the final leg of its 9-year journey to the Pluto system. As its distance from Pluto and its moons decreases by the day, the veil on these distant worlds are slowly being lifted.

By the time of the closest approach, you can be assured that the internet will be inundated with close-up pictures of Pluto and its moons Charon, Nix, Hydra, Styx and Kerberos. New Horizons will be at its closest to Pluto in 14 July 2015.

To be a planet, an object must satisfy two conditions:

1) It must be big enough to have a rounded shape. This distinguishes them from comets and asteroids, which have irregular shapes.
2) It must orbit the Sun directly. 

Personally, I find this second condition unfair to undeniably planet-like objects like the awesome Europa, which NASA plans to visit soon, or Ganymede, which is even bigger than Mercury. Some scientists argue that such big, planet-like moons should be called "satellite planets". Our own Moon would be eligible for such a title.

Back in 2006, in the same year New Horizons was launched to what was then the "ninth planet" Pluto, the International Astronomical Union (IAU) decided to add a third criterion a body must pass to be considered a "major planet":

3) To be a major planet, a body must be massive and large enough to have cleared its path of debris.

It was this third criterion that led to Pluto being kicked out of the club of major planets, which now only includes eight bodies: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. But contrary to what many people believe, this did not strip Pluto of its planethood.

Because it satisfies conditions 1) and 2), Pluto is still a planet. It's just a dwarf one.

Also, Pluto is not a loner in the new club of dwarf planets, but is joined not only by Ceres but also by other distant worlds like Eris, Haumea, Makemake, and Sedna.

 Many scientists suggest that the list of dwarf planets should include more bodies in the Asteroid Belt and the Kuiper Belt. The Asteroid Belt is between Mars and Jupiter, while the Kuiper Belt is beyond Neptune's orbit.

Image credit: Karl Tate, contributor
Some of you might be asking themselves, "What's the point of visiting dwarf planets?" Well, what's the point of exploring a new place? We visit dwarf planets because they present new horizons of discovery and scientific adventure.

We visit them because they are intriguing and unique. We visit them because curiosity and wanderlust are twin impulses that are at the core of who we are as humans. In other words, we visit them because doing so is just awesome.

Visiting dwarf planets also has a lot of fringe benefits. Aside from the usual side benefits of space exploration, knowing more about dwarf planets can help us piece together the puzzle that is our origin story.

Because they straddle the boundary between the major planets and asteroids and comets, dwarf planets can teach us a lot about what makes a planet a planet, and provide us hints about how planets are formed. Because they are smaller than the major planets, dwarf planets are also relatively less disturbed; the ice and rocks that compose them are closer to the original ones that formed our Solar System. Despite being relatively small, dwarf planets are actually a big deal.

The Dawn and New Horizons missions to two very different dwarf planets are part of a grand detective work in search of our place in the universe. It's hard to think of a nobler endeavor than that. 


1. Jet Propulsion Laboratory (2015). Ceres spots continue to mystify in latest Dawn images. Retrieved from the JPL website:
2. National Aeronautics and Space Administration (2015). New Horizons. Retrieved from the New Horizons webpage of the NASA website:
3. National Aeronautics and Space Administration (2015). DawnRetrieved from the Dawn webpage of the NASA website:
4. Seeds, MA. (2003). The Solar System. 3rd ed. CA: Thomson Brooks/Cole. 

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