Much of the public is still pretty upset that Pluto was demoted in 2006 from a small frigid planet to a puny little dwarf planet.
I disagree with the general public’s idea that this re-classification must be a demotion. Astronomers estimate that there could be as many as 2,250 dwarf planets in the Kuiper Belt and I can assure you we will learn many amazing things about them when the New Horizons mission finally reaches its Pluto fly by in 2015 and then spends the next 5 years or so encountering other objects in the Kuiper Belt. There’s no doubt that our idea of what defines a planet (or even a dwarf planet, now) will change again in the upcoming decade.
Hubble Portrait of the “Double Planet” Pluto & Charon
This is the clearest view yet of the distant planet Pluto and its moon, Charon, as revealed by NASA’s Hubble Space Telescope (HST). The image was taken by the European Space Agency’s Faint Object Camera on February 21, 1994 when the planet was 2.6 billion miles (4.4 billion kilometers) from Earth; or nearly 30 times the separation between Earth and the sun.
Image Credit: Dr. R. Albrecht, ESA/ESO Space Telescope European Coordinating Facility; NASA
Knowing that this definition may soon be outdated, what exactly is a planet?
According to the International Astronomical Union (IAU), for an object in space to be a planet, it has to be in orbit around a star, it has to have enough mass to pull itself into a spherical shape with its own gravity and it has to have cleared its neighborhood in space of other orbiting bodies. There are still many objects with similar size and mass that are crowding Pluto’s orbital plane because the tiny planet isn’t large enough to either fling these other bodies into space or suck them in for a lethal collision. Pluto still has a chance to reclaim the planet fame, though, if it crashes into a smaller Kuiper Belt object and gains mass from it. That could very well lead to Pluto being able to rid its orbital plane of any other object similar in size and mass and, if the idea of what makes a planet hasn’t changed, we would be able to call Pluto a planet once again.
Now that Pluto has acquired this new status, what is a dwarf planet?
A dwarf planet is defined by the IAU as a celestial body orbiting a star that is massive enough to be rounded by its own gravity but has not cleared its neighboring region of smaller bodies and is not a satellite. That description fits Pluto perfectly and, to further emphasize the point, astronomers had found another dwarf planet, called Eris, in the Kuiper Belt that is 25% more massive than Pluto. The other 3 known dwarf planets are Ceres, Makemake and Haumea.
The IAU recognized Pluto’s special place in our solar system by designating dwarf planets that orbit the sun beyond Neptune as plutoids. Eris, which orbits far beyond Neptune, is a plutoid while Ceres, which orbits in the main asteroid belt between Mars and Jupiter is a dwarf planet – NASA Solar System Exploration: Planets: Dwarf Planets: List of Dwarf Planets.
I remember being upset about Pluto being re-classified as a dwarf planet and it was amplified by me personifying Pluto as a sentient being that could somehow have its feelings hurt by us changing its status. We all love Pluto and I felt bad for the tiny little planet until I began digging deeper into the infamous demotion and realized that by changing Pluto to a dwarf planet we have given much-needed attention to other dwarf planets that are hiding deep within the Kuiper Belt and maybe even beyond that. We have the New Horizons mission currently en route to study Kuiper Belt objects “first hand” and as our anger dies off and we accept Pluto’s fate as the most well-known dwarf planet we will continue to have an insatiable interest in what lies beyond our family of 8 planets.