16 years ago today Pluto was demoted. During a meeting of the IAU (the International Astronomical Union) members voted to downgrade the then ninth Planet to “Dwarf Planet” status. Since then, the reassignment has been one of astronomy’s most controversial topics.
There is a group of researchers who ask whether or not Pluto should be reclassified as a planet again in addition to several other similar objects in the solar system. The astronomers claim that Pluto was unfairly “maligned”. The group says we should consider the solar system has more than 150 planets.
The dissenters state that Pluto should be classified as a planet as was the case used by astronomers and astrologers since the 1500s that planets are any geologically active body in Space. If those were the only requirements, then we would have to reclassify other objects as planets. These would include large asteroids like Ceres, and the planetary moons Enceladus, Europa, and Titan. Do the dissenters think the more planets the merrier?
Although I’m emotionally attached to calling Pluto a planet, I’m going to go out on a limb with my unprofessional opinion and disagree with the dissenting astronomers. According to the IAU’s definition, a planet must meet three criteria. That is, the object must be spherical, orbit its star, and have gravitationally cleared its orbit of other objects.
“If you slid Pluto to where Earth is right now, heat from the Sun would evaporate that ice, and it would grow a tail. Now that’s no kind of behavior for a planet.”–astrophysicist, Neil deGrasse Tyson
Although Pluto appears to meet two of the requirements by being spherical and in orbit around the Sun, it fails to qualify because Pluto shares its orbit with countless astroid-like objects called “plutinos”. The eight remaining planets continue to satisfy the three planetary requirements. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune are all spherical, orbit the Sun, and have mostly cleared their orbits of extraneous objects.
Because of the diversity of stellar planetary architectures, it’s important to have a baseline. This will be increasingly important as the James Webb Telescope and the Hubble Space Telescope discover other star systems with exoplanets throughout the Milky Way and other galaxies.
Although Pluto and similar objects in orbit around the Sun are large enough to evolve into spheroids, many experts are reticent to call them planets. They do deserve to be called “Dwarf Planets”.
The Blue Jay of Happiness quotes engineer, planetary scientist, and Principal investigator of the New Horizon’s Pluto mission, Alan Stern. “Just because Pluto or comets aren’t as big as Jupiter doesn’t mean they are not scientifically important–indeed, just the reverse is often true. Sometimes, great things come in small packages.”