On August 24, 2006, many were greeted with the devastating news that Pluto was no longer classified as a planet. Many sad children had to rip the planet off of their solar system models, posters had to be changed, and some were so upset that protests raged in universities worldwide. So, why isn’t Pluto a planet and what other mysteries does this non-planet hold?
To understand why Pluto was no longer a planet, we need to go back to the new definition of a planet. According to the International Astronomy Union (IAU), a planet must orbit a star (in our case, planets orbit the Sun), it must be large enough to have enough gravity to force it into a spherical shape, and it must be large enough that its gravity can get rid of any other objects that’s of similar size near its orbit around the Sun. The latter means that Earth, for example, must be able to get rid of another body, like an asteroid and not have it join its orbit around the Sun.
Pluto didn’t meet the definition of a planet because it couldn’t clear its orbit of other bodies, meaning that other objects, like asteroids, joined its orbit around the Sun. Instead, it is now classified as a dwarf planet, which has the same definitions as a planet except it doesn’t clear its orbit of other celestial bodies.
Ever since Pluto was classified as a dwarf planet, it has greatly interested scientists. Many discoveries have been made since then and even a space mission was launched to study the dwarf planet called New Horizons. So what mysteries is this small body hiding?
One important thing to note about this planet is its size: it is incredibly small compared to most planets and dwarf planets. It is around 1,400 miles wide, about half the width of the United States! It’s even smaller than Earth’s Moon but luckily Pluto’s moons are smaller than the dwarf planet, otherwise it would be facing a lot of gravitational problems. (We will revisit this in a future blog.)A day on the planet is around 6.4 Earth days while a year is 248 Earth years–ample time to work on those New Year resolutions!
One of the most important questions scientists ask when uncovering planets and dwarf planets is whether they can house life. Unfortunately, the surface of Pluto is extremely cold reaching temperatures as low as -240 degrees Celsius, making it highly unlikely to sustain Earth life or even house any life of its own. However, there are traces of water being present on the dwarf planet and because its inner core is much warmer, there are chances of there being oceans and potentially housing marine life but this theory is to be visited more with upcoming space missions.
Another reason why Pluto wouldn’t be able to support human life is due to its composition. Its atmosphere is thin and expands when it approaches the sun and collapses as it moves away. Its molecular composition is nitrogen however, molecules of methane and carbon monoxide have been detected which makes scientists wonder if this dwarf planet can house life afterall. One question remains, how can Pluto’s atmosphere change as it approaches or moves away from the Sun? As the dwarf planet approaches the Sun the surface ice sublimates (it goes from solid to gas) and it rises to temporarily form a thin atmosphere. Its low gravity also allows it to extend farther in altitude. As it moves away from its star, the atmosphere freezes over and falls back down as snow.
One last thing I’ll leave you all with is the naming of the dwarf planet. One day in 1930, a grandfather and his 11-year-old granddaughter were eating breakfast and the grandfather told the girl that a new planet had been discovered. The girl thought a fitting name for the planet would be Pluto after the Roman God of the Underworld. This suggestion was sent to Lowell Observatory and was adopted immediately. So if you hear about the discovery of an exoplanet, maybe you can send in a name suggestion. (They’d probably use your suggestion since the names they’re giving to planets are quite odd these days, ahem HD189733b!)
Stay tuned for future blogs where we will explore more dwarf planets like Pluto.
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