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Happy Birthday Pluto!

Exactly 80 years ago today, the one-time ninth and smallest planet, Pluto, was discovered by Clyde Tombaugh. In this time, Pluto has gone through a lot of changes, both in space and in its reputation here on Earth. Let’s take a moment to celebrate everyone’s favorite dwarf planet by getting to know it a little better.

Disruptions of Uranus

The driving force that lead to Pluto’s discovery began when late nineteenth century astronomers noticed a strange disruption in the orbit of Uranus. They speculated this disturbance had to be caused by another planet beyond Neptune and in 1906, Percival Lowell started to seek out this so-called “Planet X” in the Flagstaff, Arizona observatory he founded fifteen years earlier. Lowell and his team worked diligently for the next seven years, but when the researcher passed on, the project was forced to temporarily close due to a messy legal battle with Lowell’s widow. Interestingly, during those seven years, the researchers did capture the first ever images of Pluto on March 19, 1915, but the team did not recognize them for what they were. In 1929, the search started back up again with promising, and young, astronomer Clyde Tombaugh (seen above) at the lead. His job consisted of looking at images of the sky taken weeks apart and then look for any shift in position of the objects imaged. On February 18, 1930, he discovered a moving object seen on photos taken in the past January.

Renaming Planet X

As soon as the news hit the papers, the observatory began receiving suggestions for the new planet’s name. The planet’s future name eventually came from 11 year-old Venetia Burney. She was quite the fan of mythology and thought “Pluto” (another name for the god of the underworld) would be an appropriate title for a cold planet so far away from the sun. Her name was officially selected on March 24 and she was given five pounds as a reward. Part of the reason the title was selected was based on the fact that the initials for the new planet would then share the initials for the man who started the whole project, Percival Lowell.

Sounds Like Hell

People were immediately enchanted with the newest member of our Solar System and quite a few things were named in the planet’s honor, including Disney’s newest character, a certain yellow dog you may be familiar with. A little over a decade later, a newly discovered element was named after the planet as well --you  may know it as plutonium. While most cultures use the name Pluto, a few languages have interesting translations for the dwarf planet. In Chinese, Japanese and Korean, the name is translated into “underworld king star” and in some Indian languages, the planet is named after Yama, the guardian of Hell in Hinduism. Image by Gene Duncan of Walt Disney World.

The Myth Of Planet X

Even though the discovery of Pluto occurred while astronomers were searching for a Planet X that would have thrown off the orbit of Uranus, Pluto is not Planet X. It only happened to be in the right place at the right time to get discovered, but its mass is not large enough to disrupt the orbit of Uranus. Scientists now believe there is actually no Planet X, which makes Pluto’s discovery all the more lucky. In 1992, data from Vogager 2 gave scientists new data on the mass of Neptune, which helped them recalculate its pull on Uranus, which eliminated any remaining suspicions about the existence of Planet X.

Getting to Know You

Eighty years after Pluto’s original discovery, scientists still know very little about the dwarf planet. Because it is so far from the Earth, investigation is difficult, in fact, NASA has compared it to trying to examine details in a soccer ball that sits over 40 miles away. They still have made some fascinating discoveries about Pluto though, including its composition, its rotation period, its orbit and the existence of three moons around the planet. One day on Pluto is equal to a little over six days on Earth. The planet rotates on its side, along its orbital plane, which makes for very extreme seasonal variations. During the solstice, one hemisphere remains entirely in the dark, while the other remains in permanent daylight. Because of these factors, the distance from the sun and Pluto’s chaotic orbit, the dwarf planet is said to be one of the most contrastive objects in the solar system. While it is impossible to directly photograph Pluto’s surface details, scientists have been able to process images using pictures from the Hubble Space Telescope. These images show that Pluto’s color seems to change between blue, black, orange, red, brown and white. Even stranger, the images show that it has increased in redness dramatically between 2000 and 2002. The changes are so dramatic, that the astronomer responsible for assembling the pixel-sized images into an actual picture has originally believed he made a mistake.

Knowing What Its Made Of

Using spectroscopic analysis, researchers have been able to discover that Pluto’s surface is made of 98% nitrogen  ice with methane and carbon monoxide. The planet’s strange axis orientation and small size make it always oriented towards the Charon moon with the same face at any given time. Interestingly, the face that is always oriented toward this moon contains more methane ice, while the other side has more of the carbon monoxide ice. The surface is believed to look something like what you see above. Image via L. Calcada [ESO]

A Chaotic Orbit

The orbit of Pluto cannot be calculated as far into the future as other planets because it has a somewhat chaotic orbit. Scientists can predict its position for the next 10 million years or so, but small changes in the Solar System can throw the orbit off. Despite the chaos though, certain factors ensure that the object will never stray too far or collide into another planet. Interestingly, Pluto’s wide elliptical orbit appears to put it in line to crash into Neptune, as it periodically crosses Neptune’s orbit and comes closer to the sun than the eighth planet. It stays within Neptune’s orbit for about 20 years and this occurrence happens only once every 248 years, or about once every Plutonian year. The last time Pluto entered Neptune’s orbit was in 1979 and it left this orbit in 1999. While it looks like Pluto could hit Neptune when the orbits are viewed from above, the dwarf planet is far from the planet in a 3D plane. In fact, it actually comes far closer to Uranus than Neptune.

Size Matters

As the popular shirt in the Neatorama store reminds us, Pluto provides ultimate proof that size matters. The dwarf’s entire mass is about a fifth of our moon’s and one third of its volume. When put up on a map of the Earth, like the one seen at right, the diameter of Pluto is just barely bigger than the length of the U.S. from north to south (Charon, it’s largest moon, is pictured beside it). From its initial discovery, Pluto has continued to “shrink” as scientific calculations help better estimate its size. Originally, astronomers calculated the size based on its perceived effect on Neptune and Uranus, but once it was proven that Pluto was not Planet X, its size was re-estimated. In 1955, calculations stated that it was around the size of Earth. Then, in 1971, it was estimated to be closer to the size of Mars. In 1976 though, astronomers in the University of Hawaii discovered that the planet contained methane ice, which meant it had to be highly luminous for its size and could not be more than 1% the size of the Earth. In 1978, when Charon was discovered as Pluto’s moon, it allowed scientists to properly estimate the mass of the dwarf planet. Now that we have a good idea of Pluto’s size, we know it’s smaller than seven of the moons of other planets. Image by Calvin J. Hamilton [Solar Views]

To Be, Or Not To Be (A Planet)

Here’s the part you all knew was coming, the controversial discussion about the little object’s role in our Solar System. As stated before, Pluto is relatively small. As a result, letting it stand as a planet would mean we would at least also have to add Eris as a planet, as this object also directly orbits the sun and is larger than Pluto. Pluto’s role as a planet began to come in question in the seventies, once scientists started to figure out just how small it was. It wasn’t long after that lots of objects that were similar to Pluto began to be discovered, including the aforementioned Eris. In 2006, the International Astronomical Union (IAU) released the first official definition of a planet, which met cutting Pluto from the lineup. Their rules for planethood said:


  1. The object must be in orbit around the Sun.

  2. The object must be massive enough to be a sphere by its own gravitational force. More specifically, its own gravity should pull it into a shape of hydrostatic equilibrium.

  3. It must have cleared the neighborhood around its orbit

While Pluto met the first two requirements, it did not clear the neighborhood around its orbit because it was part of the Kuiper belt of objects orbiting the sun. As a devastating consolation prize, Pluto was added to a newly defined group of objects called “dwarf planets.” Contrary to popular opinion, Pluto was not the first object to lose its planetary status. That titled belongs to Ceres, which was originally named the eighth planet in the Solar System in the 1800s, but lost its title when it was decided to be an asteroid instead. When Pluto’s planetary status came into question though, Ceres was once again on the table for being reinstated to its former planetary glory.

A Stellar Controversy

Many scientists still argue that the object should keep its status. One of the factors that has kept this argument so strong is the little amount of support, even amongst astronomers, as to the actual definition of a planet. As a matter of fact, only 5% of over 9000 astronomers in the IAU even voted for the definition, and of that number, not all voted in favor of the resolution. To make matters worse, many astronomers say this definition of a planet (particularly the part about clearing its own orbit) cannot not be applied to solar systems outside of our own, making it essentially useless. NASA leader Alan Stern has argued that the definition cannot even work in our own Solar System, as Earth, Mars, Jupiter and Neptune all share their orbits with asteroids. After the definition, some members of the California state assembly denounced the IAU for “scientific heresy,” while New Mexico and Illinois both passed resolutions declaring that Pluto is still a planet under the states’ skies. After the reclassification of Pluto, the American Dialect Society chose the word “plutoed” as its word of the year, meaning “to have demoted or devalued someone or something.”

Where Does It Come From?

Pluto’s origin was a subject of many theories since its discovery. One early hypothesis said that it was escaped moon of Neptune, but scientists criticized this idea since Pluto never comes within close contact of the planet. In 1992, astronomers found a whole population of icy objects beyond Neptune that seemed similar to Pluto. They named this group of objects, the Kuiper Belt (seen in the above image). Pluto is the largest of these objects, but it is believed that Neptune’s moon Triton was originally a part of the belt as well. Scientists now agree that Neptune likely underwent a sudden migration at one point and this helped it grab a moon and knocked some of the objects in the belt (like Pluto) into chaotic orbits. This theory may also explain Pluto’s unique relationship with its moon, Charon. As stated before, the dwarf planet and its relatively large moon are tidally locked together and always face the same side of one another. Image by WillyD [Wikipedia]

New Horizons For the Dwarf Planet

Up until recently, there were no serious attempts to explore Pluto more in depth. In 1992, NASA started working on a program called the Pluto Kuiper Express that would allow a closer investigation of the object, but the project was canned in 2000. In 2003 though, a new project started up called New Horizons. The craft was launched in 2006 and some of the ashes of the man who discovered Pluto, Clyde Tombaugh, were included on the spacecraft. New Horizons will make its closest approach to the planet in 2015, but only time will tell what well will learn of our little dwarf planet, or if it will even hold the same classification by the time the work begins. Sources: Nasa, Wikipedia, Nine Planets, Solar Views, ZD Net, Wired


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The word 'planet' is derived from a Greek word that means 'wanderer'. It referred to the fact that several of the lights in the sky at night seemed to move around with respect to the others, which otherwise marched across the heavens in an orderly fashion during the passing of the seasons.

The term was used for centuries before the discovery of Pluto, and even before the discovery of the fact that the Earth was round.

Given its original meaning, the classification of Earth as a planet is not correct. Because the Earth cannot be seen as a point of light from its own surface and can therefore not wander.

Why don't we go back to first principals on all this and just start classifying Solar System objects in a similar way to that used for stars, for which we use luminosity, spectra, size, color, etc.. Solar system objects have different properties against which they can be grouped, but why not?
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Thank you for accurately pointing out that the debate over Pluto's planet status is far from settled. Umost objects in the Kuiper Belt, it has attained hydrostatic equilibrium, meaning it has enough self-gravity to have pulled itself into a round shape. When an object is large enough for this to happen, it can become differentiated with core, mantle, and crust, just like Earth and the larger planets, and can develop the same geological processes as the larger planets, processes that inert asteroids and most KBOs do not have. Ceres meets this criterion and should have its planet status restored as well.

Not distinguishing between shapeless asteroids and objects whose composition clearly makes them planets is a disservice and is sloppy science. As of now, there are three other KBOs that meet this criterion and therefore should be classified as planets—Haumea, Makemake, and Eris. Only one KBO has been found to be larger than Pluto, and that is Eris.
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