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Pluto: Not An Ordinary Dwarf Planet

by Dani Johnson

July 25, 2013

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Above Image: A Hubble Space Telescope image of Pluto and its moons. Charon is the largest moon close to Pluto. The other four bright dots are smaller moons discovered in 2005, 2011 and 2012.

Image Credit: NASA

Since its discovery in 1930, Pluto has been thought of as the ninth and smallest planet in the Solar System. Since its reclassification to a dwarf planet in 2006, the public still hasn't stopped debating on whether or not the "demotion" was the right choice. Still, it wouldn't have been discovered when it was if it weren't for an American astronomer hell-bent on discovering "Planet X". Now, Pluto is the second largest known dwarf planet in the Kuiper Belt, and there's even a mission on its way to encounter the icy world.

Name & Discovery:

American astronomer Percival Lowell predicted the existence of "Planet X" at the beginning of the 20th century based on strange movements he observed in the orbits of Uranus and Neptune. He used the Lowell Observatory that he built in Flagstaff, Arizona to take pictures of the same spot in the sky at different times to compare to each other and hopefully find movement that indicated another planet. Unfortunately, Lowell died before he discovered the planet, but he left behind a million-dollar legacy to fund the continuation of the search.

With no formal education, amateur astronomer Clyde Tombough sent his planetary sketches to the Lowell Observatory in hopes of landing a job. The observatory was so impressed by his work that they hired him to help look for "Planet X", which was a painstaking job to say the least. Like Lowell, he would take multiple pictures of the same part of the sky at different times and place the photographic plates in a device that would rapidly switch between images. He spent 10 months bent over this device, switching plates back and forth to compare the vast amount of stars in a single swatch of sky to one another in hopes of finding minute movement. He finally found what he was looking for in February 1930.

After the discovery, the Lowell Observatory invited the public to submit name suggestions for the new planet. An 11 year old girl submitted the name Pluto and it was chosen because the cold, distant planet resembles the home of the Roman god of the Underworld.

We have since made observations that have shown us that Lowell's measurements were incorrect and that Pluto isn't actually massive enough to cause any disturbances in the orbits of the neighboring gas giants. Still, his enthusiasm provided fuel for the public imagination and the construction of the Lowell Observatory lead to many significant findings, including his "Planet X".

From the discovery of Pluto until it's reclassification in 2006 it was called a planet even though it was much smaller and its orbit much different than any other planet in our Solar System. After discovering multiple bodies as far out as and similar to Pluto, and even one that is slightly bigger than Pluto (Eris), the IAU felt that it was time to reclassify what exactly makes a planet. Pluto is now defined as a dwarf planet.

Quick Facts:
  • Pluto orbits the Sun well past the eighth planet within a zone called the Kuiper Belt.

  • Pluto is the second largest dwarf planet.

  • Pluto is thought to be about the same age as the Solar System, which is thought to be about 4.5 billion years old.

  • Pluto is, on average, almost 6 billion kilometers from the Sun. That's almost 50 times farther from the Sun than the Earth.

  • Pluto's sidereal orbital period, or year, is about 248 Earth years.

  • Pluto's sidereal rotation period, or day, is a little more than 6 Earth days. Much like Venus and Uranus, this little dwarf planet rotates backwards, or retrograde, compared to all the other planets and moons in the Solar System.

  • Pluto and Charon, like Uranus, are tipped on their sides and seem to roll around the Sun instead of spin.

  • Pluto has a diameter of about 2,360 kilometers or about two-thirds as wide as Earth's Moon.

  • The minimum/maximum temperature on Pluto is about -233/-223 °C (-387/-369 °F).


So far, four missions have traversed interplanetary space and made it to the farthest reaches of our Solar System. Unfortunately, none of them have had the opportunity to study Pluto, so NASA's New Frontiers program sent New Horizons to study the Kuiper Belt.

New Horizons will be the very first space craft sent specifically to observe this part of our Solar System. In 2015, the spacecraft will begin its 150-day flyby to study Pluto and its moons and then the craft will venture on to study other Kuiper Belt objects.

Above Image: The best-ever Hubble images of Pluto.

Image Credit: NASA/ESA/SWRI

Physical Characteristics:

Pluto is so small and so far away that it is rather difficult to gather any good data. The best images from Hubble can still only show us vague, pixelated shapes on a tiny reddish dot in space. From what we can tell, Pluto is about 2/3rds the diameter of Earth's Moon yet only about 1/6th the Moon's mass. The dwarf planet probably has a rocky core surrounded by water ice, but we won't have good data on these kinds of details until 2015 when New Horizons is due to fly by.


When Pluto's wacky orbit brings it inside of the orbit of Neptune, the temperatures rise just enough for a frozen layer of ice to thaw and temporarily form a thin atmosphere around the dwarf planet. The atmosphere seems to be made of mostly nitrogen with traces of methane. Since Pluto's gravity is so much lower than Earth's, its atmosphere actually extends much higher in altitude than Earth's. The formation of this temporary atmosphere makes Pluto very similar to a comet, since similar processes are what cause the tail we see trailing behind comets.

Orbit and Rotation:

Pluto's eccentric orbit takes it up to 49 AU away from the Sun, which is almost 50 times farther from the Sun than the Earth! The eccentric orbit also causes Pluto to get closer to the sun than Neptune for part of its 248 Earth-year orbit.


Above Image: Hades and Persephone

Image Credit: Sandara on DeviantArt

In Greek and Roman mythology, Pluto is the ruler of the Underworld. He captured Persephone, the daughter of Demeter and Zeus, and demanded that she be his wife. Demeter, the goddess of agriculture, searched for her daughter and wouldn't allow anything to grow until she found her. She demanded that Zeus force his brother to give Persephone back, but she had already eaten the seeds of a pomegranate so he compromised that she stay half of the year with Pluto and the other half with Demeter. Demeter is so distraught when her daughter stays with Pluto that she refuses to let anything grow while she's away, causing the seasons.


The five known satellites of Pluto are Charon, Nix, Hydra, Kerberos and Styx. It is thought that a very young Pluto was struck by a similar sized body and the resulting debris turned into its moons.
"The moons form a series of neatly nested orbits, a bit like Russian dolls," said team lead Mark Showalter of the SETI Institute in Mountain View, California.
Charon [kair-uhn] was discovered in 1978 by Naval Observatory astronomer James W. Christy while making routine measurements of photographic plates of Pluto. Charon is almost half the size of Pluto, which is so big that they are sometimes referred to as a dual dwarf planet system. In Greek mythology, Charon is the ferryman on the river Styx. Hermes brings souls to the river and Charon ferries them across into the Underworld.

Nix and Hydra were both discovered in 2005 by a team at Hubble Space Telescope. In Greek mythology, Nix is the goddess of the night, and is one of the first-born elemental gods called Protogenoi. Hydra is the nine-headed serpent that Heracles (or Hercules in Roman mythology) battled as the second of his twelve labors. The middle head of the monster is the only one that is immortal, but every time Heracles cuts off one of the other heads two more grow in its place. With the help of his servant Iolaus, he burned off 8 of the heads of the Hydra, then buried the immortal one under a gigantic rock.

Kerberos [ kur-ber-uhs] was discovered in 2011 by a team at the Hubble Space Telescope. In Greek Mythology Kerberos is the gigantic three-headed hound that guards the gates of the Underworld so that no soul may ever leave. He has a serpent's tail, a mane of snakes and lion's claws.

Styx [stiks] was discovered in 2011 by a team of scientists that was searching for potential hazards for NASA's New Horizons mission that is due to encounter the dwarf planet in 2015. In Greek mythology, Styx is the river that separates the mortal world from the Underworld. Hermes brings lost souls to the river where Charon ferries them across if they provide a toll. If they do not have the toll, which is a coin placed on the mouth of the deceased, they are forced to wander the shores of the river for 100 years.

NASA's Significant Dates
  • 1930: Clyde Tombaugh discovers Pluto.

  • 1977-1999: Pluto's lopsided orbit brings it slightly closer to the sun than Neptune. It will be at least 230 years before Pluto moves inward of Neptune's orbit for 20 years.

  • 1978: American astronomers James Christy and Robert Harrington discover Pluto's unusually large moon, Charon.

  • 1988: Astronomers discover that Pluto has an atmosphere.

  • 2005: Scientists using the Hubble Space Telescope announce the discovery of two additional moons of Pluto. Named Nix and Hydra, the little moons may have formed at the same time as Charon did, perhaps all three splitting off from Pluto in a giant impact event.

  • 2006: NASA's New Horizons mission launches on a path to explore Pluto and the Kuiper Belt region. The spacecraft is scheduled to reach Pluto in 2015.

  • 2006: The International Astronomical Union classifies Pluto as a dwarf planet and recognizes similar worlds beyond the orbit of Neptune as plutoids.

  • 2011: Astronomers using the Hubble Space Telescope discovered a fourth moon, named Kerberos, orbiting the icy dwarf planet.

  • 2012: Astronomers using the Hubble Space Telescope to search for potential hazards to the New Horizon mission discovered a fifth moon, named Styx, orbiting the icy dwarf planet.


by Dani Johnson

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