Why Isn’t Pluto a Planet?
Pluto was named by Venetia Burney, an 11 year old, after the planet was discovered in 1930. Pluto held the status planet for 76 years until the International Astronomical Union decided that Pluto is not a planet.
So If Not a Planet, Then What is Pluto?
It is a dwarf planet; it orbits the sun but is smaller than a planet. Pluto isn’t the only dwarf planet out there. There are four other Dwarf planets recognized by the IAU: Ceres, Haumea, Makemake, and Eris. Eris is larger than Pluto which made some think it was the tenth planet.
Why Isn’t Pluto a Planet?
After Clyde Tombaugh discovered Pluto, textbooks were updated to include the newly discovered ninth planet. In the decades following this, astronomers started doubting Pluto was actually a planet. They wondered if Pluto was the first of a population small, icy bodies in the region beyond the orbit of Neptune. This region was later known as the Kuiper belt believed to contain comets, asteroids and other small bodies made mostly out of ice. (However, only in 1992 did David C. Jewitt and Jane X. Luu discover the candidate Kuiper belt object (15760) 1992 QB1; the first object after Pluto and Charon (one of Pluto’s moons) to be discovered in the Kuiper belt.) Pluto’s orbit is unusual for a planet in the solar system and is more like the orbits of the Kuiper Belt objects.
When Pluto was discovered, it was thought to be bigger than Mercury. Afterward, astronomers calculated Pluto’s mass and found that it is smaller than Earth’s moon, yet ten times larger than the largest asteroid, Ceres. Also, Eris, being (mistakenly) thought to be larger than Pluto at that time, also deserved the status of Pluto. These discoveries were the reason the International Astronomical Union needed to set a definition of planet.
In 2006, the IAU held a discussion in Prague, the Czech Republic to do exactly that. They stated a planet is a celestial body which:
- is in orbit around the Sun,
- has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
- has “cleared the neighborhood” around its orbit.
Pluto met the first two criteria but failed to meet the last. Clear the neighborhood means either vacuum up or eject all large objects in its vicinity; Pluto shares its neighborhood with Kuiper Belt Objects, therefore was relegated to the status of dwarf planet.
Only 10% of the 2,700 scientists who attended the 10-day meeting were there when the Pluto vote took place. Many people objected to this decision, Prof Alan Stern, chief scientist for the New Horizons mission (more on New Horizons later), called the outcome “an awful decision” and described the new definition as “internally inconsistent”.
That’s It? Is It Really Over?
Nope, it’s not over yet. NASA sent a probe named New Horizons to Pluto in Jan. 2006. After a decade-long trip, in the summer of 2015 finally reached it’s destination and photographed Pluto. By studying those photographs, scientists have noticed what they think are clouds. If it has clouds, it’s got to be a planet, right? Prof Alan Stern, same Prof that called IAU’s decision awful, told the New Scientist that the possibility of clouds and a weather system on Pluto certainly makes Pluto’s case stronger. Though the IAU still may not reverse Pluto’s status.
Other Cool Things About Pluto
Why did an eleven-year-old get to name Pluto?
Basically, she had some connections. Venetia Burney was the granddaughter of the librarian of Bodleian Library of the University of Oxford, Falconer Madan. He read about the discovery of Pluto in The Times and mentioned it to his granddaughter. SHe suggested the name Pluto –the Roman God of the Underworld who could make himself invisible. Falconer forwarded the suggestion to astronomer Herbert Hall Turner, who told his American colleagues at Lowell Observatory. Clyde Tombaugh, Pluto’s discoverer liked the name because it started with the initials of Percival Lowell who had predicted the existence of Planet X, which was thought to be Pluto.
Other cool things about Venetia Burney: The asteroid 6235 Burney and Burney Crater on Pluto were named in her honor.
Pluto has five moons: Charon, Styx, Nix, Kerberos, and Hydra. Charon is the largest, and it is massive enough that it is sometimes considered a double dwarf planet.
Russia is bigger than Pluto.
Pluto’s surface area is about 6,813,163 square miles, and Russia is 6,592,771 square miles.