P is for Pluto

I know, I know, I’m way behind on my A to Z posts. Life hit me hard in April but I’m doing my best. I have very much enjoyed writing these and so, even though I will finish pretty late, I will finish them. Thank you for reading :) 

If you have been following along on my little A to Z Astronomy posts you might have noticed the occasional mention of Pluto and my conflicting feelings on the demotion of Pluto from “planet” to “dwarf planet”. I thought that since this tiny little ball of rock and ice, lonely out there orbiting so far from the sun, held such an important place in my heart, it deserved a post all its own.

The first color image of Pluto and Charon made by New Horizons By NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute [Public domain], via Wikimedia Commons
Pluto officially discovered in 1930 but it’s existence was predicted earlier. It all started when a man named Urbain Le Verrier was studying the orbit of Uranus. He predicted that there was some other planet out there messing with Uranus’ orbit, that planet turned out to be Neptune. Then when astronomers were studying Neptune’s orbit they found that there had to be something else messing with Uranus’ orbit too.

In 1906 a man named Percival Lowell founded the Lowell Observatory and started a project to look for the mysterious “Planet X”. Lowell died in 1916 before the official discover of “Planet X” but before his death, and unknown to him, there had been two images captured of Pluto. Nobody recognized them for what they were though.

Because of a legal battle with Percival’s widow regarding Lowell’s portion of the Observatory, the search for Planet X did not resume until 1929 where the job was handed off to Clyde Tombaugh. Tombaugh would take pictures of portions of the night sky in pairs and at different times. Then he would study the images to see if any of the celestial objects had moves. In 1930, after a year of looking at pairs of photographs Tombaugh found “Planet X” in a pair of images taken January 23rd and 29th of that year. The discovery was telegraphed to the Harvard College Observatory on March 13,1930.

The discovery made headlines across the globe.

Original plates from Clyde Tombaugh’s discovery of Pluto. (Apparent magnitude +15.1) Credit: Lowell Observatory Archives, from.

The name Pluto came from the Greek god of the underworld and was suggested by an eleven-year-old girl named Venetia Burney (1918–2009). She suggested it to her grandfather Falconer Madan, a former librarian, who passed it on to astronomy professor Herbert Hall Turner, who then cabled it to colleagues in the United States.

A vote was held at the Lowell Observatory, who had naming rights, and the name Pluto received every vote. The other options were Minerva and Cronus. I like the name Pluto but I might have voted for Minerva.

In 1930 Walt Disney named Mickey Mouse’s companion Pluto, but it has never been confirmed that the name was inspired by the planet. In 1941, Glenn T. Seaborg named the newly created element plutonium after Pluto, in keeping with the tradition of naming elements after newly discovered planets, following uranium, and neptunium.

Pluto orbits the sun about every 248 Earth years. Unlike the planets of the Solar System which have nearly circular orbits, Pluto’s orbit it highly eccentric and inclined, which means oval in shape and tilted. The eccentricity also brings it closer t the sun than Neptune for a portion of it’s orbit.

Pluto has five known moons, the largest of which is Charon with a diameter just over half that of Pluto.Pluto and Charon are sometimes described as a binary system because the barycenter of their orbits does not lie within either body, or more simply, one does not seem to be entirely orbiting the other, they kinda orbit each other. The IAU doesn’t actually have a definition for binary dwarf planets though. The other moons are Nix, Hydra, Kerberos, and Styx. 

The surface of Pluto

By NASA/ESA/M. Buie (Southwest Research Institute) [Public domain], via Wikimedia Commons
Currently there don’t seem to be many detailed images of Pluto’s surface. The above images were both taken by the Hubble telescope, the first (top) was taken in 1994, the second (bottom) is from 2002/2003.. As you can see the images aren’t sharp at all and we can’t exactly see any craters or mountains, but we can see that the terrain is varied. The white areas are frost and the dark areas are carbon-rich residue left over from the sun breaking up the methane in the surface.

From what we can tell Pluto isn’t floating around cold and dead out there, it’s actually quite a dynamic world. It’s atmosphere rises and falls with it’s seasons that are driven by not only it’s long and eccentric orbit around the sun but also it high axis tilt. Pluto “rolls” along it’s orbit path which means for its seasonal changes are quite extreme. At its solstices, one-fourth of its surface is in continuous daylight, whereas another fourth is in continuous darkness

Due to it distance from Earth studying Pluto has been difficult but that is about to change. On July 14th of this year Pluto will be visited by the New Horizon’s Probe which will take more detailed measurements and images of the dwarf planet and it’s moons before heading off to visit the rest of the Kuiper belt.

Speaking of the Kuiper belt, it was the discovery of this region beyond the planets, and Pluto’s place in it that prompted the demotion of Pluto from planet to “dwarf planet”. That and the 2005 discovery of another “trans-Neptunian object” named Eris, which was found to be approximately the same size as Pluto put the IAU in a pretty awkward position. Should they add Eris to the list of planets? Or create a new classification?

The debate came to a head in 2006 when the IAU created an official definition for the term “planet”:

  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.

It is the the 3rd criteria which sealed Pluto’s fate. Pluto’s orbit has many more objects then the planets of the Solar System, it just doesn’t have the mass to pull those objects in to eject them out with it’s gravity. After the definition for planet was finalized the new “dwarf planet” category was created for Pluto, Eris, and the other objects like them.

While the decision makes sense there has been push back from both the public and other members of the astronomical community. One valid point was that only five percent of astronomers voted to demote Pluto, therefore the vote does not represent the views of the entire astronomical community.

There has also been an emotional reaction from the public, myself included. I have mixed feelings about Pluto not being a planet anymore. On the one hand I grew up in the late 80s and early 90s and when I was in school I was taught that Pluto was a planet. Growing up I always felt bad for lonely little Pluto out there cold and alone. For me it’s demotion was just sad. On the other hand I am a “science person” which means when facts do not support something, I have to let it go. I agree that Pluto is not like the other planets, and I agree that it makes sense to reclassify it.

But deep down I have this spot in my heart for little Pluto, and I imagine I always will.


Image: Shining brightly in this Hubble image is our closest stellar neighbour: Proxima Centauri. Proxima Centauri lies in the constellation of Centaurus (The Centaur), just over four light-years from Earth.

Although it looks bright through the eye of Hubble, as you might expect from the nearest star to the Solar System, Proxima Centauri is not visible to the naked eye. Its average luminosity is very low, and it is quite small compared to other stars, at only about an eighth of the mass of the Sun. However, on occasion, its brightness increases.

Proxima is what is known as a “flare star”, meaning that convection processes within the star’s body make it prone to random and dramatic changes in brightness. The convection processes not only trigger brilliant bursts of starlight but, combined with other factors, mean that Proxima Centauri is in for a very long life. Astronomers predict that this star will remain middle-aged — or a “main sequence” star in astronomical terms — for another four trillion years, some 300 times the age of the current Universe.

These observations were taken using Hubble’s Wide Field and Planetary Camera 2 (WFPC2). Proxima Centauri is actually part of a triple star system — its two companions, Alpha Centauri A and B, lie out of frame. Although by cosmic standards it is a close neighbour, Proxima Centauri remains a point-like object even using Hubble’s eagle-eyed vision, hinting at the vast scale of the Universe around us.

ESA/Hubble [CC BY 3.0], via Wikimedia Commons


8 Replies to “P is for Pluto”

  1. I’m not as devastated by Pluto’s demotion as many others seem to be. It’s not like Pluto went anywhere. It’s still out there. It still exists. It will continue to exist. And now it has a special designation, a place different from the other bodies we study in our solar system. It’s special in its own way.


  2. There’s every chance that Pluto will be re-elevated to planet status. As I understand it, there had been an earlier attempt to develop a different definition that would have kept Pluto a planet. In the end, only 424 members of the IAU, out of about 10,000, actually voted for the definition of ‘planet’ that led to the demotion of Pluto. The vote was passed with a majority of 80. So it could come up again. From the scientific perspective, it’s not a wonderful definition. I think the main problem is that it tries to over-classify what is actually quite a range of diverse objects across the solar system.


    1. Oh that would be great! I agree that the definition isn’t perfect for the same reason you stated, there is quite a bit of diversity among planets and the current definition may prove to be too narrow.


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