Pluto: To Catch an Icy King


  • Author: Patrick Rhodes
  • Date: 27 Jul 2015
  • Copyright: All graphs and artwork created by Patrick Rhodes, images of Pluto appear courtesy of NASA

Sly as a fox, it is. Mysterious and diminutive, it has eluded us for decades. Despite what we've learned about Pluto, constant debate continues to rage over its classification. From the moment it was discovered, astronomers have bickered over this icy body and its place in our solar system. Was it Planet X? Is it a planet at all? Did it really 'have it coming'? We've all longed to know more about this categorization-resistant body which has stirred up so much controversy in news and astronomy circles alike. How did we get so riled up about an icy rock so far distant? To understand that, we must start at the beginning.

thumbnail image: Pluto: To Catch an Icy King

Planet X

Before there was Pluto, there was Planet X.

Allow me to set the scene for you: It is the mid-1800s in Europe and North America. People are migrating to cities en masse, lured by the economics of the Industrial Revolution. As the number of mechanical monstrosities increase, so too does the pace of scientific discovery. Charles Darwin has just published The Origin of Species (original full title and certified mouthful: "On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life") which inflames the science-vs-religion debate. The planet Neptune is discovered. This, coupled with Uranus' prior discovery in the late 1700s raises the possibility that more, undiscovered worlds exist in our solar system.

Neptune was actually seen by Galileo two hundred years before its official 'discovery[1]'.

When the orbit of Uranus is further analyzed, it seems to be 'off'. In fact, it was perturbed enough that astronomers of the time infer that another planet - a massive planet - must be responsible[2]. Given that Neptune was discovered in a similar manner, it seems to be a reasonable deduction. Since this Uranus-influencing-planet had yet to be discovered[3], it is given the moniker of 'Planet X', which, admittedly, sounds very cool.

mass estimates of pluto over time

Figure 1
(click image to enlarge)

One astronomer, having suitable wealth and possessing an insatiable curiosity, is deeply committed to finding Planet X. His name is Percival Lowell and he is nothing if not a romantic visionary. Having first been preoccupied with the mapping of Mars, he then turns his attention to this hypothetical planet. At the observatory which he funded and built in Flagstaff, Arizona in 1894, he scans the night skies in often below-freezing temperatures in the area calculated to be the home of Planet X. Sadly, he never finds anything and dies in 1916, nearly fifteen years before Pluto would finally be discovered.

Percival Lowell was convinced there were canals on Mars, put there by a desperate civilization tapping into the polar ice caps for water. He published three books on Mars detailing his observations and suppositions: Mars (1895), Mars and Its Canals (1906), and Mars as the Abode of Life (1908).

Searching the heavens for a planet at this time is extremely tedious. It is done by hand using a 'blink comparator', a device which flips two photographs back and forth in quick succession, allowing the human eye to more easily spot any differences; such work is generally left to those who are just starting their careers. One such novice, named Clyde Tombaugh, is a Kansas farm boy possessing a meticulous nature coupled with an intense interest in astronomy. He is hired to perform this work and he performs it well. About a year after he is hired, he discovers Pluto on February 18, 1930.

Once discovered, Pluto has a devastating effect on the perception Planet X. Due to its faintness and difficulties resolving its disc, its previous size estimations were far too large. Therefore, astronomers are forced to calculate that its mass is roughly equivalent to that of Earth - a far cry from the original 18-Earth-Mass projection from the mid 1800s. Much later, astronomers further investigate the data and realize that Neptune's orbit was, in fact, fully explained[4] without interference from 'Planet X'. Further calculations continue to downsize Pluto's mass to its present estimation, about 0.218% that of Earth. Perhaps there is a lesson here relating astronomy to the technological hype cycle.

Pluto was named[5] by an eleven-year old girl named Venetia Phair (née Burney).


Magic Carpet Ride

While reading this section, and to immerse you in the heady days of the space age, have a listen to "Magic Carpet Ride" by Steppenwolf. It works wonderfully.

Before 1961, it was deemed 'impossible' to travel to the outer reaches of the solar system. In order to generate the velocity needed to escape the Sun's gravitational pull - not to mention Earth's own gravitational field - a spacecraft would need generate enormous amounts of thrust. Enormous amounts of thrust require enormous amounts of fuel. Enormous amounts of fuel is prohibitively heavy. Prohibitively heavy objects require enormous amounts of thrust. And around and around we go in a vicious circle. Hence, the problem launching such a vehicle into the outer solar system.

NASA has spent billions of dollars attempting to (unsuccessfully) develop a spacecraft that uses nuclear energy[6].

Enter Michael A. Minovitch, a young graduate student originally hired at NASA to work on an unrelated problem involving free-fall trajectory between two bodies influenced by the Sun. After solving that, he goes after more difficult problems such as the "Three-Body Problem" including motion through the solar system. Prior to this, Minovitch had never studied the problem of space propulsion which probably allowed him a fresh approach to space travel, as he was unencumbered by previous assumptions. During this time, he discovers the notion of propulsion generated by the planets themselves, without the use of rockets. It becomes what we now colloquially refer to as 'gravity assist[7]'.

In short, send a spacecraft to a nearby planet and use that planet's energy to catapult the vehicle to a distant planet, such as Pluto. Hence, gravity assist.

new horizons gravity assist to jupiter and pluto

Figure 2
(click image to enlarge)

With the formerly 'impossible' wall of interplanetary travel now breached, NASA set its sights on the outer solar system. By the early 1970s, they use gravity assist to send space probes to Jupiter and beyond, beginning with Pioneer 10. As technology improves, NASA sends more probes, visiting Jupiter, Saturn, Uranus and Neptune (and using each of those planets for gravity assist maneuvers). Eventually, it is decided to send a probe to far-away Pluto; an ambitious project. On January 19, 2006, New Horizons is launched, arriving at the Pluto system in mid-July, 2015 and bringing us to the present day. Indeed, it has been a magic carpet ride.


The Ice King

Pluto lies within a band of objects called the Kuiper belt, which is itself about 30-50AU (Astronomical Units) from the Sun. Statistically speaking, objects as large as Pluto are extremely rare in that region of space. In fact, they are so rare, it helps to employ a bit of statistics to more clearly see the picture. Astronomer David Jewitt showed that objects in the Kuiper belt (called KBOs) are distributed by size according to the Power Law[8]. Simply put, as KBOs grow in size, there are exponentially fewer of them. For example, objects that are 1/10 the size of Pluto outnumber objects that are equal to the size of Pluto by a factor of 1000 (if there is one object the size of Pluto, there will be 1000 objects that are 1/10 the size of it and 1,000,000 objects that are 1/100 the size of it, etc.).

Pluto the Kuiper belt and the power law

Figure 3
(click image to enlarge)

Out of all those KBOs, we have just visited the largest one we know about, the King. However, Pluto wasn't thought to be the biggest kid on the block, if you include objects beyond the Kuiper belt called 'trans-Neptunian objects' (TNOs). Out there, Eris lurks; and it was Eris that, given our estimates, was the actual King.

Like Pluto's label, things change.

As of July 13, 2015, the New Horizons flyby upwardly adjusted Pluto's diameter by 70 km, making it the largest TNO (or KBO, or Plutoid, or dwarf planet) that we know about. It's diameter is now 2,370 km, compared with Eris which is estimated at 2,326 km. That makes Pluto #1, Eris #2. Of course, should we ever visit Eris, then we might see that Eris is in fact larger. But for now, Pluto takes the title. It's good to be the King.

The total mass of the Kuiper belt is estimated to be about 10% that of Earth[9].


To Be or Not To Be... a Dwarf Planet?

What is Pluto, exactly? Commonly, it's known as a 'dwarf planet', which is what I've called it for some time now. However, in a recent email exchange with Jewitt (who co-discovered the Kuiper belt with Jane Luu circa 1992), he had this to say about that term:

"Even the most devoted dwarf planet aficionados agree that Pluto is a large Kuiper belt object. The latter is the most meaningful label for Pluto."

- Dr. David Jewitt

There it is, then. Pluto's best categorization is a large Kuiper belt object. I've done my part in advancing the term. Truth be told, however, astronomers only have themselves to blame for the confusion surrounding Pluto's label[10]. "Planet" is a far more powerful term than "Kuiper belt object" and astronomers are just like the rest of us when it comes to capitalizing on popular vernacular.

venn diagram pluto kuiper belt ceres makemake eris haumea dwarf planet

Figure 4
(click image to enlarge)


New Horizons

We've finally caught the 'King' and the New Horizons mission has completed its flyby of the Pluto system. For the first time, we have detailed images of the planet.

While zooming through the Pluto system, New Horizons has mostly been in an information-gathering mode. After all, it was only there for a short time, given the probe's velocity with no way to slow down. Therefore, it's focus has been on performing the science at hand, taking in about 100 times more data than it can send. By the time July comes to a close, the probe will have left Pluto far behind and will continue into the outer reaches of the solar system and eventually, interstellar space. The majority of what we will learn will come in the following years as the probe transmits enormous amounts of data back to Earth[11].

What has New Horizons learned since the flyby? Here are a few:

  1. Pluto has mountains - big ones (3500 meters in height).
  2. Charon has a youthful terrain and dark area named 'Mordor'.
  3. No additional moons were discovered orbting Pluto (surprising).
  4. Pluto has a polar ice cap.
  5. Pluto is larger than previous estimates by about 70 km.

This is just the tip of the iceberg, in terms of what we will learn in the years to come. It's an amazing achievement to study such a distant object up-close, given the raw state of space travel a mere fifty years ago. In spite of the problems which plague human society, sometimes we come together and accomplish truly amazing feats when we put our collective mind to it. Stay tuned to the New Horizons project to keep in touch, it's going to be fun.



Recent images of Pluto (courtesy of NASA and the New Horizons mission):

(click image for details)

"Dynamic Duo" "Pluto's Big Heart" "Ice Floes"
July 9, 2015 July 13, 2015 July 24, 2015



  1. Jewitt, David and Luu, Jane. "Pluto, perceptions & planetary politics." Daedalus 136:1 (2007). Print.
  2. Tyson, Neil de Grasse. The Pluto Files. New York: W. W. Norton & Company, 2009. Print.
  3. Jewitt, David and Luu, Jane. "Pluto, perceptions & planetary politics." Daedalus 136:1 (2007). Print.
  4. Standish, E. M. "Planet X: No Dynamical Evidence in the Optical Observations." The Astronomical Journal 105:5 (1993). Print; available from
  5. "NASA - Transcript: The Girl Who Named Pluto." NASA. 17 January 2006. Last accessed June 14, 2015; available from
  6. "NASA Eyeing Nuclear Fusion Rockets for Future Space Exploration." Last accessed July 1, 2015; available from
  7. Minovitch, Michael. Gravity Assist. Last accessed July 1, 2015; available from
  8. Jewitt, David. "Kuiper Belt Objects." Annual Review of Earth and Planetary Sciences 27 (1999): 287-312. Print; available from
  9. Ibid.
  10. Jewitt, David and Luu, Jane. "Pluto, perceptions & planetary politics." Daedalus 136:1 (2007). Print.
  11. Stern, Alan. "What Will New Horizons See - and When?" Sky and Telescope. 2 June 2015. Last accessed July 10, 2015; available from

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