Artists concept of the young Solar System. Credit: NASA

The Pluto Paradox
Why We Struggle With Celestial Definitions

By: Thomas M. Ciesla, July, 2008

Knowledge, or more precisely the increase of knowledge, is in a sense, iconoclastic. Increased knowledge requires us to regularly discard certain 'truths' for new ones. Such is the nature of science as a self-correcting endeavor, perhaps no better represented than in the field of astronomy.


For thousands of years, mankind's perspective on the Universe was radically different from our current view. Initially we thought the world was flat, and then it wasn't (except for the few somewhat eccentric holdouts today). The ancient Greeks recognized seven 'planetes' – Greek for 'wanderers': Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn. Earth is not listed as a planet because the early Greeks considered it the center of the Universe. Then along came Copernicus, relegating the Earth to the lesser role of a planet; followed by Galileo's discovery of Jupiter's moons, which demoted our moon from a planet to well, Earth's Moon.

In 1801, an Italian astronomer accidentally discovered Ceres, which was immediately regarded as the missing planet between Mars and Jupiter based on predictions of the Titus-Bodes Law. Over the next fifty years, more than a dozen other celestial objects were found floating along with Ceres. Scientists began to understand that Ceres was not a planet, but the largest of millions of rocky objects orbiting the Sun in what became known as the Asteroid Belt. Ceres was large enough for gravity to make it assume a spherical shape, while most of its neighbors remained odd-shaped rocky bodies.

Fast forward to 1930, when Clyde Tombaugh, working with a device known as the 'blink comparator', discovers another 'wanderer'. Initially called a celestial object, 'Pluto' went on to become the ninth planet despite our incomplete knowledge of this point of light at the (presumed) edge of the Solar System. Pluto quickly captured the public's imagination and in the decades to come, millions and millions of American children, eagerly learned about the nine planets. Later in that century, another Pluto, Walt Disney's loveable cartoon character mesmerized children in the movies and then on television. Pluto became a duality, mixing entertainment and science under one name.

Now let's fast forward to the end of the twentieth century. Except for Pluto, robotic spacecraft have explored all the major planets and their satellites. By this time our knowledge of Pluto and the Solar System in general had changed dramatically. Pluto, at 2,300 kilometers in diameter, proved to be much smaller than expected , and its mass was six times less than Earth's Moon. Its orbit was highly tilted from the plane of the ecliptic (See Note 1), and it's axis inclined at 122°. In short, Pluto didn't behave like the other planets, and at one point in its orbit was closer to the Sun than Neptune – that is, it actually crossed Neptune's orbit. The only objects known to cross planet orbits were asteroids and comets. Scientific eyebrows were slowly being raised when discussions turned to Pluto planetary status.

In 1978 we discovered that Pluto had a moon – Charon -- that was approximately fifty-three percent of Pluto's size. At that ratio, the barycenter of the Pluto/Charon system did not lie within the sphere of Pluto, but in the space between the two bodies (See Note 2). There was conjecture that the Pluto/Charon was in fact a 'binary planet'. In 1994, the Hubble Space Telescope allowed scientists to directly see details on Pluto's surface. According to Dr. Marc Buie of Lowell Observatory in Flagstaff, Arizona, and part of the Pluto imaging team, “Hubble has brought Pluto from a fuzzy, distant dot of light, to a world which we can begin to map and watch for surface changes. [6 ] Soon afterward, new Hubble images showed two additional moons around Pluto, later named Nix and Hydra.

Figure 1:
The surface of Pluto as imaged by the Hubble Space Telescope (HST). Scientists can now monitor Pluto for changes in its surface coloring. Photo credit: Eliot Young, Richard Binzel & Keenan Crane

Figure 2:
Pluto with Charon (two lower dots), along with the moons Nix and Hydra. Photo credit: NASA/ESA and HST Pluto Companion Research Team.

Table 1: Types of Trans-Neptunian Objects

A family of small solar bodies found between Jupiter and Neptune. Their orbits suggest these objects recently moved from the Kuiper Belt. Most of these die by being ejected from the Solar System or by striking a planet or the Sun.

Objects with roughly spherical orbits close the plane of the ecliptic. (i.e. Chaos, Deucalion, Quaoar, and Varuna)

A subset of Transition Objects, the inactive nuclei of comets recently emplaced in the planetary region of the Solar System, probably from the Oort Cloud.

Main Belt Comets
Objects with orbits in the region usually occupied by asteroids but with physical characteristics of comets (tail and/or coma).

TNO's that exhibit prominent spectral bands due to the presence of methane (i.e. Pluto, Eris, 2005 FY9).

Oort Objects
Objects whose orbits lie beyond the outer edge of the Kuiper Belt (i.e. Sedna).

Objects with more elliptical orbits, traveling in a 2:3 resonance with Neptune, meaning they orbit the Sun twice for every three times Neptune does. (i.e. Pluto, Huya, Ixion, Orcus, Rhadamathus)

Scattered Disk Objects (SDO)
Objects with aphelion distances near Neptune and semi-major axes greater than 50 AU.

Transition Objects (TO)
Comets that lack activity (coma/tail) due to depletion of near-surface volatiles.

Objects in a 1:2 resonance with Neptune, traveling around the Sun once for every two Neptune orbits

The Kuiper Belt Objects

As scientists learned more about Pluto, they were also learning more about the area beyond Pluto known as the Kuiper Belt, named after astronomer Gerard P. Kuiper . It wasn't until 1992, however, before scientists discovered the first Kuiper Belt object , also called Trans-Neptunian objects (TNO's) and the preferred term in this article. Since then, over 1,000 such objects are known. In 2005, a discovery by Prof. Mike Brown and his colleagues at the California Institute of Technology would be the beginning of the end for Pluto's status as a planet. Brown had discovered a TNO that was at least as large as Pluto,[3] and shortly afterward was confirmed to be twenty seven percent more massive than Pluto and 700 km (435 miles) larger in diameter. [4]


Table 2: Manjor TNO Objects
Object Name Diameter(km) Known Moons Type of TNO
MakeMake 1500 -- Cubewano
2003 EL61 1380 2 Other TNO
Eris 3000 1 SDO
Ixion 800 -- Plutino
Orcus 1601 1 Plutino
Pluto 2253 3 Plutino
Charon 1212 -- Satellite of Pluto
Quaoar 1250 1 Cubewano
Sedna 1700 -- Oort Object
Varuna 600-900 -- Plutino


The discovery of UB313, later named Eris(See Note 3), prompted scientists to approach the International Astronomical Union (IAU) to define the term 'planet'. If Eris was simply an 'object' in the Solar System, and larger than Pluto, then logically Pluto should also be classified as such. In 2006, the IAU issued 'Resolution 5: Definition Of A Planet In The Solar System'. In order to be a 'planet', a celestial body: (A) is in orbit around the Sun, (B) has sufficient mass to assume a round shape, and (C) have sufficient mass to assume a near sperical shape.

Because Pluto had not cleared other objects from its orbit in the Kuiper Belt, it was reclassified as a 'dwarf planet', as defined in the IAU resolution. To add insult to injury, the IAU Minor Planet Center quickly assigned Pluto a number just as they do for a run-of-the-mill asteroid. Pluto, once the ninth planet, is now object # 134430. [7] 'Dwarf planet' is a somewhat paradoxical term, though considering the outcry from the general public over the Pluto controversy, that term may have been used to recognize that 'planet' is not only a scientific term, but a social construct, much like the term 'continent', for which there is no official definition.

Why not simply call dwarf planets TNO's? It turns out that our Solar System is not the neat and tidy place we all grew up thinking. Besides the eight planets, Asteroid Belt and Kuiper belt (See Note 4), there are all sorts of rocks, dust and comets whizzing chaotically through the system. The Asteroid belt, the Kuiper belt, and the Oort Cloud (See Note 5) are considered reservoirs for the flotsam-and- jetsam leftover from the formation of the Solar system; each containing thousands if not millions of objects. Ceres' location in the Asteroid Belt prevents it from being called a TNO, and is now classified as a dwarf planet.

Figure 3:
The 'Old' Solar System with the Jovian planets in blue and Pluto in red. Image courtesy of Michael Brown, Caltech

Figure 4:
The 'New' Solar Systtem with the Jovian planets in blue, aPluto in red, and TNO's in green. Image courtesy of Michael Brown

In 2003, Mike Brown and collaborators discovered the farthest planet-like object – Sedna, which at its farthest point is 86 AU from the Sun, compared to 39 AU for Pluto. According to Brown, “The Sun appears so small from this distance that you could completely block it out with the head of a pin.” [5] Sedna, the second reddish object in the Solar System after the planet Mars, is described by its discoverers as an Oort Cloud object, because it never enters the Kuiper Belt, existing in a region between the Kuiper Belt and the Oort Cloud. [2] Therefore, Sedna must be labeled an Oort object rather than a TNO. See the sidebar for an explanation of the various types of TNO classifications.

For some astronomers, it seems a bit provincial to base a definition of a planet solely on what we can observe in our own Solar System. Over the last decade, hundreds of exoplanets (See Note 6) have been discovered, and as technology and techniques improve, the range of exotic planets is likely to increase. The majority of 'planets' found around other stars today are gas giants (usually larger than Jupiter), and brown dwarfs. This is due to technological limitations, but most astronomers are confident that we will be able to discern earth-sized planets within the next decade. Besides exoplanets, scientists have discovered the existence of objects known as 'free-floaters', objects not in orbit around a star, but drifting alone through the galaxy. Are these planets that were ejected from their orbit around a star? Are they brown dwarfs or the more exotic gray dwarfs? Or did these objects form outside of a planetary system, coalescing from an isolated disk? The simple answer is: we don't know.

Because new discoveries in astronomy and cosmology are constantly changing our perspective of the Universe, we are enslaved by the expression, “based on our current level of knowledge.” The term 'planet' has been around for about 400 years and its definition has changed many times. While an ideal definition of 'planet' would not change as time goes on, this may be an unattainable goal. Long before the IAU's Resolution 5, an objects location in our Solar System has much to do with what it is called. For example, if Mercury (D= 4,878 km) were placed in orbit around Saturn it would be a moon of that planet and smaller than Saturn's largest moon Titan (D=5,150 km). In the Jupiter moon system, Callisto (D=4,800) is roughly the size of Mercury, while Ganymede (D=5,276 km) is larger. It's philosophically intriguing that these celestial bodies are called moons, while being larger than the 'dwarf planets'. Location, location, location!

We struggle to permanently name celestial objects because despite advances in our knowledge of the universe over the past few centuries, we possess a nascent understanding of the cosmos. Of course, that is the nature of exploration: trying to understand the larger physical reality around us. Every discovery leads to question the scientific and cultural 'truths' we come to accept as definitive. The scientific community is usually more inclined to accept change such as planetary 'status' than the general public. The cultural – or societal — construct represented by a term is often more powerful than many realize. As professor Michael Brown suggested while discussing his discovery of Eris, “There are sometimes professional arguments over whether the discovery merits such attention, based on the definition of “planet”. We care in part because society cares.” And surprisingly, society does care.

Case in point, those millions and millions of children in the second half of the 20th century who memorized (at least we hope they did) the nine planets in our Solar System. Those children went on to become one of the most powerful cultural forces in American society – the baby boomers. If they never learned anything else about astronomy after elementary school, they probably could still recite the planets as adults. Does anyone really think that an edict from the IAU (that most people have never heard of) is going to convince almost 80 million people that Pluto is no longer a planet; a planet they have grown up with? Unlikely. In general, people have no problem accepting terms such as 'black-holes', 'brown-dwarfs', or 'exoplanets' because these are new, unfamiliar terms. Most of us have no 'tradition' associated with such terms, unlike the term 'planet.'

Artist concept of Sedna. Credit: NASA
Art Souza, West Hawaii's District Superintendent of Education has an interesting perspective on the Pluto-as-planet-controversy, “I wonder if Pluto becomes just a note in a textbook 'correcting an error' or if it can be used as the basis of deep and reflective conversations in classrooms across America? Pluto gives us an opportunity. Can we make this a larger moment, a teachable moment that allows kids to think about the consequences of this discussion in the framework of larger implications for research and scientific knowledge?” Getting children to think about science and the scientific method as opposed to simple memorization for test taking – what a radical perspective!

So a final time, “what is a planet?” 'Planet' is just a word and words mean whatever we say they mean at a given moment in time, morphing as our collective reality changes for better or worse. I turn to my 1911-edition of The Century Dictionary for these definitions:

planet: 1. a star other than the fixed stars, a star revolving in orbit.
star: 1. any celestial body which appears as a luminous point.

My 1910 edition of Webster's Imperial Dictionary offers these definitions:

planet:   a celestial body which revolves about the Sun or other center, or a body revolving about another planet as its center.
star:   1. popularly, any of the heavenly bodies, excepting the Sun and the Moon; more specifically a celestial body which from a distance appears to be a luminous point.

My 1982 edition of The Random House College Dictionary is a bit more specific:

planet: any of the nine large heavenly objects revolving about the sun and shining by reflected light.
star: n 1. any of the heavenly bodies, except the the moon, appearing as luminous points in the sky at night.   2. Astron. any of the large self-luminous, heavenly bodies, as the sun.

Today these definitions may seem a bit childish. The Century Dictionary seems at first to be saying that a planet is a star (the definition then goes on to describe in more detail the major planets orbiting around the Sun); while the Webster's Imperial Dictionary seems to include moons in its definition. I thought it would be interesting to also include the definition of a star, since surely, everyone knows what a star is, right? Note that each definition uses the term 'appears as a luminous point.' To the unaided eye, many of the planets and their moons appear as 'luminous points', as did Pluto until the Hubble Space Telescope pictures. Even more interesting, each definition excludes the Sun as being a 'star'; it is much more than a 'luminous point' from our perspective. For the word 'planet', The Random House Dictionary makes the important distinction, 'shining by reflected ight,' and includes 'self luminous' in the definition of a star.

Artist concept of Quaoar. Credit: NASA
Science defines words and releases them into the population to be understood, discussed and absorbed. Some of these words strike a chord and take on a life of their own, a meaning that is often resistant to change. The words that are raised to this level of societal constructs typically take on dual meaning to accommodate the old and the new definitions. For many of us today, Pluto is both a planet and a 'dwarf planet'. This is not as odd as it may sounds, in astronomy there are examples of objects with dual classifications, such as Chiron. Chiron was originally classified as an asteroid. When comet-like activity was later observed, it was classified as a comet, without being removed from the asteroid catalogue.

Eventually, just as the definitions in my old dictionary tell us, words take on new meanings that become accepted by the masses in relatively short spans of time. Future scientific definitions are likely to be victim of ever-shortening lifetimes as the pace of technology and discovery continues to accelerate. Change is good, even when it means a planet is demoted. As professor Brown observes, “We should feel no embarrassment about learning more and modifying what we previously meant when we said 'planet'' [1]

In 2015, NASA's New Horizons probe will arrive at the Pluto system, the first spacecraft to reach this remote part of the Solar System and the first to ever visit a binary object. Does Pluto's demotion to a dwarf planet lessen the importance of the mission? Hardly. Pluto is still a fascinating celestial body. Its exploration will tell us much of what was happening in the early formation of the Solar System.

The piano-sized probe will then speed towards the Kuiper Belt to advance our understanding of TNO's as well as the structure of this debris belt. The data from this mission will greatly advance our understanding (and our definitions) of what it's like at the edge of the Solar System. As we learn to deal with our constantly changing perspective of things around us, one can only hope that Walt Disney's Pluto will forever remain that cute cartoon character, entertaining future generations of children.



(1) The ecliptic is an imaginary plane extending out throught the Solar System from the Sun's equator. Most solar bodies orbit the Sun at, or within a few degrees of the ecliptic. The inclination of Pluto's orbit to the ecliptic is seventeen degrees.

(2)The barycenter of the Earth/Moon system lies roughly 1,707 km below the Earth's surface. Both the Moon and the Earth itself rotate around that point.

(3) Eris' orbit is 45 degrees inclined to the ecliptic and the Kuiper Belt, which is why it's listed in the Scattered Disk object (SDO) part of the Trans-Neptunian Object table. On its closest approach to the Sun, Eris is 38 AU away, while when at its farthest from the Sun it is 97 AU away.

(4) The Kuiper Belt is a collection of icy bodies orbiting the Sun from between 30 AU - to - 50 AU. Far larger than the Asteroid Belt, the Kuiper Belt was once considered to be a prime source of short-term comets. Astronomers now believe that short-term comets are actually Scattered Disk Objects.

[5] The Oort Cloud is actually a theory, there has been no direct observation of it to date. Some astronomers believe, however, that Sedna is an inner Oort Cloud object.

[6] Exoplanets (also referred to as extra-solar planets) are planets found orbiting around stars other than the Sun. Over 270 exoplanets have been discovered to date. At this time, the closest one to our Solar System is Epsilon Erandi b, at 10.5 light years away.



1.Basri, Gibor and Brown, Michael. "Planetisimals To Brown Dwarfs: What Is A Planet?", Ann. Rev. Earth Planetary Science. 34 (2006) 193-216

2.Brown, Michael, E., Trijillo, Chadwick, and Rabinowitz, David. "Discovery Of A Candidate Inner Oort cloud planetoid" ApJ Letter, 10 August 2004

3.Brown, Michael, E., Trijillo, Chadwick, and Rabinowitz, David. "Discovery of a Planetary-Sized Object in the Scattered Kuiper Belt" The Astrophysical Journal, 635:L97-L100, 2005 Dec 10.

4.Bertoldi, F., Altenhoff, W., Weiss, A., Menten, K.M., & Thum, C. 2006. "The trans-netunian object UB313 is larger than Pluto" Nature 439, 563-564

5.Caltech Media Relations, News Release "Most distant object in Solar System discovered; could be part of never-before-seen Oort cloud" March 15, 2004

6.Hubblesite News Release Archive. “Hubble reveals surface of Pluto for first time” News Release # STSci-1996-09

7.Jewitt, Dave and Luu, Jane X. "Pluto,Perception & Planetary Politics" Deadalus Winter 2007 pps 132-136. American Academy Arts & Sciences.

8.NASA. "Dwarf Planet Eris Is More Massive Than Pluto" ScienceDaily 18 June 2007. 8 May 2008