Our present, somewhat muddy, view of what "planet" means has been created by our current perception of the universe. Without this perception, our definition becomes meaningless. The way our view of "planet" has changed since the word was invented is the result of continuous 'updating' of the meaning to make it match our current views.
The Original Planets
The word planet was originally used to describe something that moves across the sky independent of the motion of the stars. The stars all moved together, their position with respect to each other never appeared to change. It was as if they were all attached to some big sphere with Earth in the middle. But there were seven objects up there that moved around on their own (other than objects like clouds, meteors, and comets which were considered transient phenomena, rather than objects.) They wandered around, and didn't move like they were fixed in place like the stars.
Because they wandered, they got the name "planetes" from the Green word for "wanderer" (root plan-). So a "planet" was something that was seen to wander across the sky. The planets were:
- The Sun
- The Moon
Notice that the Earth was not a planet. What a laughable idea! The Earth's not in the sky. But notice that the Sun is a planet. Of course, it is in the sky, and it moves independently of the stars.
The fact that there were seven planets tied heavily into the perception of seven as some sort of special number. There were seven known metals, after all, and each was associated with one of the planets (gold, silver, copper, tin, lead, iron, and mercury.) Sevens were found all over the place, in part because of the number seven being tied to the planets. It was an early attempt at identifying unifying rules behind the universe.
One that the ancients appear to have missed is the planet Uranus. It is actually bright enough at times to be seen by eye. But it appears as most as a very dim star, and its motion against the other stars is slow enough it would not have stood out as easily at the other planets.
Also, there were times when Venus was thought to be two planets. Its appearance in the morning, on one side of the Sun was thought to be one planet, called Phosphoros, and its appearance in the evening another, called Hesperos. Pythagorus of Samos recognised it as a single object, which became known as Aphrodite as the knowledge spread through Greek culture. It had been known as a single object long before, by the Mediterranean cultures, but for a time Greek culture saw it as two objects. Later, when the Romans adopted Greek astronomy, they translated Aphrodite to their own goddess, Venus, which is the name we use today.
The Sun-Centered Universe
After the word planet had become well established, a new way of looking at the universe came about. Originally, there were two places that defined the universe, Earth and Sky. While the idea that the Sun may be a more central place to the universe than Earth goes back long before Copernicus, it was still something remote and "out there" rather than "down here", so the idea of Earth as having more kinship with Mars and Venus than the Sun never really took off. They were all "out there."
But with the general acceptance of the Copernican concept of calculating the calendar by the motions of objects circling the Sun, the Earth became one of those objects circling the Sun. There was no general word for "things that circle the Sun", but five of those things were already called planets, the sixth circled an object that circled the Sun (the Moon) and the seventh was the Sun itself.
Plus, everything associated with the rejected idea of things going around the Earth was ready for disuse. Why not just repurpose some of the terms? Like, say, "planet."
Planet was redefined as "things that circle around the Sun." Now the Sun was no longer a planet, and neither was the Moon, since it didn't directly circle the Sun, but the Earth. But Earth joined the new short list of six planets.
Later, when Galileo found little objects circling Jupiter he gave them a name appropriate for little things hovering around the King of the Gods, he called them "attendants", or, as the word he used has come to us, "satellites". This gave a new category for the Moon to fall into, it now became Earth's "satellite."
Later, more objects were found to be circling the Sun. They, too, were planets, as the definition of "planet" was "something that circles the Sun." They were defined as planets by their motion around the Sun. This included Uranus and a bunch of small objects between Mars and Jupiter, in the wide gap where Bode's Law said there ought to be a planet.
Planetary Orbits from Recreations in Astronomy, 1886
These objects were located by their motion. They were dim, but they moved independently of the "fixed stars", and their motion was described as "planetary". That is, they were moving around the Sun.
It wasn't too long before about 200 of these objects were found. To help keep track of them, they were given numbers as well as names. Some special ones were found that weren't in orbits in the space between Mars and Jupiter, like Eros. Eros has an orbit that goes from outside the orbit of Mars to inside it, nearing Earth. For a long time Eros was the object known to come closest to the Earth, aside from the Moon.
Are Asteroids Planets?
In the astronomy books from the time of the discovery of Ceres and Pallas, up to the 1930s, the asteroids are usually given, as a group, the same treatment as any other single planet. In the chapter or chapters detailing the planets of the solar system, the asteroids appear among the other planets.
In the 1869 tenth edition of Outlines of Astronomy by Sir John Herschel the asteroids are described definitely as planets. He acknowledges that very little is known about them, and that assertions that some of them possess an atmosphere is highly questionable. But he does speculate for a moment about possible life on these small planets:
"On such planets giants might exist; and those enormous animals, which on earth require the buoyant power of water to counteract their weight, might there be denizens of land."
-Sir John Herschel, 1869
He defines them as planets thus:
"The sun and moon are not the only celestial objects which appear to have a motion independent of that by which the great constellation of the heavens is daily carried round the earth. Among the stars are several...which...are found to change their relative positions among the rest...These are called planets. [Of those discovered since 1800] all of them but Neptune belong to a peculiar and very remarkable class or family of planets to which the name Asteroids has been assigned."
-Sir John Herschel, 1869
Recreations in Astronomy, 1886, by Rev. H.W. Warren has an extensive chapter on the asteroids, as complete as any of the sections dedicated to the other members of the solar system. In it he describes:
"...Piazzi, an Italian astronomer of Palermo, found in Taurus a star behaving like a planet. In six weeks it was lost in the rays of the sun. It was rediscovered on its emergence, and named Ceres. In March, 1802, a second planet was discovered by Olbers in the same gap between Mars and Jupiter, and named Pallas."
-Rev. H.W. Warren, 1886
The "star behaving like a planet" part is the source of the name "asteroid", which means "star-like". Through the telescope the asteroids do not show a disk, as do most of the planets. They show only a star-like point of light.
In Dr. Charles Young's Lessons in Astronomy of 1896, the planets are divided between the "Inferior" planets and "Superior" planets. The inferior planets are those whose orbits are inside the Earth's, the superior those that orbit outside the Earth's path. The asteroids have a section equal to that of the other planets, bearing the title "The Asteroids, or Minor Planets". Minor in this case means "small", rather than "unimportant." Dr. Young is very comprehensive in his discussion of the solar system, including even the Zodiacal Light with its own section among the planets, as opposed to among the sections discussing comets and meteors.
The Sun, as Seen from Planets Flora and Mnemosyne
In Dr. Simon Newcomb's Elements of Astronomy from 1900, they have a section like that of the other planets. The asteroid Eros rates its own section, because of the unusual nature of its orbit.
Fellow of the Royal Society Richard A. Procter ignores the existence of the minor planets entirely in his 1901 book Other Worlds Than Ours. His book's focus is mostly on the possible habitability of other worlds, so he not only ignores the asteroids, but throws Venus and Mercury together into a single chapter on "The Inferior Planets", while Mars, Jupiter and Saturn each rate their own extensive chapter. Uranus and Neptune share a chapter as "The Arctic Planets." Meteors and Comets even get their own chapter, looking at their influence on the formation of the solar system. But the asteroids, clearly too small to be habitable, are left out.
In 1923 Joseph McCabe refers briefly to the asteroids, not as planets, but as "planetoids." In The Wonders of the Stars he describes them as filling the space for a predicted planet between Mars and Jupiter, a planet which either broke up or failed to form.
Aside from the asteroids, it was thought that as many as four planets had been seen inside the orbit of Mercury. Lessons in Astronomy includes a brief section on Intra-Mercurial Planets, whereas Recreations in Astronomy goes a bit further by having a chapter on planet Vulcan.
In both instances the case for planets inside Mercury are stated, and in both cases the lack of evidence leaves the author unconvinced of the presence of the planet(s). Dr. Young notes that the "planets" supposedly plotted during an eclipse, if plotted only slightly differently, are in the same position as known fixed stars.
Upon its discovery, Pluto is first supposed to be the size of the Earth. Over time, its expected size has diminished. Its greatest perceived decrease in size came after the discovery of its moon, Charon. The presence of this moon allowed the mass of Pluto to be determined with some precision for the first time, using simple calculations based on gravity and laws of motion.
Pluto was found to have a mass of about 2 thousandths of the mass of the Earth. In fact, much of the mass that Pluto was supposed to have turned out to be in its satellite, Charon. Charon has about 12% of Pluto's mass.
Still, Pluto was an exciting discovery during a time when most of professional astronomy had shifted its attention outside the solar system to dealing with the stars and galaxies.
New Qualifications for Planets
Before the discovery of Pluto, as the number of known Asteroids grew, it was suggested that perhaps the Asteroids should be excluded from the class of planet. Their small size and diminutive effect on the dynamics of the solar system were cited as reasons for removing them from the list of planets, as well as the desirability of keeping the list of planets brief enough to manage without a printed list.
They were never entirely removed from the designation of planet, however. Their planetary motion, that of having independent orbits around the Sun, militated in their favor. Eventually they became known as Minor Planets, and in informal use were left off the accounting of the planets.
The discovery of an object beyond Pluto which might prove to be larger than Pluto captured the public attention as the discovery of a "tenth planet." This object was given a working name of "Xena", which became popular. Later the official name "Eris" was applied to this object. To date, it does in fact appear to be larger than Pluto. Other objects have been found to be of similar size, though to date Eris and Pluto are the largest known objects in the solar system beyond Neptune.
The discovery opened up a controversy about the definition of "planet" and the status of Pluto as a planet. Just as with the asteroids, the possibility, in fact probability, of numerous small objects like Pluto being found in the outer solar system had many questioning whether a new definition for the word "planet" should be made which would exclude these objects before they were added to the list of planets as the asteroids had been originally.
In 2006 the International Astronomical Union decided to not only assign the name Eris to "Xena", but to formally redefine the term "planet" in such a way that Pluto and Eris (and several other sizeable objects, including the asteroid Ceres) were excluded from that definition.
The Third Step
The first two requirements for the new definition of "planet" were uncontroversial. They are:
- The object must be in orbit around the sun.
- The object must be massive enough to pull itself into a spherical shape.
The first requirement is the same as has been used since "planet" got redefined from "something that moves in the sky against the stars" to "something that orbits the sun."
The second requirement is a new physical limit to set a lower size on what would be called a planet. It would formally exclude all but a few of the Minor Planets from the designation of "planet". Ceres would still qualify as a planet, but not as a "full member", rather than simply as one of the Minor Planets. Pluto would likewise retain its status under these two rules.
But under the process the IAU followed, a third rule was added:
- The object must have cleared the area around its orbit.
This rule, while unclear in many ways, had the effect of excluding any of the asteroids, Pluto, and any of the objects near or like Pluto.
Is This the Right Committee?
The process for the decision was flawed in many ways. Initially, the committee within the IAU that was responsible for the decision about naming "Xena" and developing a definition for "planet" tried to communicate clearly before the conference where the votes would be held and prepare the attendees for the choices that would be placed before them. That was good, they worked in good faith to try and bring in the scientific community as much as possible.
In the end, however, the choices that were presented ahead of time were set aside, and new ones were formulated at the last minute at the meeting, often going directly against the stated objectives of the working group itself. While no IAU rules were violated in this process, it ended up having a negative effect on the results. Community input was laid aside, many directly concerned scientists were unable to take part in the vote, and it limited what could be considered for the vote itself. A delay would have been better, though it would have meant putting off the decision, possibly for two years.
The final votes were taken after many of the scientists who had expected to participate in the decision had left the conference. Most astronomical organizations can't afford to sent their scientists to the conference for its full duration. They plan to attend only on those days most important to them. They were there for the early votes on the plans that didn't pass, Most had left before the last day of the meeting, when the final vote and decision were made.
Finally, it was not explicitly part of the IAU's charter to define the word "planet." They took this upon themselves to some degree. Not without reason, but consultation with the various national bodies from which they derive their powers, along with a higher level of public involvement, would have helped build a consensus for their decision to take this on in addition to their chartered tasks. It was their assigned job to name newly discovered objects, but not to determine what is, and is not, a planet by changing the definition.
All of these factors, plus others which are political and bear on the funding models for scientific work, had the unfortunate effect of turning the decision into a very controversial one. So far, the decision has not been formally revisited, though it should be. Either the definition could be reworked to gain a greater level of acceptance both among scientists and the public, or the present definition or something close to it should be established through a process that allows for a far greater degree of inclusion and interaction both by the scientists involved in solar system studies and by the public.
Before any further vote, the IAU needs to make its case that defining the word "planet" is even in its power, and that involved organizations outside the IAU accept that and any process it plans to use to formulate that definition. Educational groups and a variety of scientific organizations both in and outside astronomy, and government organization associated with science should be approached on this basis. The definition of "planet" affects far more people in society than astronomers.
Adding to the Controversy
As if the problems with the 2006 IAU decision were not enough, the name chosen for objects which just fail the definition of "planet" on the third criterion were given the designation "dwarf planets." Why is this a problem? Because the noun applied to them is still "planet". A chihuahua may be a "small dog", but the adjective "small" doesn't remove it from the class of "dog."
So, formally, that makes the "dwarf planets" actually planets according to the rules of English, at least. No matter that the IAU says on one hand "they're not planets", they then, confusingly, call them planets.
By that light, we presently have 12 known planets in the solar system. In order from the Sun by average distance they are:
With the controversy has come a range of emotional reactions on both sides of the debate. One is a false concern over how many planets there "ought" to be. The concern is that if all Plutoids are recognized as planets, then the list will grow too long for schoolchildren to memorize, or some other such vague upper limit that would seem to suggest that the number of planets is somehow important. Another is the use of the term "demoted" with respect to Pluto, suggesting that it is being punished, or its supporters for its definition as a planet are being punished by the act of redefining "planet."
There are many other such things that get drawn in as well. On these two subjects, a clear answer to the first may be whether an upper limit should be placed on the number of U.S. Presidents or U.K. Sovereigns to make it easier to memorize their names? Perhaps we should redefine President to drop the one-termers, and Sovereign to consolidate those with the same name but different numbers, or otherwise simplify the list, perhaps by dropping those who ruled for less than a decade. Perhaps we need likewise limit the number of States in the U.S., the number of elements on the Periodic Table? All nonsense, of course.
Slice and Dice
A reply to the second concern is that we can divide and distinguish the planets in different ways, choosing that which suits the task at hand. We have historically divided the planets in many different ways. There are the inferior and superior planets. There are the terrestrial and gas giant planets, now usually divided as terrestrial, gas giant, and ice giant. Plutoids is another designation that has been added, and though it appears to be a sop thrown to those who see Pluto as a planet, it can have a valuable use in naming large Kuiper belt objects like Pluto and Eris.
Realistically, there isn't a good reason for creating an exclusive definition for the word planet beyond the first, and possibly the second, terms from the IAU decision of 2006. The only reason to do so that I can perceive have more to do with emotional satisfaction than science.
Other Words, Other Worlds
We are at the point now of getting our first halfway decent look at objects smaller than suns outside our solar system. Many new terms are being coined to describe these objects. At this point, we're only capable of seeing certain types of objects well, though we're starting to get a look at a broader class of objects. The things we can see well are those which are very massive compared to the planets in our solar system, and which are in orbits with short periods, close to their sun.
Hence we have "Super Jupiters" and "Hot Jupiters" and "Super-Earths" and many other types of planets being described. We are still looking for "Earths" outside our solar system, and one after another type of planet will be proclaimed an "Earth" or "Earth-like" planet as what we find draws closer and closer to the size, temperature, orbit, and other characteristics of the actual Earth. So far, we've seen the term applied to planets with several times the Earth's mass which are far hotter than Earth and closer to their sun. By the time an actual Earth-like planet is found, the public will be sick of hearing about a "new Earth" being discovered every few weeks.
But the joy of these terms is that they are not determined by committee. No central ruling council is bothering to create a canon of terminology for these objects yet. The scientists engaged in the work are creating their own terms as an informal shorthand that allows them to avoid stating mass, average temperature, and orbital characteristics every time.
Perhaps the course of discovery and description should be allowed to go its own way within the solar system as well as outside it? Once a bureaucracy takes a task on itself, can it be expected to relinquish that task, if the organization is supposed to be for the good of science?
Maybe the scientific community, as a community, independent of the bureaucracy of the IAU, needs to take back its right to define terms appropriate to their own use. Some will see Pluto as a planet, others will not, depending on their perspective and their work. Perhaps the first error was forcing the issue to a formal vote by a committee.
Or perhaps a new solution that works will be formulated and ratified through the same vote process, and the controversy will fade into the trivia of history.
New Data Changes Old Words, Again
This seems likely, as the quantity of data that will affect our view of planets and solar systems that's coming from outside our own solar system will have its say soon. Just as the definition of planet changed decisively when our knowledge of the structure of the solar system changed due to Copernicus and Kepler, our knowledge of what a solar system is will soon change again.
Chances are we're going to be using the word "planet" in a new way very soon.