A spate of new discoveries of objects in the mass range 0.5-15 Jupiter masses (jupiters), along with the Pluto controversy, has caused increasing debate over the meaning of the word "planet" (as differentiated from "stellar or substellar" objects). A variety of uses for this word have occurred, each with a somewhat different intent. No formal definition has been officially adopted. I examine the problem in the light of current astrophysical observations and theory. The debate can be framed in terms of three distinct arenas that drive it: characteristics, circumstance, and cosmogony. By "characteristics" I mean the physical properties of an object - primarily its means of support against gravity and its source of luminosity. By "circumstance" I mean the environment in which it is found, most importantly whether it is in orbit, what primary object and other companions are present, and their orbital characteristics. By "cosmogony" I mean the origin or mode of formation of the object. The current lack of consensus derives from differing weights being applied to these three arenas in forming the definition of "planet". Each arena contains dividing lines between "planets" and "stars" which are partly matters of opinion, and end up sorting objects differently. The main purpose of this paper is to bring order to the discussion, so that we may eventually find a broadly accepted consensus definition. I cannot resist, however, offering a possible solution that steers a middle path through the various quandaries that come up in attempting a formal definition.
"When I use a word", Humpty-Dumpty said in a rather scornful
tone, "it means just what I choose it to mean neither more nor
"The question is", said Alice, "whether you can make words mean so many different things".
"The question is", said Humpty-Dumpty, "which is to be master that's all."
Lewis Carroll in Alice Through the Looking Glass (1872)
Table I. The Defining Arenas for Planets
A. Source of Internal Pressure (inferred from well-accepted models)
1. Coulomb forces (free electron, crystalline, or liquid); ideal gas law
2. Free electron degeneracy
3. Thermal pressure, supplied by fusion, due to gravity
B. Source of Luminosity (inferred from well-accepted models)
1. Internal Heat Capacity (and radioactive decay)
2. Gravitational Contraction
3. Fusion of Deuterium
4. Fusion of Hydrogen
A. Object is in Orbit (observed)
1. In circular orbit around a main sequence star
2. In any orbit around an object capable of fusion (fusor)
3. In any orbit around a more massive object
B. Object is in a system (observed or observable)
1. Object is in unique, non-crossing orbit, dynamically cleared
2. Other similar objects are in similar orbits
C. Object is not in orbit (observed)
1. Object was never in orbit (difficult to ascertain)
2. Object used to be in orbit, but was ejected (difficult to ascertain)
A. Formed in a disk around a fusor (depends on developing models)
1. Built up by agglomeration of planetesimals (difficult to ascertain)
2. Gas added by accretion onto rock/ice planetary core (difficult to ascertain)
3. Formed directly by gravitational instability in disk (difficult to ascertain)
B. Formed in a disk as a result of gravitational perturbations
1. Perturbations were due to other companions (ascertainable?)
2. Perturbations were due to passing external bodies (difficult to ascertain)
C. Formed at the center of an isolated disk (observable)
"A planet is a spherical non-fusor which is born in orbit around a fusor."
"A fusor is an object capable of core fusion at some time during
One of the most basic characteristics of astrophysical objects is their mass. It is mass which determines their structure and evolution, and often serves as the basis for classification. Does mass make sense as a basis for defining planets? It certainly plays a role in one of the few points on which there seems to be complete consensus: if an object has core conditions capable of generating nuclear fusion (which depends on its mass), it is not a planet. The proposition that we should therefore define planets as spherical objects incapable of fusion, however, has been made but enjoys no such consensus. The role of mass in the characteristics of objects is important here in two contexts: determining the dominant pressure-support mechanism in the interior, and setting the sources of luminosity (all opaque objects emit thermal radiation). These two ways do not, unfortunately, boil down to the same thing, so a mass-based definition has to combine them somehow. It also probably makes sense to continue to tell young children that planets shine by reflected light while stars make their own.
The structural divisions are fairly straightforward and sensible. Most stars (especially after their pre-main sequence phase) derive their internal support from thermal pressure. The heat may be generated by fusion, or by gravitational collapse. Of course, without support the object will just collapse further and heat up, so one might view core fusion as a strategy for keeping the core relatively cool (I am indebted to John Faulkner for pointing this out to me). As one moves to the lowest mass stars, the core densities become high enough that free electron degeneracy provides increasing pressure. It is the eventual dominance of this source of pressure that relieves the stars from "cooling" themselves by fusion, and causes the substellar realm to begin (at about 75 jupiters, depending on metallicity). Degeneracy support extends below this limit down to about 2 jupiters. At this point the internal density and pressure has decreased again enough (due to the low mass of the object), that ordinary Coulomb forces can begin to support the object. An obvious difference between "degenerate" and "ordinary" objects is that adding mass to the former make them smaller, while adding mass to the latter make them larger. All of the Solar System planets fall into the "ordinary" category.
The luminosity divisions are similar, but not identical. One problem here is that objects change their luminosity sources as they evolve, so it is harder to put an object into one bin. Almost everything is first luminous due to gravitational release of energy (by accretion and contraction). If fusion can occur, it will first occur for deuterium, then hydrogen. Main sequence stars (especially low mass ones) can burn hydrogen for a very long time. Hydrogen burning can occur in objects down to 60 jupiters (well into the substellar regime). Deuterium is much less abundant, and can only burn for a short time in any object. This occurs in objects down to 13 jupiters, early in their lives. All objects in the substellar regime derive most of their time-integrated luminosity from gravitational contraction. When an object is finally neither fusing nor contracting, its emission comes only from trapped heat inside. This happens fairly soon for rock/ice planets. Jupiter and Saturn are still contracting slightly. Although legal definitions would be hard to formulate, astronomers have been comfortable with the following categories. A "star" is an object that burns hydrogen for a "long" time, during which its luminosity is practically stable and derives exclusively from that (i.e. it achieves the main sequence). If hydrogen fusion is never the sole source of luminosity and the object continually fades, it is a (substellar) "brown dwarf". It is still a brown dwarf if it doesn't burn hydrogen at all, but only deuterium. Most would agree that the lower mass limit of brown dwarfs occurs at the deuterium limit.
At this point, cosmogony is often thrown into the mix. The deuterium boundary was originally thought to be close to the lower limit for isolated star formation as well, but that is no longer clear. Some have recently proposed that degenerate non-fusors are still brown dwarfs (or maybe "grey dwarfs" to differentiate them) if they form the same way as fusing brown dwarfs. Others would like to call such objects sub-substellar (with various names proposed), and still others would like to call them planets (without regard to cosmogony or circumstance). It has been suggested that since deuterium fusion is not really a significant long-term source of luminosity, distinctions based on it are forced. Astrophysically that is reasonable, although it is often the case that objects very near the boundary of classes are harder to differentiate than those well away from the boundary. But the importance of the fusion/non-fusion boundary is another example of a strong "cultural" influence (that stars have fusion and planets don't). Because of that, it may make sense to coin new terms, say "fusor" and "non-fusor", to make this distinction explicit (without additional baggage). Such terms would take much of the heat out of several of the current disagreements.
We can see that defining objects purely on the basis of mass has its problems. It would be nice if the degeneracy boundary coincided with fusion boundaries at both ends, but it does not. There is a mismatch at high masses between the end of stable hydrogen burning (the main sequence), and the actual hydrogen-burning limit (the latter term is often confused with the former condition). The problematic objects for our purposes are degenerate non-fusors. As regards the definition of "planet", of course, the other question is whether one is willing to use a purely mass-based definition, or must consider cosmogony and circumstance as well. Finally, to do something about the low mass limit for planets, the best choice seems to be a requirement that they have sufficient mass for gravity to force a spherical shape.
I pointed out in the Introduction that the cultural definition of planet generally includes the necessity of being "in orbit about a star". We can therefore agree that if "circumstance" is to be included in the formal definition, this is a minimal requirement. Our Solar System suggests that planets should furthermore be found in circular orbits, and this was thought to be a natural consequence of the fact that planets form in disks. Indeed, as the first extrasolar planets were found, their eccentric orbits caused some to doubt that they are really "planets". As more systems were found, however, they inspired theoretical work that makes it likely that planets can indeed inhabit eccentric orbits (even planets which formed just as Jupiter did). We realized that if one has several massive planets, it is quite easy for them to disturb each other's orbits. We further realized that disks can move planets from their birth orbits (in the most extreme cases, dumping them into the star itself). Interactions between the planet and disk can also damp or excite eccentricity, depending on the particulars. Thus, even if planets built from planetesimals really must form in circular orbits, that is not how we may find them. Furthermore, planet formation may be influenced by mergers with smaller planets, or may occur instead by direct formation through disk instabilities (see section on cosmogony); either of which might generate eccentric orbits. It now seems unwise to use a circumstance based on orbital eccentricity in a definition of "planet".
The next question is what sort of object a planet must be orbiting. This seems relatively uncontroversial; it is generally agreed that the object being orbited should be a fusor. My experience is that few raise objections if it is a brown dwarf (so long as that is defined so only fusors are brown dwarfs). The only uncertainty comes if the "central" object is a degenerate non-fusor. Both objects may be like that, or at a minimum the center of mass may be well outside either of them. Such a configuration may be hard (especially if the secondary is too massive) to differentiate from a "binary planet", so it seems safer to demand that the primary be a fusor.
The object may not be the only one in orbit about the central fusor. Indeed, it is more convincing to have several objects in a system in "planetary" orbits (meaning they at least vaguely resemble those in the Solar System), since then the system more closely resembles ours. A single object in an eccentric orbit is uncomfortably similar to a binary stellar system (though this may just reflect our Solar System bias). Nonetheless, if all the objects in the Solar System were removed, leaving only the Sun and Jupiter, it is unlikely that anyone would wish to revoke Jupiter's planetary status.
On the other hand, having too many other similar objects in similar orbits has already been fatal to an object's planetary status. Ceres was originally designated a planet (and found in the position anticipated by Bode's Law, between Mars and Jupiter). It lost its status when several objects somewhat smaller than it, and in similar orbits, were discovered. Precisely the same situation has now arisen with Pluto. If we were historically consistent, there would be no question about the demotion of Pluto to "minor planet". In Pluto's case, another strike against it is that it crosses inside Neptune's orbit. If "major" planets do that, they will inevitably perturb each other into new orbits. Stable planetary systems must have all the major planets in non-crossing orbits (by the very definition of "stable"). It is a matter of taste whether one wants to include this requirement in a definition of "planet", but it seems to make sense to call small objects in crossing orbits "minor planets" by default.
Finally, we must deal with an object which is not in orbit about a more massive object at all. Given the cultural understanding of "planet" it would seem at first that, in a discussion of circumstance, this case cannot be included in the definition of "planet". We must, however, consider the instance where an object that is born as an acceptable planet is later ejected from the system. Turning the reasoning in the previous paragraph around, if Jupiter were suddenly lost from the Solar System, would we then consider it no longer a planet? That is again a matter of opinion, but I doubt that many would consider ejection grounds to revoke its status. It should be pointed out that smaller planets are even more likely to be thrown out (witness the vast number of Oort cloud comets). An ejected Earth would certainly still be considered a planet (what else could it be?). An attractive option is to append an adjective to reflect the new situation, such as "ejected" or "free-floating", to planet. The problem here is in determining whether a free-floating object has a history like this, or was formed without a primary. If the latter is true, a clear majority prefers not to call the object a planet. Those who would like to do so don't want circumstance considered in the definition at all, and insist on a definition based purely on characteristics.
"That's a great deal to make one word mean", said Alice.
"When I make a word do a lot of work like that", said Humpty-Dumpty, "I always pay it extra."
Lewis Carroll in Alice Through the Looking Glass (1872)
Given all the above considerations, it seems difficult to compose a
definition that would satisfy the many conflicting constraints
(cultural and astrophysical), deal with the three arenas, and
encompass our new knowledge. But here is an attempt that may qualify
(without even having to pay the word exorbitant wages):
"A planet is an object that is spherical due to its own gravity, that is never capable of core fusion, and which is formed in orbit around an object in which core fusion occurs at some time". Or more succinctly: "A planet is a spherical non-fusor born in orbit around a fusor".
This captures the two clearest cultural imperatives (that a planet be in orbit, and not be a fusor). It avoids the difficulties associated with our ignorance (both observationally and theoretically) of cosmogenetic issues, and allows planets to form as they will. There is increasing evidence from the current mass distribution of "exoplanets" that a different formation mode operates below the deuterium-burning limit (or something close to it) than for more massive substellar objects, at least in proximity to solar-type stars. This definition does not conflict with that evidence. It is not so specific that it excludes any of the current objects that most would agree are planets. The definition does not include non-fusors that form "like stars" by themselves (but it is fair to say that such objects have "planetary masses"). I have modified my own previous stance on the definition (which was purely mass-based) as a concession to thinking about the overall problem in the cultural context. To apply the definition requires observations of the mass and environment of an object (or inferring these from a combination of observations and well-accepted modeling).
As an example let us take the example of HD 168443 (a solar-type star). At 3 AU there is an object which is indisputably well above the fusion boundary (with a lower mass limit of 17 jupiters). At 0.3 AU there is an object above 7 jupiters. This could also be above the fusion limit if the orbital inclination is low enough, but there is evidence from Hipparcos which makes that relatively unlikely. There is certainly no need to speculate on what to call the outer massive object it is very clearly a brown dwarf. The identity of the inner object is also easy (subject to determination of the true orbital inclination): it nicely fits my definition of "planet". It should be pointed out that there may well be other "stellar" close binaries with inner giant planets; the Doppler searchers have quite purposefully avoided examining this question by avoiding close stellar binaries as targets. At any rate, the identity of objects in this system are only puzzling to those who insist on cosmogenetic preconceptions as defining characteristics.
To the basic definition one is encouraged to add adjectives that make it clearer what one is talking about. For our Solar System, one can refer to "historical" planets (which include Pluto). As regards characteristics, one can speak of "ordinary" or "degenerate" planets (perhaps the latter could be called "superplanets" in deference to their mass). As regards circumstance, one can refer to planets as "minor" or "major" depending on whether they are in unique, non-crossing orbits or not. This may also depend on how many similar objects there are in the system. This paper does not address the question of exactly how to make this distinction. For cosmogony, one can apply adjectives for each mode of formation (once it is theoretically well-accepted and can be justified observationally). These might include "agglomerated", "core-accretion", "direct", or different and/or additional other terms.
My proposed definition has the intended "flaw" that it includes ejected planets; those subject to the following sad fate (with apologies to Gilbert and Sullivan's 1885 Mikado)
In an inner stable orbit, round a warm and yellow
with an outer fellow planet that's a huge and gaseous one.
Which will migrate ever closer, wielding gravity's sharp kicks,
whose result is cold careening to the interstellar "sticks".
To confirm such a history may be difficult in most cases. The
class of "non-fusors" is therefore, however, not congruent with
"planets", because of the possibility that non-fusors may be also be
formed as isolated free-floaters. Such objects should in
principle be given a "stellar" sort of name (such as the recently
proposed "grey dwarf") to distinguish them from ejected planets.
As a practical matter, it will be difficult to attach the proper label
to a given object (although "free-floating non-fusor" is indisputable
if the mass is known, since a fusor companion would always be found if
a non-fusor has been detected). I think it worth living with
this difficulty, because it is likely that many ejected analogs of our
local terrestrial and gas giant planets are floating out between the
stars. They deserve to be called planets despite their misfortune.
Even accepting this, the difficulty of confirmation once again
reinforces the problem with including any cosmogony in the definition.
An easy way out would be to drop "born" from the definition (and
abandon ejected planets as worthy of the name).
One must eventually deal with the question of planets versus moons, and other binary or multiple configurations. For two non-fusors in orbit about each other, it has been suggested that one differentiate between a "binary companion" and a "satellite" by demanding that the center of mass for the objects be outside both of them for the term "binary" to be appropriate. I further suggest that this be independent of the masses involved. Thus, a bound pair of objects with 9 and 11 jupiter masses would be a "binary planet", so long as the pair were born in orbit around a fusor (also independent of its mass). If they were by themselves, they would be "binary grey dwarfs" (or whatever term applies to isolated non-fusors). A substantial disadvantage with this proposed definition is that it is very "circumstantial" - to the extent that our own Moon would become a planet if it recedes sufficiently far from us. For the purpose of this paper, it is probably better to duck this issue altogether. This problem makes explicit the difficulty of the role which circumstance can play if used in the identity of objects. It is this which leads some to suggest that a purely "characteristics-based" definition makes more sense. It seems, however, that most are not ready for that conceptual change. A circumstantial component is required if we are to retain the cultural imperative of being "in orbit around a fusor" for planets.
Thus, in the end, I think it is possible to fashion a formal definition of "planet" that will accomplish the astrophysical point of having such a word. It can also fit in well with our cultural preconceptions (these are unavoidable given that we live on a planet, in a planetary system), but at some cost. This definition should be able to survive subsequent discoveries and understandings about the characteristics, circumstances, origin, and evolution of planets, and the many undiscovered configurations that Nature has probably produced, which will delight new generations of astronomers. Perhaps, as the general public becomes familiar with the new astrophysical context in which "planets" are discussed, the cultural imperatives surrounding the word will eventually change.
I welcome discussion on this paper; please send email to firstname.lastname@example.org