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What Is the Definition of a Planet?

You would think that it would be a fairly simple matter to define a planet as a “round thing that orbits a star.” However, the situation is actually very messy, partly because it has become the domain of committees, and partly because a planet is, in the end, a rather arbitrary concept. It also turns out that planets in our solar system and outside our solar system (exoplanets) are handled differently at the moment, mainly because observational information on the exoplanets and their star systems is currently far less detailed than it is for our own planets. For example, as you will see below, the official definition of a planet requires you to know whether an object is “nearly round,” but exoplanets are so far away and have an apparent size that is too small to be able to determine “roundness,” in general. In addition there is a class of objects that does not orbit a star (free-floating planets) that remains formally undefined. Such objects have been claimed to be detected, and even claimed to be more common than stars in our galaxy (e.g., T. Sumi, et al., Nature 473 (2011), 349-352). From both an observational and theoretical view, things are rather up in the air with respect to how a planet outside our solar system should be defined, and whether free-floating objects should even qualify to be evaluated for “planethood.”

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With respect to planets in our own solar system, the official “resolutions 5 and 6” of the 26th assembly of the International Astronomical Union (IAU) sets out the formal definition, and how Pluto should be categorized. One of the problems with coming up with a definition is concerned with how to treat large satellites (moons). The official definition put forward by the IAU can be summarized as follows: A planet is an object that (1) orbits the Sun, (2) is massive enough for its own gravity to make it nearly round, and (3) has cleared its orbit of debris. The definition explicitly excludes satellites, but as we shall see, there is a grey area here. Objects that satisfy the first two of these conditions and not the third are to be called dwarf planets, and Pluto is officially classified by the IAU as a dwarf planet. The shockingly vague and unscientific language of the IAU definition (“nearly round,” “cleared its orbit”) is still rather controversial.

So how many dwarf planets in our solar system are there so far? Officially there are five: Ceres (the asteroid), Pluto, Haumea, Eris, and Makemake. However, there are many more candidate dwarf planets amongst the many rocky/icy objects beyond Neptune and Pluto, in the so-called Kuiper Belt, located at roughly 30 to 50 AU from the Sun. However, for the time being the IAU accepts only five as official dwarf planets. The other objects are known as KBOs (Kuiper Belt Objects), or TNOs (Trans-Neptunian Objects). Their relatively small sizes are very difficult to measure. To complicate things further, the term plutino is used to describe objects in the inner Kuiper Belt that follow similar orbits to Pluto in the sense that the orbital period is locked in resonance with Neptune's orbital period (2 plutino orbits for every 3 orbits of Neptune). Pluto is a plutino. Got that? Now, to make your head spin even faster, a plutoid is a TNO that qualifies for being a dwarf planet. This means that Ceres is a dwarf planet but not a plutoid (because it is not a TNO). Plutoids are too small to actually tell whether they are “nearly round” so the judgment is made on the basis of a theoretical guess. A plutoid is distinguished from a plutino in that it does not have the orbital resonance with Neptune that a plutino does. Pluto is a plutoid. Satellites of plutoids are not themselves classed as plutoids (refer to the discussion about Charon for the obvious problems with this). You can find more details on the dwarf planets and keep up with the latest findings at another IAU website that also includes details of the satellites of the dwarf planets, as well as details of the eight “classical” planets and their satellites.

Finally (as far as the solar system is concerned), what are “small solar system bodies”? Basically the IAU defines this term as the official designation of anything that misses the definition of a being a planet, a dwarf planet, or a satellite. This includes asteroids and comets.

Does the Classification of a Solar System Object Depend on How It Was Formed?

No: the IAU explicitly did not include a “formation criterion” because there are too many theoretical uncertainties. In other words, formation of solar system objects is not yet sufficiently understood.

What Are the Mass Limits for a Planet In General?

The IAU definition of a planet also does not specify an upper boundary on the mass of an object. However, it is known that if the mass of an object exceeds about 12 to 13 times the mass of Jupiter, it may begin to burn what is known as deuterium (hydrogen with two neutrons per atom instead of one), and such nuclear burning would then classify the object as a brown dwarf (e.g., see W. B. Hubbard, A. Burrows, and J. I. Lunine, Annual Reviews of Astronomy and Astrophysics, 40 (2002), 103-136). The exact “critical” mass is subject to theoretical uncertainty because the calculations involve uncertain parameters and assumptions. Objects that exceed the critical mass are known as brown dwarfs, which are essentially extremely dim stars. Note that nuclear burning of deuterium occurs before hydrogen because the temperature required for the ignition of nuclear burning is lower for deuterium than it is for hydrogen, even though deuterium is far less abundant than hydrogen.

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What about the lower end of the mass range? Is there a minimum mass that can qualify as a planet? This is essentially determined by the “round” condition in the definition of a planet. Objects that are held together by their own self-gravity are prevented from collapsing completely by various sources of opposing pressure. In the absence of internal heat sources, for objects that have densities that are typical of planets, the principal sources of resistance are the electrostatic forces in the atomic and molecular structure. Every part of a self-gravitating structure is always trying to fall and collapse to the center. If this self-gravitational force is sufficient to overcome the resistive forces then the matter falls radially until new resistive forces increase enough to prevent further collapse, and opposing forces balance each other again. Since the gravitational force is always directed towards a single “point,” the center of mass, the resulting equilibrium structure will tend towards being spherical. However, if the object was not massive enough and compact enough to begin with, gravity will not be able to crush and redistribute the matter and the equilibrium configuration will not be spherical. In general, estimating the critical mass and radius for an object to become spherical is complex. It depends on the detailed composition and the physical state of the matter (for example, whether the object is rocky or not, and whether the rock was molten at the time of formation). The asteroids provide good examples: the smaller asteroids have no particular shape, whilst some of the most massive ones (including Ceres) are spherical. For objects that are further out in the solar system, which are typically made predominantly from ices, a critical size of about 400 kilometers wide has been estimated (see http://web.gps.caltech.edu/~mbrown/dwarfplanets/). Although the physical principles that determine the lower limit of the mass of a planet are straightforward, in practice it is not simple to calculate that lower limit.

The next question is, what is the maximum mass that a brown dwarf can have before it becomes a fully-fledged star, burning regular hydrogen in nuclear fusion reactions? The question is important because if a candidate exoplanet has mass measurement uncertainties that straddle the planet/brown dwarf boundary (13 Jupiter masses), we would like to know whether we can rule out the object being a star. To become a fully-fledged star an object must be massive enough for the pressure from nuclear burning to balance the tendency to collapse under its own gravity. In other words, the object is not supported by the intermolecular electrostatic forces, nor by nonnuclear thermal pressure, against gravitational collapse. Calculations show that the critical minimum mass for a star is about 80 times the mass of Jupiter (but the exact value depends on the chemical composition and other factors; e.g., see W. B. Hubbard, A. Burrows, and J. I. Lunine, Annual Reviews of Astronomy and Astrophysics, 40 (2002), 103-136).

So What Is an Exoplanet?

We see from the above discussion that the first condition that must be met for an object to qualify as a planet is that its mass must be less than the critical mass at which thermonuclear burning of deuterium can begin. At the lower mass end, the definition of a planet is consistent with that for our solar system: the object must be massive and compact enough that its self-gravity makes it “nearly round.” The condition of “clearing the orbit of debris” also carries over. However, in addition, there is the issue of whether an object that is not associated with a star (i.e., a “free-floating” object) should be called a planet (even if it has a mass of less than 12 to 13 Jupiter masses). For the moment, such objects are not labeled as planets, but are labeled as “subbrown dwarfs.” However, sometimes they are referred to as “planemos” (short for “planetary mass objects”).

In practice, it may not be possible to measure the required observables for an exoplanet to unambiguously determine its formal qualification as a planet. In particular, only a small percentage of planets have yet been imaged, and it is not possible, in general, to say with confidence that an object is “nearly round,” and the issue really begs a more quantitative notion of what exactly is meant by “nearly round” anyway. There is a document that was produced by the “IAU Exoplanet Working Group” in 2006 that describes the ins and outs of the classification of exoplanets, and it is still quite an ongoing affair.

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File under: Defintion of a Planet; Definition of an Exoplanet or Extrasolar Planet; What is a Planet? What Is an Exoplanet; What Is an Extrasolar Planet?

© Tahir Yaqoob 2011.