Definition of planet

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Although planets are the principal component of the solar system other than the sun, a precise definition of the term is surprisingly elusive. This article details the questions that may arise when trying to formulate a strict definition of the word.

For most astronomers the issue will be decided by the International Astronomical Union (IAU). According to a published report from Nature magazine, the discovery of 2003 UB313 (which is a Kuiper Belt object bigger than Pluto) has forced the issue. An IAU committee which had already been working on a definition is now expected to promulgate one soon.

History and etymology

There has never been a precise and unchanging definition for the word "planet." It began life as a series of loosely intensional definitions that singled out their subjects by their shared inherent qualities, but in recent years has devolved into an extensional definition, or arbitrary list. Today, the pace of scientific discovery both within and without our solar system has blurred the clear demarcations that once separated planets from other astronomical objects, and debate now rages over how to create a new intensional definition that takes account of recent data.

When originally coined by the ancient Greeks, a planet was any object that appeared to wander against the field of fixed stars that made up the night sky (asteres planetai "wandering stars"). This included not only the five "classical" planets, that is, Mercury, Venus, Mars, Jupiter and Saturn, but also the Sun and the Moon (the "seven heavenly objects"[1]). Sometimes a distinction was made in terminology so that if one said the "five planets" it would mean all of the "naked eye" ones except for the sun and moon. There was no need for as exact a classification of a planet in those days and so the sun and moon could be included or excluded as planets in discussions.

Eventually, when the heliocentric model was accepted over the geocentric, Earth was placed among their number and the Sun was dropped, and, after Galileo discovered his four satellites of Jupiter, the Moon was also eventually reclassified.

The orbital criterion

From that point, a central aspect of any definition of a planet was that it must orbit a star. However, the manner and nature of the orbit has been disputed.

Asteroids

The first challenge to the orbital definition of a planet occurred in 1801, with the discovery of Ceres.

Bode's Law, a mathematical function which generates the size of the semimajor axis of planetary orbits, predicts a body between Mars and Jupiter. Ceres was found to lie at almost exactly the required distance, and was initially referred to as a new planet.

Then in 1802, Heinrich Olbers discovered Pallas, a second "planet" at roughly the same distance from the Sun as Ceres. The idea that two planets could occupy the same orbit was an affront to centuries of thinking. Eventually these "planets" numbered in their thousands, and William Herschel gave them their own separate classification, asteroids, thus reserving the concept of "planet" for larger bodies in unique orbits.

The Pluto controversy

The relative sizes of the Earth on the left, with (from top to bottom) the Moon, Pluto and its moon Charon, Sedna and Quaoar on the right.

The discovery of Pluto by Clyde W. Tombaugh in 1930 had little initial effect on the idea of what a planet should be, since, despite being smaller than Earth's Moon, and in a more eccentric orbit than any other planet (sometimes it is closer to the Sun than Neptune), it still appeared to broadly fit the contemporary definition of a planet. Then, beginning in 1992, astronomers began to detect large numbers of icy bodies beyond the orbit of Neptune that were similar in composition and size to Pluto. They concluded that they had discovered parts of a long-suggested region of icy/rocky planetesimals that comprised the source for most if not all comets. Exploration of the trans-Neptunian region and the objects it contained placed Pluto in perspective. The first formation within this region to be discovered was the so-called Kuiper Belt (sometimes called the Edgeworth-Kuiper Belt), a band of icy debris that is the source for "short-period" comets; those, like Halley, with orbital periods of up to 200 years. Pluto's orbit was found to lie right in the middle of this band and thus its major-planet status was thrown into question, with some scientists claiming that it should be reclassified as a member of the Kuiper Belt, and that a "planet" should be redefined as, in the words of Mike Brown of Cal Tech, "any body in the solar system that is more massive than the total mass of all of the other bodies in a similar orbit." There has been outcry at suggestions that Pluto be reclassified, and the International Astronomical Union officially voted to retain Pluto's classification as a planet.

Since the late 1990s the discovery of several objects not much smaller than Pluto, such as 50000 Quaoar and 90377 Sedna, kept the debate over Pluto's planetary status active. On July 28, 2005 astronomers announced the discovery of a trans-Neptunian object as large as, and probably larger than Pluto, designated 2003 UB313; although its discoverers (and many in the news media) immediately referred to it as the tenth planet, it is officially designated as a minor planet -- the provisional designation 2003 UB313 referring to its official listing in the minor-planet archive as the 7827th object found in the second half of October 2003. Such discoveries parallel the discoveries of other asteroids after Ceres, which was itself originally referred to as a planet. This highlights the fact that there is a precedent for demoting a planet to non-planet status.

Double planets

Pluto is not the sole suspect in this case of planetary puzzlement. Pluto and its satellite Charon, are the only planet/moon pair in the solar system whose barycenter lies above the planet's surface, which means that both orbit each other. In addition, this is the only case where both the planet and its moon are tidally locked with respect to each other. If one were to stand on the surface of either Pluto or Charon, one would see its opposite partner hanging forever in the sky, perpetually still. As a result, it is common for astronomers to refer to Pluto/Charon as a double planet; two objects orbiting the Sun in tandem.

Even our own Moon, surprisingly, could be considered a partner in a double planet system, since, though it certainly orbits the Earth, the timing of its orbit round the Earth with the Earth's orbit round the Sun means that, looking down on the ecliptic, the Moon never actually loops back on itself, and in essence orbits the Sun in its own right [2].

There is even some disagreement about whether all moons in the solar system, even those that do not orbit the Sun directly, should be classed as "secondary planets", since they often exhibit features in common with true planets. Jupiter's moon Ganymede and Saturn's moon Titan are both larger in terms of diameter (though not mass) than Mercury, and Titan even has a substantial atmosphere, thicker than the Earth's. Moons such as Io and Triton demonstrate obvious and ongoing geological activity, and Ganymede even has a magnetic field.

The size criterion

Kuiper Belt objects, along with asteroids, centaurs and other small solar system bodies, are known collectively as minor planets, with the implication that "minor planet" is not a type of planet, but an independent category, though the point when one becomes the other is still not clear. Isaac Asimov suggested the term mesoplanet be used for planetary objects intermediate in size between Mercury and Ceres (such as Pluto), but for decades astronomers have used the term planetoid with broadly the same meaning.

Some have argued that the cut-off point between "planet" and "minor planet" should be when an object becomes spherical under its own gravity. Many astronomers favour this defintion because it would allow Pluto to retain its status as a planet. Yet it would not only let Pluto into the planetary club, but also Ceres, to say nothing of a dozen, possibly hundreds, of Kuiper Belt and Oort cloud objects. To refer to these objects both as planets and as members of their own distinct populations would be to single them out with an additional term that conveys only that they are round, and nothing of their origin, composition or location, all of which attributes they share with other, non-spherical members of their populations.

The fusion criterion

The dividing line between star and planet has been blurred considerably since 1995, with the discovery to date of over 150 extrasolar planets; planet-sized objects in orbit around other stars.

One of the principle elements of any definition of "planet" has been that it must be too small to commence nuclear fusion at its core. The ability to fuse hydrogen is seen as the sole province of stars. However, stars such as brown dwarfs have always existed at the boundary of that distinction. Too small to commence sustained hydrogen fusion in their cores, they have been granted star status on their ability to fuse deuterium. Although such fusion does occur, this process lasts only a tiny fraction of the star's lifetime, due to the relative rarity of that isotope, and hence most brown dwarfs would have ceased fusion long before their discovery.

Since binary stars or other multiple-star formations are fairly common, many brown dwarfs are likely to orbit other stars, and since they would not be producing energy through fusion, they could be described as "planets". Similarly, white dwarfs, which have also ceased fusion, could be reclassified as planets. The recent discovery of a "planet" around a brown dwarf star has ignited a debate as to wether it should be referred to as a planet or a natural satellite.

But the confusion does not end with brown dwarfs. Zapatario Osorio et. al. have discovered many objects in young young star clusters of masses below that required to sustain fusion of any sort. [3] These have been described as "free floating planets," but also "grey dwarfs" or "sub-brown dwarfs", as they could have formed in the same fashion as stars. If that is the case, then it raises the question about whether such an object, even one as familiar as Jupiter or Saturn, should be referred to as an orbiting low-mass star, rather than a planet. After all, apart from size and relative temperature, there is little to separate Jupiter from its host star, the Sun. Both have similar overall compositions; hydrogen and helium, with trace levels of heavier elements in their atmospheres.

The IAU has officially released a statement to define what constitutes an extrasolar planet and what constitutes an orbiting star:

  1. Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.
  2. Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed nor where they are located.
  3. Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).

The definition, like any definition of a planet, is uncertain and ambiguous. By ignoring the process of formation, it is essentially saying that any star too small to commence nuclear fusion in its core is a planet, thus placing it in the same company as Earth, Mars and (at this time) Pluto.

History vs. science

Ultimately, a strict scientific definition of what a planet is may prove different from the determination provided by convention and history. Mike Brown, since his discovery of 2003 UB313, has shifted his postion on what constitutes a planet. "Scientists have not yet realized that the term planet no longer belongs to them. But, quite clearly, it does not... The word "planet" has been around much longer than modern science." [4] It may very well be that children will continue to learn of the nine planets in school while scientists work in a solar system of eight, or hundreds, or even abandon the term "planet" altogether. For now, "planet", like "continent," is a word caught between the scientific and cultural worlds, without a clear meaning.

References

Further Reading

Planet Quest by Ken Croswell

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