Eight is Enough?

August 16, 2008

Creative Commons License photo credit: CommandZed

Two years ago this month, the International Astronomical Union adopted a new definition of ‘planet’ which excludes Pluto. Not only do I, as Planetarium Astronomer, continue to get questions about Pluto’s ‘demotion’, but scientists themselves continue to debate it. Right now (August 14-16, 2008), a conference called “The Great Planet Debate:Science as Process” is underway at the John’s Hopkins University Applied Physics Laboratory in Laurel, Maryland. The saga of Pluto and of the definition of ‘planet’ offers some insight into our solar system and into how science works.

northern tier sky
Creative Commons License photo credit: truello

The definition of ‘planet’ has changed before. Ancients looked at the sky and saw that certain ‘stars’ in the sky changed position, while most stars seemed to form the same patterns all of the time. The Ancient Greeks called the moving stars ‘planetes‘, or wanderers–this is the origin of the word. The Moon, too, appears near different stars each night. The Sun’s apparent motion is less obvious, since we don’t see the Sun and stars at the same time. Careful observers, however, can see that different stars rise and set with the Sun at different times of year. The full list of ‘planetes’, then, included the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn. (Astrologers still use this archaic definition of planet).

Thanks to Copernicus and Galileo, people began to realize that the Sun, not the Earth, was the center of the solar system. The definition of ‘planet’ changed from ‘object which moves against the background stars’ to ‘object in orbit around the Sun’. The Sun and Moon, which had been planets, no longer were.

The position of Uranus, discovered in 1781, seemed to fit a pattern described by astronomers Johann Titius and Johann Bode. That same ‘Titius-Bode rule’ also predicted a planet between Mars and Jupiter, so when Giuseppe Piazza discovered Ceres at just the right distance in 1801, it was considered a planet. By 1807, four new ‘planets’ had been found between Mars and Jupiter (Ceres, Pallas, Juno, and Vesta). By the middle of that century, however, dozens of these new objects were being discovered; up to 100 had been found by 1868. It thus became clear that astronomers had in fact found a new category of solar system object. Astronomers adopted the term ‘asteroid‘, which William Herschel had recommended in 1802; ‘planet’ was redefined to exclude very small objects that occur in bunches. This is how science works; we must constantly revise even long standing definitions as we learn more about the universe around us.

In the late 19th century, astronomers noticed that Uranus and Neptune seemed to deviate ever so slightly from their predicted positions, suggesting that another planet was perturbing them. in 1906, Percival Lowell started a project to find the culprit, which he called ‘Planet X’. In 1930, Clyde W. Tombaugh located Pluto in sky photographs he took at Lowell Observatory in Arizona. It soon became apparent, however, that Pluto was not massive enough to influence the orbits of Uranus or Neptune. Throughout the mid 20th century, astronomers continued to revise Pluto’s estimated size downwards. From 1985 to 1990, Pluto’s equator was edge on to us, such that we saw its moon Charon pass directly in front of and behind Pluto’s disk. This allowed scientists to measure Pluto’s diameter more precisely, proving that it had not been the Planet X that Percival Lowell sought. Pluto’s diameter is just under 2400 km, a little less than the distance from the Rio Grande to the US/Canadian border. Pluto’s discovery, it turns out, was an accident.

In addition to small size, Pluto has an unusual orbit. Planetary orbits are ellipses rather than perfect circles. The eccentricity of an ellipse indicates how ‘out-of-round’ it is on a scale from 0 (perfect circle) to 1 (parabola–far end at infinity). Pluto’s orbit has an eccentricity of about 0.25, much greater than that of planets such as Earth (0.01) or Venus (0.007). The planets orbit nearly (but not exactly) in the same plane; Mercury‘s orbit, inclined by 7 degrees, is the most ‘out of line’. Pluto’s orbit, however, is inclined by 17 degrees.

Released to Public: Solar System Montage (NASA)

Behold: a pluto-less solar system.
Creative Commons License photo credit: pingnews.com

We divide the planets of our solar system into two categories: the inner planets (Mercury, Venus, Earth, and Mars) which are made mostly of rock, and the outer planets (Jupiter, Saturn, Uranus, and Neptune) which are gas giants with no solid surface. Pluto, however, fits in neither of these categories, as it is made of ice and rock (by some estimates, it’s 70% rock and 30% ice; by others, it’s about 50/50).

With its small size and abnormal orbit and composition, Pluto was always a misfit. Textbooks noted how Pluto fit in with neither the rocky inner planets nor the gas giants in the outer solar system. Still, Pluto remained a ‘planet’ because we knew of nothing else like it. There was simply no good term for what Pluto is.

That began to change in 1992, when astronomers began finding Kuiper Belt objects. The Kuiper Belt is a group of small bodies similar to the asteroid belt. Kuiper Belt objects (KBOs), however, orbit beyond Neptune’s orbit. Also, the Kuiper Belt occupies more space and contains more mass than does the asteroid belt. Finally, while asteroids are made mostly of rock, KBOs are largely composed of ice, including frozen ammonia and methane as well as water–just like Pluto. In addition to the Kuiper Belt proper, there is a scattered disc of objects thought to have been perturbed by Neptune and placed in highly eccentric orbits. Objects in the Kuiper Belt, scattered disc, and the much more distant Oort Cloud are together called Trans-Neptunian Objects (TNOs)

With the discovery of more and more KBOs, astronomers began to wonder if Pluto might fit better in this new category. Not only was the composition similar, but there is even a group of KBOs called plutinos, with orbits similar to Pluto’s. In the Kuiper Belt and the scattered disc, astronomers began to find objects approaching Pluto’s size, including Makemake, Quaoar, and Sedna.

Pluto can't get no respect
Pluto takes advantage of the wildly (?)
popular LOLcats to plead its case
with mankind.
Creative Commons License photo credit: the mad LOLscientist

To call Pluto a planet, but not these others, seemed arbitrary.

Finally, in 2005, a team of astronomers located Eris, which is slightly bigger than Pluto. Clearly, Eris and Pluto are the same kind of thing; either both are planets or both are not. If they both are planets, however, then should we include Quaoar et al., above? We have only just begun to explore and understand the Kuiper Belt and the scattered disc. Might we eventually find dozens of new ‘planets’ like Eris? Hundreds? Thousands?

This is what led the International Astronomical Union to reconsider the definition of ‘planet’ two Augusts ago. The IAU decided it was simpler to limit the number of planets to eight (Mercury through Neptune) and classify Pluto (and Eris, Quaoar, et al.) among the Trans-Neptunian objects. A new term, “dwarf planet,” includes the biggest asteroids and TNOs–those big enough to have assumed a spheroid shape. Still, other astronomers remain dissatisfied, hence the discussion going on in Maryland now.

There are two things we must keep in mind if we’re wondering when the Pluto question will be ‘resolved.’ First, decisions and conclusions of scientists are not holy edicts to be obeyed and never questioned. Quite the contrary, all such conclusions are provisional, pending new discoveries and better information. Any new decision reached this weekend is likely to be revised when the IAU meets again in 2009, and again in 2015 when the New Horizons mission arrives at Pluto. If it were any other way, science could not function.

Secondly, all categories which help us organize and understand things in our minds (including ‘planet’) are pure human inventions that only roughly correspond to nature. Although we need to categorize the things we see, nature does not; no matter how we classify objects, nature presents us with borderline cases that challenge us. Pluto is the same thing today as it was in 2005 or even before it was discovered in 1930. We need to distinguish our need for neat categories from our need to explore and describe nature.

Proud to be a space cadet? Learn more about astronomy:
Dust off your telescope – or visit the George Observatory – to see what’s in the night sky this month.
Ten billion trillion trillion carats – the universe has great taste in diamonds
If it blew a hole in your roof, you’re on the right track – how do you tell a rock might be a meteorite?

Authored By James Wooten

James is the Planetarium Astronomer at the Houston Museum of Natural Science. He teaches students every school morning in the planetarium, and also answers astronomy questions from the public.

6 responses to “Eight is Enough?”

  1. Eight is most certainly not “enough” or correct regarding the number of planets in our solar system. That number is constantly in flux as new discoveries are made; at the moment, it stands at a minimum of 13. Both Pluto and Eris are planets as well as Kuiper Belt Objects. The IAU definition crafted two years ago makes no sense, as it states that dwarf planets are not planets at all. This, along with the criterion that an object “clear its orbit” to be considered a planet, is vague, confusing, and downright sloppy. None of the eight big planets fully clears its orbital zone of nearby asteroids, and Neptune does not clear its orbit of Pluto. The general consensus at this conference was that any object in hydrostatic equilibrium, meaning it has enough gravity to have pulled itself into a round shape, is a planet. Hydrostatic equilibrium is important because the mass necessary to produce this state is what is needed to start geological processes. Asteroids, unlike Pluto, Eris, and Ceres, are not in hydrostatic equilibrium and therefore are geologically inert.

    Here is the real issue. There are three, not two categories of planets in our solar system: terrestrial planets, gas giants, and ice dwarfs. The only reason Pluto didn’t fit is because we were unaware of the existence of this third category. This does not preclude Pluto from being a planet; it just means we need to adjust our subcategories to allow for this third grouping, which includes round KBOs as well as Ceres.

  2. Joseph Lockett says:

    Eight is fine as a number. If the term “planets” is to be useful for the general public as well as scientists, it needs to describe a small group of distinguishable and distinctive individuals. The four terrestrial and four gas-giant planets are all distinct entities; most of what I’ve seen indicates that they are more different from each other, in size, distance, and composition, than are the bevy of asteroids or the cloud of Kuiper objects. The IAU’s concern was that we not arrive at a situation where we suddenly had 20-odd planets or more, many of them near-indistinguishable siblings of similar size, orbit, and composition.

    The resistance to Pluto’s “demotion” is purely the human resistance to change. Writers and historians can point out that it’s happened before, as with the newfound asteroids — apparently Ceres and Vesta even had planetary symbols invented before they were kicked out of the club! But Americans, particularly, aren’t very good at looking at history for comparisons, instead preferring to concentrate on the limited view of our own personal experience. Pluto’s demotion is a great victory for logic, long views, and the living scientific method.

  3. Erin says:

    Thank you, Laurel and Joseph, for some really interesting comments. I noticed more on the planet debate over the weekend: The Bad Astronomer wonders if this is all merely a semantic issue, and some researchers at the University of Bonn are making the case that brown dwarfs need their own class.

  4. No, Pluto’s demotion is not any sort of victory for logic in any way. We should not simply accept change for the sake of change, especially when that change makes no sense. Why does the number of planets need to be small? If our solar system has 20 plus planets, then that is what it has. We don’t attempt to limit the number of elements in the Periodic Table or the number of stars and galaxies because “there will be too many for students to memorize.” The universe was not designed for our convenience.

    Objects in hydrostatic equilibrium are very much distinguishable geologically from inert, shapeless asteroids. The former are geologically differentiated into core, mantle, and crust, just like Earth and the other major planets and different from shapeless asteroids that do not have these characteristics. Actually, Ceres should be classified as a planet as well since it is clearly in hydrostatic equilbrium. Vesta appears to have been round at one time and subsequently impacted by an asteroid that knocked off its south pole. When the Dawn mission gets there, it will likely show Vesta to be differentiated as well.

    Dwarf planets are a subclass of planets. This is where the IAU definition needs to be amended. Stating that opposition to the current IAU definition is simply resistance to change is nothing more than an ad hominem attack and a straw man–both being logical fallacies.

  5. James Wooten says:

    The rule that a planet must have cleared its orbial path strikes me as a way of saying that a ‘planet’ should not be just one of a large group of similar objects that orbit together. Pluto’s reclassification is clearly based on the discovery of many objects similar to it and dissimilar to the eight major planets.

    Jupiter, Saturn, Uranus, and Neptune clearly represent one group of objects–the gas giants. Mercury, Venus, Earth, and Mars represent another group of objects–the terrestrial planets. What’s more, these categories appear to be closed; we expect to find no other members of either category in our own solar system. Asteroids form a large group of similar objects distinct from these eight, as do comets and the Kuiper Belt objects and scattered disk objects beyond Neptune. The eight distinctive objects orbiting the Sun need a term that sets them apart from those bodies that occur by the thousands. The unqualified term ‘planet’ is as good as any, as has the advantage of describing most of these objects since antiquity.

    It seems as though some people consider a ‘minor planet’ or a ‘dwarf planet’ less interesting than a plain old ‘planet’, but I’m not quite sure why that would be. If Pluto’s size, composition and density, as well as the eccentricity and inclination of its orbit, are more similar to Kuiper Belt objects than to Jupiter, Neptune, Earth or Mars, then calling it a member of the Kuiper Belt isn’t really a demotion. We’re just beginning to fully understand the Kuiper Belt; the reclassification may spur a renewed interest in Pluto. Further, the terms ‘minor planet’ and ‘dwarf planet’ clearly denote sub-categories of ‘planet’.

    Finally, the New Horizons mission will usher in a new era in understanding Pluto–one where actual measurements and data taken on site will guide theory. I expect the IAU to revise its definition based on what we learn. All definitions we throw around now are only temporary.

  6. Olka says:

    Where I can to find posts on this topic?

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