The Universe in the Classroom

A Good Definition of the Word "Planet": Mission Impossible?

2) The Problem with Pluto

One of the remaining problems is illustrated by the "Pluto controversy". There has been a heated public discussion in the past few years about whether our ninth planet is really deserving of the name. Since Pluto orbits a star, and is much less luminous, it certainly satisfies the definition we have above. The problem is actually not with Pluto, but with a very large number of other bodies in our Solar System which also satisfy this definition, but which are not called planets. Perhaps the most notable of these is Ceres, the largest asteroid. Ceres was discovered in a deliberate search that was motivated by Bode's Law of planetary spacing (which we still don't understand, and doesn't really work with new discoveries). The planets known in the 18th century all satisfied this law, but the fourth one seemed to be missing (between Mars and Jupiter). When Piazzi found an object in about the right orbit, it was hailed as the fourth planet. It was surprisingly small, but then Mercury is not much larger than our Moon anyway (and smaller than the largest moons of Jupiter and Saturn).

Bodes's Law
Expected distance in AU Actual distance Planet
(0 + 4) /10 = 0.4 .4 Mercury
(3 + 4) /10 = 0.7 .7 Venus
(6 + 4) /10 = 1.0 1.0 Earth
(12 + 4) /10 = 1.6 1.52 Mars
(24 + 4) /10 = 2.8 2.1-3.5 asteroids
(48 + 4) /10 = 5.2 5.2 Jupiter
(96 + 4) /10 = 10.0 9.5 Saturn
(192 + 4) /10 = 19.6 19.2 Uranus
(384 + 4) /10 = 38.8 30.1 Neptune
(768 + 4) /10 = 78.0 39.6 Pluto

For more about this relationship, go to: (A new window will open.)

The status of Ceres as the fourth planet came under attack only a couple of years later, when Vesta and Juno (smaller asteroids) were found in similar orbits. Herschel (who had previously been the only one to have found a planet, Uranus, by chance) began questioning whether any of them were really planets, since all the "proper" planets had their orbits to themselves. Such a concept, of course, had not previously been part of the understanding of "planet" (and didn't appear in our definition above). He won the day, however, as asteroids continued to be discovered. One suspects that this was largely because it just wouldn't do for our Solar System to have hundreds (or even thousands) of planets (ask your students why not?).

Gaspra Ceres
Asteroids (left to right) Eros, Ida (with its satellite Dactyl), and Gaspra have all been photographed by spacecraft and none of them is round. Ceres (far right) is the largest asteroid and is round.

Furthermore, all but the largest asteroids aren't even round -- they just look like rocks (some are admittedly extremely large rocks). That is because the planets are round due to the fact that their self-gravity is strong enough to overcome all material forces; the object must assume its most efficient shape (a sphere). Ceres is just over this limit (and so is round), while most asteroids are not massive enough. Comets (which also satisfy our definition above) are like icy asteroids. Comets tend not to have orbits like planets; their paths are not circular, may well cross several of the planets' orbits, and often have orbital planes that lie well out of the Solar System's plane (the ecliptic).

Pluto (like Ceres) was found as the result of a deliberate search. It was the same sort of search that had turned up Neptune. This relied on the observation that an outer planet had wobbles in its orbit which seemed to be caused by an undiscovered body further out. Careful calculation of Uranus' wobbles had led directly to the location of Neptune. Now Neptune also seemed to be wobbling, and a location for Pluto was calculated. It was very approximate, and Tombaugh (under Lowell's direction) had to search a big area before he found it. We now know that Pluto is far too small to cause any wobbles in Neptune (they weren't real), and Pluto's discovery is much more analogous to that of Ceres than Neptune. Like Ceres, Pluto is surprisingly small compared to the other planets, and further turned out to be much smaller than first thought. In fact, it is barely larger than Ceres, and smaller than many of the major moons. Its orbit is also somewhat comet-like (as is its composition), since its path is neither circular nor in the ecliptic. Nonetheless, for decades it was accepted as the ninth planet.

The complaints began when, as with Ceres earlier, other objects began to turn up in very "Pluto-like" orbits. These are all part of the Kuiper Belt -- the remains of the outer disk that originally formed the Solar System. Its presence had been predicted, but because the Kuiper Belt Objects (KBOs) are small and much further away than the asteroids, there was a gap of six decades rather than two years between the discovery of the largest example and smaller ones. Now, however, we know over 100 of these KBOs, and the next larger one is more than half Pluto's size. We may well find a larger KBO in the near future. A sizeable contingent of astronomers feels that if we are not going to call Ceres a planet, we really shouldn't call Pluto one either. The only real difference between them is that Pluto had a much longer grace period before its orbital companions were found. Alternatively, we should restore planethood to Ceres. Everyone agrees that we shouldn't call all objects orbiting the Sun "planets", so we have to have a reasonable way to exclude objects that are "too small" (or too low in mass). As with Godzilla, size matters, but where should we draw the line? Is "roundness" is a good criterion? (To find out more about these KBO's try this link:

Trans-Neptunian Objects

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