Our Solar System of Eight Planets

August 2006  :  Jim Hendrickson

On August 24, 2006, the General Assembly of the International Astronomical Union in Prague voted to change the formal definition of “planet” in such a way that excludes Pluto from being what are now the eight “classical” planets. Not since 1930, the year of the discovery of Pluto, has the number of planets in our solar system changed.

Under the new definition, Pluto is now classified as a dwarf planet, one of three categories that all sun-orbiting object fall into along with classical planets and small solar system bodies.

The debate of whether Pluto should be classified as a planet has been ongoing for several years because the previous definition was too vague and the recent discovery of an object in the outer solar system clearly larger than Pluto raised questions of whether the object, 2003 UB313, should be the 10th planet. The rate of discovery of these outer solar system bodies has been accelerating since the first one was spotted in the early 1990’s, making it increasingly likely that more will be found that are similar in size or larger than Pluto. Would these also be considered planets? And if they are, what makes them planets but objects just a bit smaller than them not planets? And what about 1 Ceres, the largest of the main belt asteroids discovered over 2 centuries ago? It was clearly becoming more difficult to say what should be classified as a planet.

The vagueness of the definition of planet comes from the fact that in ancient times, a planet was simply a star that did not remain stationary in the sky with respect to the background stars. At the time there were only 5 planets, Mercury, Venus, Mars, Jupiter, and Saturn. These were the only “wandering stars” as the ancient Greeks called them that were visible to the naked eye, as there were no telescopes to spot the remaining two. Earth itself wasn’t considered a planet until the Copernican model of the solar system was devised in the 16th century.

By the time Uranus was discovered in the late 1700’s it clearly fit the model of what a planet should be, by this time understood to be large spherical bodies orbiting the sun.

A bit of a conundrum arose at the opening of the 19th century, however, when an object was found orbiting the sun between Mars and Jupiter. At first this object, named Ceres, was considered to be a planet, but telescopic observations showed nothing more than a point of light, so it clearly did not appear to be like the other planets. It was then categorized as a new type of object known as an asteroid, meaning “starlike” due to its appearance in telescopes.

Neptune was discovered in the mid-19th century by mathematical prediction based on the perturbations observed in the orbit of Uranus. Neptune was undoubtedly a planet due to its appearance as a distant blue ball in a telescope.

After Neptune was discovered it was widely believed that there was still an undiscovered, large planet beyond the orbit of Neptune. The search for Planet X ensued.

It wasn’t until 1930 when Clyde Tombaugh found a new planet by examining photographic plates he exposed using a telescope at Lowell Observatory. Named Pluto, this dim point of light which can only be detected visually in telescopes of about eight inches and larger was only detectable by noting the change in its position against the background stars over the course of 2 nights of observation. Once its orbit was calculated, it was determinted that Pluto was a small object, quite unlike the larger planets of the outer solar system. It certainly wasn’t the Planet X being searched for, but it was a planet nonetheless. Many decades passed and no new planets were discovered, but many thousands of small solar system bodies were found. Since then the solar system has consisted of no more or no fewer than nine planets. Today we have eight. So what are the consequences of “losing” a planet?

As it turns out, even though Pluto has been stripped of its planetary status, its discovery was significant in the fact that it was the first of a new class of solar system bodies and it would be more than six decades before the next one was found. These are the objects that we know today as the Kuiper belt, or trans-Neptunian objects. And among these Kuiper belt objects, Pluto holds more prominence than it ever did as a planet, being the second largest one discovered to date and also being comparatively close to the sun.

For decades its ephemeris has been published in books and journals so that backyard astronomers could track its progress across the sky as it makes its 248 year orbit around the sun. With planetarium software and the web, it will always be possible to find Pluto at any given time.

Until recently, no interplanetary space probe had ever been sent to Pluto, making it the only planet to not have been explored up close by a robotic visitor from Earth. That changed in January 2006 when the New Horizons mission was sent on a journey to Pluto and points beyond; and until 2015 when we finally get our first up-close look at it, Pluto will remain the enigma at the edge of the solar system.

Planet or not, it is not likely that astronomers will lose interest in Pluto, and perhaps the biggest casualty of its reclassification is the old grade-school mnemonic used to remember the names and order of the planets: “My very educated mother just showed us nine planets.”

The IAU therefore resolves that planets and other bodies in our Solar System be defined into three distinct categories in the following way:
  1. A classical planet1is a celestial body that
    1. is in orbit around the Sun,
    2. has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
    3. has cleared the neighbourhood around its orbit.
  2. A dwarf planet is a celestial body that
    1. is in orbit around the Sun,
    2. has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape2,
    3. has not cleared the neighbourhood around its orbit, and
    4. is not a satellite.
  3. All other objects3 orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”.
  1. 1 The eight classical planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
  2. 2 An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
  3. 3 These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

The IAU further resolves:
Pluto is a dwarf planet by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects. This category is to be called “plutonian objects.”