The Demotion of Pluto

Poor Pluto! It used to be a planet, and now it’s not. The underdog of the solar system is championed by fourth graders and Boomer traditionalists alike.

When I give talks on astronomy, one of the most common questions I get is about why Pluto is no longer classified as a planet. Let’s take a look.

Why was it called a planet in the first place?

The first planet discovered by telescopic observation (Mercury, Venus, Mars, Jupiter, and Saturn are visible to the naked eye and were known to the ancients) was Uranus, in 1781. (The NASA approved pronunciation is YOOR-un-us, by the way.) Observations of its orbit suggested that it was being perturbed by another planet that lay beyond it, even farther from the sun. Calculations based on Newtonian mechanics led to Neptune’s discovery in 1846.

In the image below, Uranus is on the left and the slightly smaller Neptune on the right.

As Neptune was observed over several years, astronomers began to speculate that yet another planet lay beyond Neptune, perturbing its orbit as it had perturbed that of Uranus. Thus began the search for Planet X.

It was centered at Lowell Observatory in Flagstaff, Arizona. In 1930, the newly hired Clyde Tombaugh found a small dot that shifted its position over a few days. Planet X had been found!

What led to its demotion?

Beginning in 1992, the advent of digital cameras and computers that could rapidly analyze the images obtained led to the discovery of a number of objects that were, like Pluto, found beyond the orbit of Neptune. These are members of the Kuiper Belt, a collection of relatively small and icy bodies left over from the formation of the solar system.

The known Kuiper Belt Objects (KBOs) now number in the thousands, and more than 100,000 KBOs over 100 km (62 mi) in diameter are thought to exist. Pluto, the first such object discovered, is just the largest.

What is the definition of a planet?

The original word comes from the Greek and means “wanderer.” To the ancients, this was anything that moved against the background of the fixed stars. The sun and the moon, along with the five naked eye planets we recognize, all were considered to be planets. (These seven objects account for the seven days of our week.) In time, we recognized that the sun is a star and the moon is a satellite. A planet came to mean anything orbiting the sun and not another planet. When the first asteroid orbiting between Mars and Jupiter was discovered in 1801, it was initially designated as a planet. But as more such objects were discovered, a new category of asteroid was created.

When Pluto was declared to no longer be a planet by the International Astronomical Union (IAU) in 2006, it necessitated a definition of exactly what constituted a planet. According to the IAU, a planet in our solar system:

1. is in orbit around the Sun,
2. has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
3. has “cleared the neighborhood” around its orbit.

The new designation of dwarf planet to which Pluto was assigned met the first two of these criteria but not the third.

Is this a problematic definition? Yes it is. How round does it have to be? And what does it mean to “clear the neighborhood?” Jupiter has asteroids that both lead and trail it in its orbit. Has Jupiter “cleared its neighborhood?”

In the end, I think we have to accept that despite the best efforts of the IAU, the definition of “planet” is historical, not scientific. There are eight in our solar system. In order of increasing distance from the sun, they are Mercury, Venus, Earth, Mars (the smaller rocky “terrestrial” planets), Jupiter, Saturn, Uranus, and Neptune (the larger gaseous and icy “Jovian” planets).

How many dwarf planets are there?

Most KBOs are too small for their self-gravity to pull them into a spherical shape. And only one asteroid—Ceres—is. The great distance of KBOs makes accurate determinations of their physical characteristics difficult. The five on which there is general agreement (some astronomers would add others) are shown below.

The largest KBOs are shown below at the same scale with the Earth. Many of them, as you can see, have moons orbiting them. Haumea’s odd shape is due to its very rapid rotation, thought to be due to the collision that also formed its moons.

“There are more things in Heaven and Earth, Horatio, than are dreamt of in your philosophy.”  Act 1 Scene 5 of Shakespeare’s play, Hamlet. Indeed, even in the astronomically cozy neighborhood of our solar system, much surely remains to be discovered.