Pluto: A Rose by Any Other Name?

by Devon Hamilton PhD - Senior Scientist / Physics
30/04/01

"What makes a planet a planet?"

A seemingly innocent question such as this opens a proverbial can of worms for astronomers. For the ancient Greeks, the planets were "wandering" stars-five stars that slowly changed their positions when compared to all the other stars that filled the night sky. These wanderers became associated with the gods and mythologies of many cultures. Not until 1610, when the Italian scientist Galileo Galilei first pointed his new telescope at these wandering stars, was their true nature as other worlds revealed. (1)

In the nearly 400 years since Galileo stunned the world with his discoveries about the nature of our universe, the number of planets known to us has steadily increased. In 1781, the English astronomer William Herschel discovered the planet Uranus, and in 1846 Neptune was added to the list by Johann Galle (although its existence had been predicted by both John Adams and Erbain Leverrier). Pluto, the last planet in our solar system, was discovered in 1930 by American astronomer Clyde Tombaugh.(2) And so it remained until the 1990s; when astronomers suddenly began finding dozens of planets orbiting other stars. At last count about 70 of theses "exo-planets" had been discovered.(3)

However, there are some questions as to the "planet-ness" of many of these celestial bodies. Traditionally, a planet is thought to be a body orbiting a star, with enough mass to gravitationally pull itself into a spherical shape, but not enough mass to fuse the hydrogen isotope deuterium. This sounds straightforward enough, doesn’t it? But hold on. The asteroid Ceres, and other large asteroids, orbit the Sun and have enough mass to become spherical. Is Ceres a planet? Well, yes and no. Ceres and the other asteroids are defined as "minor planets", which is where some of the confusion arises. In fact, many astronomers have been asking whether or not the planet Pluto is really a planet. There are some astronomers who wish to designate it a minor planet like Ceres and the other asteroids. Other astronomers want to give it a sort of dual citizenship – as both a planet and an asteroid. The problem is, it’s not really like an asteroid, either.(4)

Pluto and its moon Charon

Pluto is the runt of the litter of planets in our solar system. It is by far the smallest, and the oddest. The inner four planets of our solar system are all rocky, small worlds (although still larger than Pluto); the next four planets are all swollen gas giants. All eight of these planets follow elliptical orbits that are almost circular, and they all orbit in roughly the same plane about the Sun. On the other hand, Pluto has a comparatively steeply inclined and elongated orbit that actually brings it inside Neptune’s orbit.

The asteroid Ida and its tiny moon Dactyl.

In composition, Pluto is more comet-like than any of the other planets. It consists largely of various ices (water, carbon dioxide and methane) with a few rocks thrown into the mix. In size it is smaller than many of the moons of our solar system, including the Earth’s Moon, Ganymede, Callisto and Titan. Pluto does have a moon of its own, Charon (discovered in 1978), which orbits close to Pluto and is almost half of Pluto’s size. However, some asteroids are known to have satellites (or "moons") of their own, most notably the asteroid Ida, which is orbited by the tiny asteroid Dactyl.(5)

So what is Pluto?

Well, it turns out that this corner of our solar system is a region known as the Kuiper Belt. This disk-shaped region just beyond the orbit of Neptune is home to almost all the comets we periodically see swinging through the inner solar system. By last count there were some 70,000 icy objects with diameters greater than 100 kilometres in the Belt – Pluto has a lot of company. These "trans-Neptunian Objects" or "Kuiper Belt Objects" all bear remarkable similarities to Pluto. In fact, some even occupy very similar orbits, and are known as "Plutinos" (the largest of which is about one quarter the size of Pluto)(6). There are some who think that Pluto would be better described as "King of the Kuiper Belt" rather than being known as the "runt of the litter". However, when the International Astronomical Union considered reclassifying Pluto in 1999, they were buried in outraged e-mails. Apparently, Pluto will always be a planet in the public’s mind.(7)

But what about the other half of the "planet" definition: the limit to mass? Because of the detection techniques used to find exo-planets, we preferentially select for larger ones. And astronomers are surprised at how large these planets are. Many of these planets are several times the mass of Jupiter, near the boundary of how most people define what a planet is. This creates a little bit of a problem because it appears that there may be an overlap between planets and a group of stars called Brown Dwarfs.(8)

Brown Dwarfs are "wannabe" stars – they have insufficient mass to start the sustainable thermonuclear fusion reactions that power normal stars, although they have enough mass to burn the rare hydrogen isotope deuterium for a short period. They are believed to form in a similar fashion to normal stars-out of a giant collapsing cloud of gas and dust-unlike planets, which are believed to be formed out of the leftover material when the parent star has formed. So how are we to distinguish between a small brown dwarf and a large exo-planet? Some astronomers argue that if the body orbits another star, it must be classified as a planet, and if it doesn't, then it must be a brown dwarf. However, numerous brown dwarfs have been found orbiting other stars and recently, "lonesome" free-floating planet-sized objects (smaller than brown dwarfs, 5 to 15 times the size of Jupiter) were detected in a nearby star-forming region.(9) Uh oh.

While all of this may sound as if astronomers can’t make up their minds, it is actually a great example of science in action (not to mention an embarrassment of riches). One of the cornerstones in the pursuit of scientific understanding is taxonomy, or classification. The term taxonomy is usually associated with biologists and palaeontologists. The system of taxonomy as a means of classification for plants and animals was initiated by the Swedish biologist Carolus Linnaeus in 1737(10), who grouped together different species of plants and animals based upon their physical resemblance. Similarly, other branches of science also have taxonomies. In geology, rocks are often grouped by their appearance (fourth-grade lessons on the differences between igneous, metamorphic and sedimentary rocks lurk in my memory). In astronomy, different classes of stars are formed based upon their colour and other characteristics of their light. Galaxies are grouped based on their shape and dust content. At its roots, astronomy is a passive or historic science. Astronomers cannot change the compositions of stars, or alter the orbital positions of planets. We rely instead on making as many observations as we can, and using these observations to comprehend the underlying physics. One of the key steps done in this process is classifying the objects we are observing, grouping them by their characteristics, binning them by common observed criteria – much as 18th and 19th century taxonomists grouped animals into species, genus and phylum. When stars are classified, the groups or bins are based upon the appearance of their spectra. If the light of a star is passed through a prism, the different wavelengths of radiation are bent (or refracted) by different amounts, yielding a spectrum (we see the Sun’s spectrum as a rainbow after a spring rain). The characteristics of a spectrum reflect a star’s temperature and size. This means that by grouping stars by their spectrum, we are also grouping them by their physical characteristics, just as biological taxonomists do when comparing the appearance and structure of plants or animals. (How many legs? Does it have a backbone?)

Just as the taxonomists and biologists discovered as they classified animals and fossils, astronomers have found that the boundaries between "species" of astronomical objects can be a little bit fuzzy. The difference between an elephant and a mouse or between the Earth and Jupiter are easily apparent, but the transitions between lions and tigers, or Pluto and Plutinos are not as clear. When we call Pluto a planet, we are drawing boundaries on a continuum. Astronomical objects range in size over a continuum, from microns to light-years. Each one of our boxes of stars covers a range of temperatures, and there will obviously be an overlap at the edges of our boxes. The fact that we try and force some of these objects into boxes with particular labels like "planet" is just a reflection of how we go about doing science. By classifying and subdividing those classifications even further, we gain insight into why we see this diversity in properties; why there are Brown Dwarfs and Plutinos; why there are rotwiellers and terriers. It’s near these box edges that sometimes we learn the most interesting things, which is what science is supposed to be about.

1. To learn about Galileo and his time, try going to http://es.rice.edu/ES/humsoc/Galileo/
2. To learn about Clyde Tombaugh and the Lowell Observatory, try http://www.lowell.edu/ and http://www.jpl.nasa.gov/ice_fire//9thplant.htm
3. To find out the latest news in exo-planet hunting, go to http://exoplanets.org/
4. A great amount of background about the Pluto debate can be found at Mark Kidger’s page http://www.iac.es/galeria/mrk/Pluto_debate.html
5. More about Ida and Dactyl and the Galileo Mission can be found at http://www.jpl.nasa.gov/galileo/assets/astimages.html
6. To find out about Plutinos, try http://www.ifa.hawaii.edu/~jewitt/kb/plutino.html and http://www.space.com/scienceastronomy/solarsystem/plutino_001024.html
7. For more about Kuiper Belt Objects and trans-Neptunian Objects, go to Dave Jewitt’s page at http://www.ifa.hawaii.edu/faculty/jewitt/kb.html
8. A good place to start on Brown Dwarfs is a Scientific American article here http://www.sciam.com/article.cfm?articleid=000276D4-33FB-1C75-9B81809EC588EF21
9. Find a biography of Linnaeus at http://www.ucmp.berkeley.edu/history/linnaeus.html