What’s Up With Pluto?

By Raymond Benge, Associate Professor of Physics and Astronomy, Northeast Campus

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto! Those were the nine planets that I learned as a child. In elementary school, my father bought me a solar system science set, with cut-outs of the various planets and information about them. Granted, at the time, much of that information was a guess, since no spacecraft had yet been to any of the planets other than Venus and Mars, and there was not even telescopic detail available for Mercury, Uranus, Neptune or Pluto. But, as I got more involved in astronomy later in life, I realized that Pluto was unlike the other planets. Still, in early 2006, NASA launched a spacecraft called New Horizons to visit Pluto, the only planet not yet visited by a spacecraft. New Horizons passes by Pluto during the summer of 2015. But, in 2006, astronomers at a meeting of the International Astronomical Union removed Pluto from the list of planets. The decision sparked a widespread outcry from many people outside of the astronomical community, most of whom never even bothered to find out about why Pluto was removed from the list. So, what happened? Why was Pluto removed from the list? And, is the New Horizons mission still relevant if Pluto is not a planet?

To answer these questions, we need to understand a few things about the definition of the word, planet. The ancient Greeks looked up into the sky and saw stars and constellations. They also noticed that a few of those “stars” moved. The word, planet, comes from the Greek for “wanderer.” The Greeks identified the wanderers that we know as Mercury, Venus, Mars, Jupiter and Saturn. (Earth, of course, wasn't in the sky!) The Greeks also considered the Sun and Moons wanderers. The Romans named the planets other than the Sun and Moon after some of their gods. Eventually, the seven wanderers became the seven planets that the weeks are named after (Sun's day, Moon's day, Saturn's day, etc.). In English, we used other than Roman gods to name the other days of the week, but in French and Spanish it is clear that Tuesday is Mars' day, Wednesday is Mercury's day, and so forth. Eventually, astronomers came to realize that the Moon goes around the Earth and that Earth revolves around the Sun, as do the other planets. After some debate, they came to acknowledge that Earth is a planet, and the other planets that orbit the Sun are worlds, perhaps even like Earth. The solar system became Mercury, Venus, Earth, Mars, Jupiter and Saturn.

William Herschel's discovery of Uranus in 1781 shocked the astronomical community because it showed that we did not really know everything out there in the solar system. Perhaps there was more to find. In fact, astronomers noticed that there was a large gap in the solar system between Mars and Jupiter. After an extensive search, astronomers found four objects in that gap. These were quickly hailed as four new planets: Vesta, Juno, Ceres and Pallas. So, the solar system was then 11 planets: Mercury, Venus, Earth, Mars, Vesta, Juno, Ceres, Pallas, Jupiter, Saturn and Uranus. But, soon other tiny planets were found between Mars and Jupiter, and all were tiny. The largest, Ceres, is not even as big as the state of Texas in diameter, and a much larger object, Neptune, was discovered in 1846 that was near the size of Uranus. So, astronomers decided that the tiny objects between Mars and Jupiter were simply not big enough to rate as planets in light of the larger bodies, and they were removed from the list, leaving eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

The story is not complete without an understanding of how these bodies were found. Uranus was found by sheer chance. Ceres was found while searching for something in the gap between Mars and Jupiter. Pallas was found by accident while trying to observe Ceres. Juno and Vesta were similarly found by chance while looking in the general area of the sky covered by the gap between Mars and Jupiter. Neptune was found after mathematicians observed that the orbit of Uranus was not quite right. John Couch Adams and Urbain Le Verrier independently computed that another body's gravity was influencing the orbit of Uranus. A search for said object yielded Neptune.

The innermost planets (Mercury, Venus, Earth and Mars) are small compared with the outermost planets (Jupiter, Saturn, Uranus and Neptune). The inner planets are rocky, solid worlds. The outer planets are mixture of liquid and gas. In fact, the dominant composition of the outer planets is substances known as gasses on Earth, and so those worlds are often called “gas giants.” Giant is a good description of these worlds, for Jupiter could hold nearly 1,000 Earths inside of it. Even the smaller gas giants, Uranus and Neptune, could hold several dozen Earths inside of them. The tiny things between Mars and Jupiter became known informally as “asteroids.” They, too, were solid rocky bodies much like Earth and the inner planets. The asteroids didn't seem to fit with the rest of the solar system, and astronomers had many theories about why there were so many of them between Mars and Jupiter. One popular idea (now known to be totally incorrect) was even that this was somehow a planet that had gotten blown apart (much like the planet Alderaan in Star Wars). This was the solar system, as astronomers knew it, at the beginning of the 20th century.

By the 20th century, European astronomers were joined by astronomers in America studying the planets. But American astronomers and European astronomers had two different ways of measuring planetary positions: one measured positions relative to the Sun, and the other measured positions relative to the Earth. But not all astronomers correctly compensated for the different measuring systems. Looking at old measurements of Neptune's position relative to Earth and comparing them to new measurements relative to the Sun, some American astronomers mistakenly thought that perhaps there was a wobble in Neptune's orbit similar to that found nearly a century earlier in Uranus' orbit. This led some to suggest that another planet might exist beyond Neptune. During a search for such a planet, in 1930, American astronomer Clyde Tombaugh discovered a tiny speck that was eventually given the name Pluto (a name suggested by Venetia Burney, a girl in an elementary school in England). At the time of discovery, only rocky worlds and gas giants were known. Pluto was small, so it was no gas giant, so astronomers assumed it was rocky. No telescope on Earth at the time could tell just how small Pluto might be. Since rocky bodies like Mercury, Mars or asteroids reflect a small percentage of the light that falls on them, astronomers had to estimate Pluto's size based on its brightness. The guess, based on an assumption that is made of rock, made Pluto nearly the size of Mercury or Mars — clearly planet-sized.

But, right away, astronomers noticed that something was wrong with Pluto. Most planets orbit the Sun with orbits that are almost in the plane of the Sun's equator. Pluto's orbit, however, is tilted with respect to the orbits of the eight other planets. Furthermore, Pluto's orbit is among the most elliptical in the solar system, coming closer to the Sun than Neptune for about a decade each of its 248 year-long orbit. Much larger telescopes still failed to measure its size, showing that it was smaller than anyone had guessed in the 1930s. Eventually astronomers were able determine the composition of Pluto. Instead of being rocky, Pluto is icy. Ice reflects far more light than rock, so Pluto turned out to be much, much smaller than first thought. Modern measurements of Pluto put it at about 1,450 miles across — a bit larger than the distance across Texas. This is only a little larger than the largest asteroid. Thus, many astronomers, myself included, argued that Pluto really didn't fit with the rest of the planets. It is only slightly larger than an asteroid. It is not a gas giant planet, nor is it a rocky planet. I told my students from the mid-1980s onward that it simply did not fit with the rest, and would never have made the list of planets had its true size and composition been known right away. Astronomers are a rather conservative lot, so once on a list of planets, Pluto was destined to remain on the list unless there were good reasons to remove it.

That reason came in the late 1990s though. Better equipment led to the discovery of dozens of icy objects beyond Neptune with orbits similar to that of Pluto. Many of these objects were nearly the same composition as Pluto and only slightly smaller. It didn't seem to make sense for Pluto to be a planet, but an object almost identical except for half the size to not be a planet (for comparison, Mars is nearly half the diameter of Earth and Mercury is even smaller, so half the size is plenty close enough to be the same class of object). Eventually, in 2005, an object like Pluto, but larger was discovered (later to be named Eris). Thus, if Pluto were a planet, so should Eris be a planet. And, if both Pluto and Eris are planets, then about a dozen similar but only slightly smaller objects are also planets. Likewise, Ceres, Vesta and a number of larger asteroids would once again regain planet status due to being the same size. In fact, they'd have an even better claim to planet status since they are rocky like the four inner planets. This is what led the International Astronomical Union to finally define planet in terms of a body large enough to have sufficient gravity to perturb all similar sized objects out of its orbit. That leaves Pluto, Eris, Ceres, and the others off of the list. But, the International Astronomical Union did create a class of object in-between planets and asteroids, called dwarf planets. Dwarf planets are objects large enough for gravity to pull them into roughly spherical shapes, but they are not large enough to clear their orbits of similar sized objects. Currently, there are five recognized dwarf planets: Ceres, Pluto, Eris, Makemake and Haumea. Another half-dozen bodies are almost certainly dwarf planets, but we are awaiting final confirmation as to their size and shape, and about a dozen more are possible candidates, depending upon what we learn about them.

When people talk about wanting Pluto to be a planet, they are also lobbying for all of these other objects to be planets. I often give presentations at schools, and children seem to like Pluto for some reason, so many of them insist that they want Pluto to be a planet again. But, when I tell them that instead of memorizing eight planets without Pluto, they would have to memorize perhaps twenty planets with Pluto, these children almost universally decide that Pluto is definitely not a planet! And, recall that Pluto was not the first planet to be kicked off of the list. Ceres, Pallas, Juno and Vesta were kicked from the list about a century and a half earlier.

But, what does this mean about the New Horizons mission? If Pluto is not a planet, is it still relevant to study? Was the New Horizons mission, launched months before Pluto was booted from the list of planets, a waste of time and money? Surprisingly, the answer is that our current understanding of Pluto and the asteroids makes the study of Pluto even more relevant than if it were simply a tiny planet.

Astronomers have debated for many years how planets form. The current best understanding is that the cloud of gas surrounding the forming Sun (the disk is called a proplyd) provided the raw material for planets to form. Jupiter and Saturn probably formed straight from the proplyd. However, the other planets had a different mechanism for formation. Small clumps of material ran into one another to make larger bodies. Eventually the largest of these bodies began to resemble planets themselves, and we would call such an object a planetesimal. Most of the planetesimals then ran into one another to make the planets. Near the forming Sun, where it was warmer, most of the planetesimals were rocky, so the planets formed would be rocky. Far from the forming Sun, it was cold enough for ice to be stable, so the planetesimals were composed of both ice and rock. Ice and rock mixed together allowed for planets such as Uranus and Neptune to gather enough mass to pull in hydrogen and helium and to become much larger than the inner planets. Eventually, though, the orbits of the largest planets, Jupiter and Saturn, shifted. As Jupiter shifted inwards, the process of planet formation stopped, leaving a number of bodies only partially formed. These are the asteroids. A few leftover planetesimals in the asteroid belt are the largest bodies there: Ceres, Vesta, etc. Currently NASA's Dawn spacecraft is studying these relics from the formation of the solar system.

But, as Saturn shifted its orbit outward, Neptune and Uranus shifted even farther outward, hurling vast numbers of icy objects far beyond Neptune. These objects form what we know as the Kuiper Belt. The smaller Kuiper Belt objects, if their orbits get disrupted, dive close to the Sun where the ice in them becomes unstable and blows off to form a tail. This is one of the ways that we get comets! But, the largest of the Kuiper Belt objects, including Pluto, Eris, Sedna, Orcus, Makemake, Haumea, and several others are now thought to perhaps be icy planetesimals of the outer Solar System that got kicked outward before they got a chance to run into Uranus or Neptune to make those worlds even bigger. This makes Pluto even more interesting than just a tiny icy planet. Pluto is a leftover object from the earliest days of the solar system, dating perhaps even from before the current planets formed. It has been in a deep freeze far from the Sun for billions of years, and now New Horizons is about to pass by Pluto in July 2015. So, instead of simply studying the smallest planet, New Horizons is going to be studying a body that is key to the formation of the the planets themselves! After passing Pluto, New Horizons will continue onward to study other, smaller, Kuiper Belt objects in the years to come.