This week, three people have approached me wondering why Pluto isn’t a planet anymore. My brief explanation of “it doesn’t meet the characteristics of a planet” didn’t set too well; they grew up with Pluto, they want it back! They want their children to grow up with Pluto! I realized after these exchanges that although the fate of Pluto has been a news feature since it’s contestation began a little over a decade ago, not many people know the details behind the decision. Each of the people I spoke to this week suggested I explain the story in more complete detail on my blog. As so, by request: a brief history of the outer planets and the rise and fall of Pluto. (Left, Pluto.)
Mercury, Venus, Mars, Jupiter, and Saturn have been known for millennia. Bright and large enough to be seen from Earth with the naked eye, the Ancient Greeks recognized these planets every time they looked up to the night sky and saw wanderers among the fixed stars. In 1781, a seventh planet joined the solar system. Astronomer William Herschel observed the planet from his home garden in Bath, England using a telescope. By 1873, it was officially recognized by the Royal Society. Uranus became commonly accepted as a new planet. (Uranus as seen by the Hubble Space Telescope in 2007.)
In the early 1800s, astronomers plotting Uranus’ obit noticed inconsistencies; Uranus didn’t follow the orbit their mathematical models dictated it should. Without the technology to delve deeper into Uranus’ secrets, the only explanation was that some mass beyond the planet was exerting a gravitation pull strong enough to account for the observed deviations.
In 1845 and 1846, English astronomer John Couch Adams used Newton’s and Kepler’s laws to develop a mathematical model that predicted the orbit of a trans-Uranian planet. French astronomer Urbain Le Verrier independently predicted the same planet at the same time. Neither man, however, actually looked for their mystery planet. They each hired observatories to look for them.
Adams convinced Cambridge Observatory director James Challis to look for his predicted planet, but a lack of enthusiasm led Challis to miss the planet that turned up right in front of his face. Le Verrier had better luck. He convinced Berlin Observatory astronomer Johann Gottfried Galle to look for his planet. The very evening Galle received Le Verrier’s letter requesting his assistance, the astronomer found what is now Neptune in images previously taken at the Observatory. (Left, hurricanes on Neptune are clear in this image stitched together from tow pictures taken by Voyager 2.)
But astronomers weren’t entirely satisfied. Some still believed that Neptune alone didn’t explain Uranus’ orbit. The idea of a planet outside the orbit of Neptune – a trans-Neptunian planet – was a popular one. Using the same method of applying Newton’s and Kepler’s laws to predict the new planet, the hunt was on for Planet X.
In 1894, American astronomer Percival Lowell moved to Flagstaff, Arizona to set up an observatory – the high elevation of the small city offered clear air and the climate promised nearly year-round visibility. He had two goals – finding proof of intelligent life on Mars, and finding Planet X. Although he used models to predict Planet X’s location and search the right part of the sky, he never found it before his death in 1916. (Lowell at work at the observatory that still bears his name.)
The hunt resumed at the Lowell Observatory in 1929 when Illinois-born astronomer Clyde Tombaugh arrived. He used a blink comparator, a machine that alternated between two images of the night sky, to detect a moving object. Stars won’t move from night to night, but planets will. On February 18, 1930, Tombaugh found a tiny moving dot in two of his images. By May 1, his discovery of Planet X was accepted around the world and had been formally named Pluto.
Originally, Pluto was thought to be roughly the same size as Earth with an orbit well outside of Neptune’s. But as astronomers applied better and better technology to their study of the new planet, a very different world emerged. Importantly, it was one that didn’t agree with the other eight planets in the solar system. It’s an anomaly in almost every way.
From Mercury to Neptune, all the planets trace slightly elliptical but regular orbits on the same plane – they are all on the ecliptic. Their orbits also never cross. Pluto on the other hand traces out a much more pronounced ellipse, so much so that it spends twenty-two of its 200 year orbit inside the orbit of Neptune. It is also highly inclined to the ecliptic. It sticks out like a sore thumb. (Left, Pluto’s orbit in purple compared to the other eight planets.)
Pluto’s orbit wasn’t it’s only irregular feature. In 1978, better technology revealed previously unknown details about its size and composition. More detailed images revealed that it wasn’t as large as Earth at all. It had a moon, Charon, that looked like part of Pluto as seen by less sensitive instruments. Pluto was suddenly not only the smallest planet in the solar system, but smaller than seven moons. Jupiter’s Ganymede, Callisto, Io, and Europa; Saturn’s Titan; Neptune’s Triton; and our moon all dwarf Pluto. (Above, Hubble looks at Pluto in 2006. The dwarf planet is pictured with its moons Charon, Nix, and Hydra. It’s easy to see how Pluto and Charon could be misinterpreted as one planet.)
Pluto and Charon were revealed as an extreme case of tidal locking. Both bodies have one side that constantly face another. Many moons in the solar system are tidally locked to their planets – such as our moon – but Pluto is the only planet that is also locked to its moon.
It’s characteristics also set it apart. The eight other planets all fall into two groups: the four inner planets are rocky bodies while the four outer planets are gas giants. Pluto is neither. It has a small rocky core, a layer of water ice, and a surface of frozen nitrogen. It is an ice ball. It does have an atmosphere, but a scant one made up largely of nitrogen, methane, ethane, and carbon monoxide. The tidal locking between Pluto and Charon also affects this atmosphere; the side facing Charon has more methane while the side facing away from Charon has more frozen nitrogen and carbon monoxide.
A further oddity around Pluto was recognized not long after its discovery: it doesn’t explain the observed abnormal motions of Uranus and Neptune. This didn’t sit right with astronomers. Was there something else with mass out there? The idea that nothing at all resided beyond Pluto was equally hard to accept. How could the space beyond Pluto just be empty? (It turns out that the deviations were not due to another planet but miscalculations in the gas giants’ masses. Data gathered from the Voyager probes solved that mystery.)
Seemingly empty areas in space had been filled long before the empty space after Pluto became a problem. In the 1770s, Prussian astronomer Johann Daniel Titus worked out a mathematical formula to account for the distance between orbits of the planets. His Titus-Bode formula (Johann Elert Bode made one of the first uses of the formula so is partially credited) predicted a gap between Mars and Jupiter, and he didn’t believe it could be empty. Observations backed up the math, and his hunch. The gap was there and it wasn’t empty. At the turn of the 19th century, Chair of Astronomy at the University of Palermo Giuseppe Piazzi found a tiny moving object in an orbit within the Titus-Bode space. He named it Ceres, and it remains one of the largest objects in what has since been discovered as a very busy asteroid belt. (Left, a bird’s eye view of our asteroid belt.)
So couldn’t the same apply to Pluto’s orbit? Couldn’t there be other things out there, further things that jus needed to be found?
The idea of a trans-Plutonian “asteroid belt” had occurred to Ernst Opik. In 1932 he hypothesized that comets originated somewhere in space and that there were a lot of them. Jan Hendrick Oort took up Opik’s idea in 1950 and proposed a region of small icy bodies far from the Sun. Comets had been observed to lose material as they traveled near the Sun, a store of icy bodies explained why comets kept coming. They orbited peacefully until got pulled towards the inner solar system. The region has since been accepted and named the Oort Cloud. (Right, a diagram showing the Oort Cloud.)
But it didn’t answer the question of planets beyond Pluto, and so the hunt for the new Planet X resumed in the 1980s. In the 1990s, the effort began to pay off. In 1992, David Jewitt and Jane Luu at the Manu Kea Observatory in Hawaii found an object beyond Neptune. Its orbit is beyond Pluto’s, is less inclined relative to the ecliptic, and it does not cross Neptune’s orbit. It was the first trans-Neptunian object to be classified in the Kuiper Belt (KY-per), a region of small rocky-ice bodies that orbits close to but outside that of Pluto. In 2000, Robert MacMillan discovered Varuna, another object outside Neptune’s orbit that was also classified as a Kuiper Belt object.
But the discovery of Kuiper Belt and Oort Cloud object didn’t deal Pluto the fatal blow. Two men are credited with – or are responsible for, depending on your view – killing Pluto: Neil deGrasse Tyson, and Mike Brown.
In 2000, the American Museum of Natural History in New York City opened its new Frederick Phineas and Sandra Priest Rose Centre for Earth and Space – simply the Rose Centre – featuring the newly renovated Hayden planetarium. When it came time to organize the display of solar bodies, the museum curators grouped like objects together according to the five major types: the terrestrial planets, the Asteroid Belt, the Jovian planets or Gas Gaints, the Kuiper Belt and the Oort Cloud. Pluto didn’t fit the bill as a terrestrial planet or a Jovian planet, but it did fit with the Kuiper objects so that’s where it ended up. (Left, Neil deGrasse Tyson shows Pluto Pluto.)
A year later, visitors starting noticing something wrong with the giant scale model solar system hanging from the ceiling. Pluto was missing. The apparent discrepancy made the front page of the New York Times and a firestorm followed. As the centre’s director, Tyson took the hit as the man who demoted Pluto and became the target of the public response. People reacted strongly to the destruction of their middle-school astronomical memories. Mnemonics like “My Very Elegant Mother Just Served Us Nine Pizzas” became useless to a generation. Demonstrations were staged (right). Pluto’s status also became an emotional issue as the planet was anthropomorphized on t-shirts with slogans reading “Don’t worry, Pluto, I’m not a planet either.”
But Tyson didn’t change science books or grade school curriculum. His view of Pluto was safely in New York City. It was Mike Brown, an astronomer at the California Institute of Technology, who put the final nail in Pluto’s coffin.
Along with a small team of researchers, Brown found his first trans-Plutonian object in 2003 and named it Sedna. Observations suggested Sedna was about three-quarters less massive than Pluto with a highly elliptical orbit that it took about 12,000 years to complete. It was thought to be about three times as far from the Sun as Pluto, and it was an early contender for a tenth planet. But it wasn’t bigger than Pluto, so it joined the growing number of Kuiper objects. Browns continued to find more objects in the far reaches of the solar system, but none were bigger than Pluto so none qualified as a planet. (Left, an arrow points to Sedna in one of the images Brown used to discover the object.)
In 2005, Brown’s luck changed. He found an object larger than Pluto, orbiting well beyond Pluto’s orbit, larger than Pluto, and with its own moon. Temporarily named Xena, it was the first real contender for a tenth planet in our solar system.
It ultimately came down to the International Astronomical Union to decide if Xena was a planet or not. The problem was what this decision would do to the other large objects found in the solar system. If Xena was a planet, Sedna could be considered a planet as well. So could Varuna. So could Ceres for that matter, the large object in the Asteroid Belt. If the IAU decides that Xena wasn’t a planet, it would go down as another Kuiper Belt object and take Pluto with it. The two objects had too many similarities. What happened to one would likely happen to the other. (Right, Xena.)
The IAU’s decision was based on a members-only vote taken during the society’s 2006 General Assembly in Prague. The important resolution was on what made a planet a planet. With an agreed upon definition, classification of solar object would be much simpler and clearer. The IAU proposed solar bodies be classified as three types: planets, dwarf planets, and other, each with its own set of criteria.
Planets had to meet three criteria. A planet had to be big enough that its own gravity made it round, it had to orbit the sun and not be a satellite of another body, and it had to be massive enough to clear the area around it in space of any debris. Dwarf planets were also held to three criteria. A dwarf planet must orbit the sun, it must have sufficient mass such that it’s own gravity makes it nearly round, it must be too weak to clear debris from its surrounding area in space, and it must not be a satellite of another body. All other bodies in the solar system were held to a single criteria. As long as they orbited the sun and were neither planets nor dwarf planets, they would be known as “Small Solar System Bodies.” (Left, an artist’s concept of the view of the solar system from Sedna.)
This definitions of planet, dwarf planet, and “small solar system bodies” passed with a sweeping majority; the votes weren’t actually counted. The vote meant Xena was officially a dwarf planet, as it was subsequently renamed Eris; it’s moon, Dysnomia.
Pluto is not massive enough to clear its own area in its orbit. It failed the third criteria, making it a dwarf planet. But it had one last chance to stay in science textbooks with the other eight planets. A separate resolution was voted on to determine whether or not Pluto should stay a planet and be its own category: ice planet. (Right, an artist’s concept of Eris and Dysnomia.)
On the matter of Pluto officially designated as a dwarf planet because of its inability to meet the new “planet” criteria, the motion passed with 237 votes in favour, 157 against, and 17 abstentions. Just like that, the solar system was left with eight planets and a series of dwarf planets.
The case for Pluto’s demotion is hard to argue, but people still can’t let it go. In honour of Pluto’s discoverer Clyde Tombaugh (left), the state of Illinois passed a Resolution restoring its planetary status within the state. Popular culture has seized on the loss of a planet; t-shirts with a crying Pluto and cartoons depicting a dejected planet continue to circulate. But Pluto will never really go away. It’s story is a great one that will certainly make it stand out in middle school textbooks for generations to come.
Suggested Reading/Selected Sources
Mike Brown. How I Killed Pluto and Why It Had It Coming. Spiegel & Grau. 2010. I’ve synthesized and simplified Brown’s story; his book gives the full, dramatic details down to the fights between astronomers over who discovered Eris first.
Neil deGrasse Tyson. The Pluto Files. W. W. Norton & Company. 2009. The documentary of the same name is also worth watching.
Richard Corefield. Lives of the Planets. Basic Books. 2007.