Far from being unique, Pluto is one of many large Kuiper Belt objects (KBOs).
Credit: NASA
For thousands of years, the field of astronomy focused on making measurements and catalogues, often painstakingly built up over years or decades. But the modern astronomer's emphasis is now on understanding, rather than just measuring. How are stars born, why do they shine, and how do they die? How old is the universe, what is it made of, and what is making it expand? It's the need to answer these questions, not just to make pretty pictures of the sky, that motivates my colleagues and me.
But recently the focus of world astronomy has been back on measuring and labelling. This past week in Prague, thousands of astronomers gathered together for the general assembly of the International Astronomical Union. And amidst the debates about new scientific theories and the announcements of exciting discoveries, astronomers decided to change the number of planets in the Solar System.
This came about when a resolution passed that a planet should hereon be defined as a spherical object that orbits a star, and which has cleared the neighbourhood around it. The number of planets in the Solar System consequently drops from nine to eight; Pluto has now been demoted, and joins a variety of other small objects, such as Ceres, Sedna and Orcus, in a new yet-to-be-named category of 'dwarf planets'.
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Pluto as seen by the Hubble Space Telescope.
Image: NASA/Space Telescope Science Institute
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The new definition of a planet is simple and sensible. A celestial body can only be round if it is large enough for its gravity to pull it into a spherical shape. This excludes the vast majority of Solar System bodies, most of which are small misshapen asteroids. The requirement that a planet orbit a star also makes sense, since some of the moons of Jupiter and Saturn are otherwise big enough to qualify as planets. Finally, the key criterion that has demoted Pluto, that a planet must be able to clear out its neighbourhood, is based on our understanding that a newly forming planet around a young star clears out a large area around it as it sweeps up surrounding debris. While the first eight planets all rule their orbits, Pluto follows an unusual elongated path that crosses the orbit of Neptune. Clearly it does not make the cut.
Many people feel very passionate about Pluto's status as a planet, and will be disappointed or disbelieving that astronomers have voted to change this. But Pluto's status as a planet was only ever a historical accident. For decades, astronomers had been convinced that the orbits of Uranus and Neptune hinted at the gravitational influence of a massive, more distant body, "Planet X". When Clyde Tombaugh discovered Pluto in 1930, this exciting new object was quickly anointed as the ninth planet, before much was even known about it. But subsequent study showed that Pluto is puny, barely a quarter the size of our own Moon. And we now know that Planet X never existed, the product of some subtle errors in some century-old mathematical calculations.
So Pluto was only a planet because it was in the right place at the right time. Demoting this tiny lump of rock was a good decision. I encourage all Pluto-lovers to have a look at Pluto for themselves. Even through the largest telescope, it is barely indistinguishable from the stars around it. Compare this to the spectacular sight of Jupiter's moons and cloud bands, or the majesty of Saturn's rings. When one factors in the presence of over 200 planets now known around other stars, all with their own weather systems, moons, rings and other features waiting to be studied and understood, it's clear that it was time for little Pluto to step aside.
Bryan Gaensler is an astronomer and Federation Fellow at the University of Sydney, and a member of the Editorial Advisory Board of Cosmos.
Pluto
Good new discripion of a planet
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Thank you
What a simple but elegant explaination of the downgrading of Pluto. My son (5 years old) has discovered an interest in the Solar System and asked me about this recently. Now I can tell him the facts.
Thank you.
Pluto's demotion
Now the logic for point three: that Pluto has not swept its orbit, then means that Neptune must be demoted. Both share in the fact of having crossing orbits. Moreover, are there not Trojan asteroids that lead and lag Jupiter and Neptune in their orbits? By that logic, these too must be demoted. The argument is therefore reduced to the elliptical orbit for point three being a "demotion" factor.
Pluto is a planet
Pluto should not step aside, as it is still a planet, not just a lump of ice. Only four percent of the IAU voted on the controversial demotion, and most are not planetary scientists. Their decision was immediately opposed in a formal petition by hundreds of professional astronomers led by Dr. Alan Stern, Principal Investigator of NASA’s New Horizons mission to Pluto. And you can find audio transcripts of the Great Planet Debate, held at the Johns Hopkins University Applied Physics Lab in August 2008, where both sides of this ongoing debate were discussed, by Googling "Great Planet Debate."
One reason the IAU definition makes no sense is it says dwarf planets are not planets at all! That is like saying a grizzly bear is not a bear, and it is inconsistent with the use of the term “dwarf” in astronomy, where dwarf stars are still stars, and dwarf galaxies are still galaxies. Also, the IAU definition classifies objects solely by where they are while ignoring what they are. If Earth were in Pluto’s orbit, according to the IAU definition, it would not be a planet either. A definition that takes the same object and makes it a planet in one location and not a planet in another is essentially useless.
Pluto is a planet because it is spherical, meaning it is large enough to be pulled into a round shape by its own gravity--a state known as hydrostatic equilibrium and characteristic of planets, not of shapeless asteroids held together by chemical bonds. These reasons are why many astronomers, lay people, and educators are either ignoring the demotion entirely or working to get it overturned. Using this broader definition gives our solar system 13 planets and counting: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Haumea, Makemake, and Eris.
These are among the reasons why both scientists and lay people worldwide or either working to overturn the demotion or are ignoring it entirely.