The Debate Over Pluto Continues
Science writer Alan Boyle will soon publish a book in which he outlines his objections to the 2006 ruling by the International Astronomical Union (IAU) that caused Pluto be classified as a dwarf planet rather than a planet. His new book is called, “The Case for Pluto: How a Little Planet Made a Big Difference,” available November 9.
The Case for Pluto: How a Little Planet Made a Big Difference
The focus of Boyles dispute is the IAU's addition to the planetary definition that a planet must have cleared the neighborhood of its orbit. The addition demoted Pluto to a new class of celestial bodies known as dwarf planets. By the IAU definition, a dwarf planet orbits the Sun, is large enough that its gravity has crushed it into a sphere, but it has not cleared debris from its planetary neighborhood.
In his book, Boyle claims the IAU’s definition is too narrow and he suggests that even Earth might not be considered a planet by the IAU's definition because it too hasn't completely cleared its orbit.
Other astronomers also make the argument that the decision on Pluto comes too soon. They note that technology is only just beginning to allow the discovery of planets the size of Earth and smaller around other stars. And some astronomers believe there's still a possibility of finding objects bigger than Mercury in our own solar system.
Boyle cites the example of the "Captain Kirk rule," offered by Principal Investigator Alan Stern of NASA’s New Horizons mission to Pluto and the Kuiper belt. Stern’s rule states that if you are looking out the window of your spaceship, you’d like to know whether something is a planet just by looking at it.
When Clyde Tombaugh discovered Pluto at Lowell Observatory in 1930, it was thought to be the Planet X that astronomers had expected to find beyond the orbit of Neptune. But in the decades that followed, astronomers hypothesized that Pluto might be just the first of a number of objects beyond Neptune.
Astronomers reasoned these objects would be planetary leftovers from the early formation of the solar system and that they could exist in abundance in a region of the outer solar system we now know as the Kuiper belt (Kuiper rhymes with “viper”). The Kuiper belt, sometimes called the Edgeworth-Kuiper belt, was discovered in 1992 and named in honor of the work done by Irish astronomer Kenneth Edgeworth (1880 – 1972) and Dutch astronomer Gerard Kuiper (1905 – 1973). In contrast to Bolye and his contingency, other astronomers argue that if Pluto was discovered today, it would be classified as a Kuiper Belt Object.
Jupiter’s gravity kept the region of rocky debris known as the Asteroid Belt from forming into a planet. In that same way, Neptune's gravity has kept the icy objects of the Kuiper Belt from combining into a large planet as well.
For over fifty years, Pluto was the only object of its type to be found. Even Boyle admits that Tombaugh was lucky in finding it. The second Kuiper Belt Object (KBO) wasn't found until the 1990s, but that discovery was followed by a rash of other new KBOs. In 2005, Astronomer Mike Brown discovered an object beyond Neptune even larger than Pluto, and the IAU was compelled to vote on a new official planetary definition rather than face the possibility of having the outer solar system full of new planets.
To learn more about Pluto and the Kuiper Belt, and to learn more about Boyles new book, check out these links:
The Case for Pluto: How a Little Planet Made a Big Difference. Author Alan Boyle
NASA’s New Horizons Mission to Pluto and the Kuiper Belt, Mission Home Page