Posted: August 13, 2008
In 2006, the International Astronomical Union (IAU) reclassified Solar System objects such that Pluto became a dwarf planet, and in June of this year it was placed into the ‘plutoid’ sub-category (see our report Pluto assigned plutoid tag in new IAU classification for more details). Now astronomers have reopened the debate at this week’s Great Planet Debate: Science as a Process conference in Maryland.
Astronomers are, once again, debating the nature of Pluto and its place in our Solar System. Image: IAU.
This Thursday, Mark Sykes, the director of the Planetary Science Institute (PSI), and astronomer Neil deGrasse Tyson of the American Museum of Natural History and host of NOVA Science Now, will debate the IAU's actions in reclassifying Solar System objects into planets, dwarf planets and plutoids, as a part of the three day Great Planet Debate: Science as a Process conference at the Johns Hopkins Applied Physics Laboratory in Maryland. But despite the hype surrounding Pluto, and whether or not the little planet should be considered as part of the grown-up planetary gang or not, the scientists are more concerned about the effect the ongoing debate is having on the public’s perception and understanding of science.
"The IAU damaged the public perception of science by the high-profile spectacle of imposing, by vote, a controversial definition of a commonly used term," says Sykes. "Too often, science is presented as lists of facts to be learned from authority, instead of the dynamic open-ended process that it really is. The IAU reinforced this misconception of science".
The debate is sparked by the IAU definition of a planet, which is summed up in three points as a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. But it’s this last point that is causing a lot of confusion.
"The public and even many scientists are confused about what this means, and wonder if asteroids crossing the orbits of Earth and even Jupiter compromise their planetary status because these are objects that have not been cleared from their orbits," says Sykes. In order to clear its orbit, an object needs to be more and more massive the farther it is from the Sun.
Sykes is a strong proponent of the 13 planet Solar System, seeing Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon, Eris, and recently discovered Makemake all as ‘proper’ planets, since they are all round, and all orbit the Sun. Moreover, Pluto and Charon would be considered a double planet because they both satisfy the criterion for being massive enough to be round, and therefore are fundamentally different from small, irregularly shaped asteroids and comets. However, the IAU have an eight-planet Solar System: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune, with Ceres, Pluto, Eris and Makemake as dwarf planets, and the denomination of plutoid assigned to those dwarf planets orbiting the Sun beyond the orbit of Neptune.
The eight largest trans-neptunian objects in the Solar System, to date. Eris, Pluto and Makemake are currently classified as dwarf planets, and also fall into the sub-category of plutoids, since their orbits fall outside that of Neptune. The other bodies shown in this graphic await classification as dwarf planets (and plutoids), or will remain under the heading of 'trans-neptunian objects' or more simply 'Kuiper belt objects'. Image: NASA.
"The IAU definition, better worded, may be useful for identifying those objects that gravitationally dominate their vicinity, or perhaps those objects that managed to sweep up most of the mass of the primordial disc of the Solar System, but it does not help us understand which objects orbiting the Sun should have similar physical characteristics," continues Sykes. "And it is those physical characteristics that we go to study with spacecraft."
Some new understanding of the differences between our Solar System's planetary inhabitants and their defining characteristics will come from NASA’s New Horizons mission, which is due to arrive at the Pluto system in 2015, and the Dawn spacecraft, which will begin studying inner Solar System dwarf planet Ceres. No doubt the ‘what makes a planet a planet’ debate will begin all over again then.