Plutoids? More Confusion in Astronomy

The scoop: The International Astronomical Union has re-cast Pluto and its scrawny planetary pals into a class called "plutoids," among other changes. But are scientists going to follow suit?

The planet controversy continues. Now the International Astronomical Union (IAU) divides up the solar system into planets, plutoids (with minimum diameters of 850 km) and Ceres. In addition, it defines "small solar system objects," which were formerly asteroids and comets. Many planetary scientists and even astronomers continue to ignore their pronouncements.

The IAU has embraced a 19th century view of the solar system, before spacecraft, before rovers on Mars, before space telescopes. The clock is turned back to a time where planetary studies focused on their orbital motions and the effect that the gravity of one object had on another. The intent of the IAU definition of the term planet was to restrict the class of objects to those that gravitationally dominate the vicinity of their orbits. It failed to craft a definition that made sense to the public and even most scientists, requiring an object to have "cleared its orbit" when a simple examination of Jupiter's orbit, for instance, would seem to disqualify it. So, people rely on the IAU's ex cathedra listing of planets. An unfortunate consequence of this is the misrepresentation of science as the promulgation of "truth" by an "authoritative" body.

Planetary science has undergone a revolution during the past several decades as spacecraft and landers have brought us increasingly detailed information about other worlds, particularly Mars. We are studying geological and atmospheric processes that we also study on Earth. As a consequence, there has been a merging between terrestrial and planetary science. Geophysical models developed to explain terrestrial processes are adapted to explain the manifestation of those processes under very different conditions elsewhere in the solar system. The result is that we have a better understanding of these processes on Earth. So, planetary scientists look out into the solar system with an eye towards organizing it in a way that is useful for their studies. They naturally focus on the common physical characteristics of objects -- a perspective that does not drive the IAU.

The geophysical definition for planet is "a round object (in hydrostatic equilibrium) that orbits a star." When an object becomes large enough for its own gravity to crush it into a round shape, it has reached the point at which it is expected to manifest geology. This broader definition allows for subcategories that would embrace the IAU's perspective. However, it also allows for many more planets to be discovered in our own solar system, which the IAU dreads. Their definition also sought to iconify the classical planets as special -- a last echo of an ancient tradition that associated them with gods.

We look forward to the IAU joining the 21st century, but if they are unable to move out of their comfort zone, it won't stop planetary scientists from moving forward.

Mark V. Sykes is the director of the Planetary Science Institute in Tucson, Arizona and is a member of the science team for Dawn, a spacecraft currently en route to study Vesta and Ceres in the Asteroid Belt. The views expressed are the author's alone and do not represent the official position of the Discovery Channel.

MORE EXPERTS