14 DAY TRIAL // The International Astronomical Union definition of what a planet is has attracted intense criticism from some astronomers. The official definition holds that "a planet is 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 neighborhood around its orbit."
The controversy involves the last criterion. What does it mean that the body has "cleared the neighborhood around its orbit"?
Alan Stern, who heads NASA's New Horizons mission to Pluto and works at the Southwest Research Institute in Boulder, Colorado, US, told New Scientist that "the definition introduced is fundamentally flawed. As a scientist, I'm embarrassed." According to Stern, neither Earth, Mars, Jupiter nor Neptune have cleared their neighborhood (see image and this article). "This is a half-baked criterion for planethood," he says.
But other astronomers point to the following error in Stern's argument: Although Earth, Mars, Jupiter and Neptune have not completely cleared their neighborhood of other bodies, the amount to which they've done it puts them in an entirely different class from asteroid Ceres or Kuiper Belt object Pluto.
Steven Soter has devised a quantitative measure of the degree of clearing. He writes: "A planet is an end product of disk accretion around a primary star or substar. I quantify this definition by the degree to which a body dominates the other masses that share its orbital zone [i.e. bodies that cross each others' orbits]. Both theoretical and observational measures of dynamical dominance reveal a gap of five orders of magnitude separating the eight planets of our solar system from the populations of asteroids and comets."
What this means is that each of the eight planets accounts for 99.98% or more of the mass in its zone, whereas Ceres accounts for only 25% and Pluto or "Xena" for even less.
Thus, considering Pluto a dwarf-planet (which is to be understood as a unified concept - "dwarf" is not an adjective to "planet") has a definite physical meaning and reflects the processes involved in its formation. Pluto and "Xena", as well as Ceres, are a part of a group of many objects, unlike the planets. The story of how these bodies have formed is different from the story of how planets have formed.
"We made a mistake 76 years ago," says Mike Brown, an astronomer at the California Institute of Technology who discovered several massive objects in orbits beyond Pluto's. "I thought that people would be too scared to demote Pluto. It is the right scientific decision."