The biggest astronomical debate of the young millennium culminates this Thursday at International Astronomical Union's (IAU) meeting in Prague, where national representatives will give thumbs up or down to IAU's latest planet definition proposal.

Last Wednesday, the IAU Executive Council (EC) offered a controversial planet definition which would confirm Pluto's planetary status, and instantly promote Pluto's moon Charon, the asteroid Ceres, and the newly-discovered UB313 to planets as well. This proposal was voted down by a wide margin two days later in an internal vote open exclusively to the planetary community. It lost 60% of the votes to an alternative definition proposed by Gonzalo Tancredi and Julio Fern?ndez. But the real vote comes this Thursday.

Although the EC's original proposal seems simple and based on physical concepts (a planet is massive enough to be rounded by its gravity, orbits a star, and is neither a star nor a satellite of a planet), the definition stirred controversy among the planetary community. The challenge: under the new definition, there could soon be dozens of new 'planets' in our solar system. That struck many astronomers as the wrong result.

At the same time, two important papers dealing with the planethood definitions appeared on the online preprint site arXiv.org. In the first paper, Dr. Steven Soter (Dept. of Astrophysics, American Museum of Natural History, NY) detailed the concept of the gravitational dominance in the orbital zone of the planetary body, while Croatian scientist Dr. Bojan Pečnik (Dept. of Physics, Univ. of Split, Croatia) argued that the necessary planethood criterion should be ability to keep an atmosphere.

The most popular alternative proposal, by Tancredi and Fern?ndez, requires the planet to be by far the largest body in the local population, and massive enough to be round. A body rounded by its own gravity but accompanied by others of similar size would not qualify and be called something else. This would kill the planetary aspirations of Pluto, along with other trans-neptunian objects (TNOs), and all asteroids.

Reasoning similar to the alternative proposal was applied by Steven Soter in a work submitted to the Astronomical Journal. Soter argues that "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."

Croatian scientist Dr. Bojan Pečnik agrees, saying "Gravitational dominance in one's orbital zone around a star or stellar remnant should be required to be a planet, but a planet should also posses an intrinsic physical property, irrespective of the dynamics of its environment".

Currently, the most popular property is roundness caused by object's own gravity. The problem with that definition is that celestial bodies can be made round through processes other than those considered by the EC definition. For example, violent kinetic event can shatter a potato-shaped asteroid into the spherical rubble pile (which is what most likely happened to Dactyl, a 1 kilometer moon of the asteroid Ida), or an iron meteorite can experience primordial melting and solidify into a sphere.

"Discriminating roundness caused by gravity from roundness due to cosmogony might prove difficult for objects on the border between major and minor planets, especially for the bodies far-out in the Kuiper Belt, or even more so for exoplanets", continues Pečnik.

That's why Pečnik prefers another physical property: ability of the body to keep its atmosphere against the vacuum of the interplanetary space. In previous work, Pečnik developed a new concept needed to quantify his criterion. This was a critical step, because earlier attempts to associate an atmosphere with planethood failed - the criterion was not quantifiable, nor it was possible to discriminate dilute atmospheres from vacuum.

According to Pečnik's work submitted to Journal of Planetary and Space Science's special issue on exoplanets, "only the ability to hold the atmosphere is required, not the atmosphere itself." Therefore, Mercury would likely, although marginally, keep its planetary status.

With so many things happening at the same time, all bets are off what will happen on Thursday in Prague. Only one thing is certain: the solar system will never be seen the same way.

George Whitesides is executive director of the National Space Society (NSS).

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