A New Definition Would Add 102 Planets To Our Solar System -- Including Pluto (washingtonpost.com)

The Grim Reefer quotes a report from The Washington Post: Is Pluto a planet? It's not a question scientists ask in polite company. "It's like religion and politics," said Kirby Runyon, a planetary scientist at Johns Hopkins University. "People get worked up over it. I've gotten worked up over it." For years, astronomers, planetary scientists and other space researchers have fought about what to call the small, icy world at the edge of our solar system. Is it a planet, as scientists believed for nearly seven decades? Or must a planet be something bigger, something more dominant, as was decided by vote at the International Astronomical Union (IAU) in 2006? The issue can bring conversations to a screeching halt, or turn them into shouting matches. "Sometimes," Runyon said, "it's just easier not to bring it up." But Runyon will ignore his own advice this week when he attends the annual Lunar and Planetary Science Conference in Houston. In a giant exhibit hall crowded with his colleagues, he's attempting to reignite the debate about Pluto's status with an audacious new definition for planet -- one that includes not just Pluto, but several of its neighbors, objects in the asteroid belt, and a number of moons. By his count, 102 new planets could be added to our solar system under the new criteria. USA Today reports: "In the mind of the public, the word 'planet' carries a significance lacking in other words used to describe planetary bodies," the proposal states. "In the decade following the supposed 'demotion' of Pluto by the International Astronomical Union, many members of the public, in our experience, assume that alleged 'non-planets' cease to be interesting enough to warrant scientific exploration."

Comments

The term "planet" is arbitrary, whatever we define

[Sique]

Originally, the planets (greek: wanderers) were those objects in the sky that didn't remain fixed in the stellar constellations, but actually wandered through them. Thus, Sun and Moon were considered planets too, and besides them, five other objects were constantly visible to the bare eye with no fixed place: Mercury, Venus, Mars, Jupiter and Saturn. And because they were seven, and seven was considered by the ancient cultures of the Mediterran to be a holy number, everything was fine. (Occasional comets which aren't constantly visible were thus considered shakeups of the celestial order and taken as bad omens.)

And then Ptolemy's geocentric model put Sun and Moon in a special group, because differently than the other planets, they never change direction in the sky, which the others do. Thus, the trajectories of Sun and Moon were easy, while the other planets needed cycles and epicycles to describe. This was one of the reasons, Nicolaus Copernicus came up with the heliocentric model, because then it made sense why Sun and Moon were "circular" wanderers, while the other planets were "epicyclic" wanderers, So, Sun and Moon were no longer considered planets, a position already shaky in the Ptolemian model. But it added Earth as a new planet. Copernicus' system didn't come up with good predictions of the planetary positions though, thus it wasn't widely accepted and even considered heretic by the Catholic Church. Johannes Kepler improved on the predictive power of the Copernican system, but Ptolemy's model was so finely tuned by now that it still was preferred for practical reasons. Galileo Galilei's discovery of the Iovian Moons gave credibility to the Keplerian model, but for navigational and other purposes, Ptolemy was still more exact. And it created a new class of celestial bodies: Suddenly, there wasn't one Moon, there were several moons out there. From a classical point of view, all moons were planets too: no fixed positions within the stellar constellations. At the end of the 17th century, Isaac Newtons Theory of Gravity gave a better model, Ole Roemer's discovery of the Speed of Light added some clues, and finally, the heliocentric model was better at predicting planetary (and lunar) positions than Ptolemy. But then a flood of new discoveries of celestial bodies clouded the view again: Uranus, Ceres and finally Neptune were discovered, and then all the other asteroids circling the Sun between Mars and Jupiter. Somehow the size of the Earth moon was used as a cut-off: Everything larger than the Moon circling the Sun was considered a planet, everything else an asteroid (which literally means "star like"). It was as arbitrary as anything else, but the Moon was close by and well studied, so for practical purposes, it made sense.

When Pluto was discovered, it became planetary status, because at first, its size could not be determined from direct observation, only because of the brightness (15 mag), it was at first considered to be Earth sized. So it got the planetary status. Later there were better pictures with larger resolution, and the estimated size shrank down to ~2500 km in diameter, and in the same way, the estimated reflectiveness (albedo) increased, so in the 1980ies, Pluto was considered a "dirty snowball", consisting mainly of water ice mixed with planetary rock. Thus the cut-off point "Moon size" was crossed, and doubts about Pluto's nature as a planet arised. It was speculated that it was a former Neptune moon losing its orbit. And when the next transneptunian objects were discovered, like Eris, with about the same size than Pluto, the whole "what is a planet" question became virulent. Simple enumeration as in "The planets are those nine celestial objects we call Mercury, Venus, Earth, Mars, Jupiter, Saturn, Neptune and Pluto" didn't work anymore, and a meaningful definition which included Pluto, but not too many other newly discovered objects, wasn't readily available.

Interesting comment from last time

[RCourtney]

The last time this was discussed on slashdot there was a very interesting series of comment posted by Rei that shed a lot of light on what the issues surrounding this are and how the situation (and redefining what a planet is) came to be.

Buried the Lead

[PMuse]

The proposed definition can be found at words 765-799 of the article.

A planet, he says, is anything massive enough that gravity pulls it into a sphere (a characteristic called “hydrostatic equilibrium"), but not so massive that it starts to undergo nuclear fusion and become a star.

The preceding 764 words are a useless regurgitation of how people feel about definitions in general and Pluto in particular. Spare me.