Many Cornellians grew up using the phrase “My Very Excellent Mother Just Served Us Nine Pizzas” to remember the order of the planets. However, the next generation of students may have to think of a new mnemonic device.
Pluto, formerly the ninth planet, was reclassified as a “dwarf planet” in an Aug. 24 resolution by the International Astronomical Union. This decision has sparked a battle between scientific rigor and historical sentimentality, igniting controversy among astronomers.
Some Cornell astronomers are disappointed by what they see as Pluto’s demotion.
Prof. Steve Squyres ’82, astronomy, wondered “why they couldn’t grandfather [Pluto] in for historical and sentimental reasons. … I don’t see why sentiment doesn’t have a place in science,” he said.
Prof. Jim Bell, astronomy, was more upset with how the vote was conducted. Only the members of the IAU who were present at their August meeting in Prague were allowed to vote, which meant that some 8,500 of the organization’s approximately 9,000 members had no say in the decision. Bell thinks that the decision will almost certainly be overturned at the IAU’s next meeting in three years.
On the other hand, Prof. Jean-Luc Margot, astronomy, is ecstatic about the decision. He thinks that it is “a triumph of science over emotion. Science is all about recognizing that earlier ideas may have been wrong,” he said.
Furthermore, he stressed that science should not conduct itself based on public opinion. People may be nostalgic for Pluto but that shouldn’t interrupt the progress of ideas. A generation from now, “[people] won’t even notice,” Margot said.
As for the argument that the IAU disenfranchised a large percentage of its members, he said that the procedure followed in August is nothing new. The IAU always resolves issues like these with a vote including whoever is at the meeting, and it was very clear when the vote would be and what would be discussed.
“Most [of the astronomical community] sided with the more reasonable definition,” Margot concluded. “Pluto is finally where it belongs.”
All three astronomers agree, though, that the decision will have no effect on how scientists conduct planetary astronomy.
Doubts regarding Pluto’s status were brought to the foreground in July 2005, when Prof. Mike Brown, planetary astronomy, California Technical Institute, announced his discovery of what he called a “tenth planet” in our solar system, nicknamed Xena. Xena is about 25 percent larger than Pluto and three times farther away, and both are found in the Kuiper Belt, a small-object-filled area of the solar system past Neptune. With the discovery of a Kuiper Belt object bigger than Pluto came the inevitable question: is Pluto really a planet? Is Xena? Is Ceres, the biggest asteroid in the asteroid belt between Mars and Jupiter? What exactly is a planet, anyway?
The IAU’s decision was the first-ever official definition of the term “planet,” first coined by the Greeks from the word for “wanderer.” The new, more stringent criteria for planethood include the stipulation that the planet “has cleared the neighborhood around its orbit.” This essentially means that to be a planet, an object must have enough gravitational oomph to either incorporate all other nearby objects into itself, whether as part of the planetary mass or as satellites, or to repel them away. When Pluto was first discovered by Clyde Tombaugh in 1930, astronomers didn’t know how many neighbors it had. It’s now obvious that Pluto has a lot of company: over 800 Kuiper Belt objects have been discovered thus far, and almost all of them were found after 1992.
From now on, objects like Pluto, Xena, and Ceres will be known as “dwarf planets,” a new category introduced by the IAU whose criteria differ from that of a full-fledged planet only in how thoroughly the object has cleared its surrounding area of other objects and space debris.