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Questions
1. What implications does Pluto's demotion have on our understanding of the Universe?
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Pluto
Pluto has been considered a planet since its discovery in 1930. Under the new guidelines, it's now considered a "dwarf planet," leaving eight planets — Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
Pluto, which is smaller than Earth's moon, doesn't fit the new criteria for a planet: "a celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a … nearly round shape, and has cleared the neighbourhood around its orbit."
Pluto doesn't qualify because its orbit is inclined relative to the rest of the solar system and crosses over the orbit of Neptune.
Not first time 'planet' demoted
It wasn't the first demotion for a body formerly considered a planet.
Ceres was considered a planet when it was first discovered in 1801.
However, after the size of Ceres was determined and other bodies were found in a belt between Mars and Jupiter, it lost its status. The term "asteroid," meaning "star-like," was coined to describe them.
Three other "planets" became "asteroids" at the same time.
Among other implications, Thursday's new definition means students will have to abandon current mnemonic devices that helped them remember how the planets are arranged in order from the sun.
For example, "Mark's Very Extravagant Mother Just Sent Us Ninety Parakeets" helped them recall that the order was Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.
Perhaps they can now return to the pre-1930 mnemonic: Mary's Violet Eyes Make John Stay Up Nights.
Report on Pluto
"Pluto is dead," said Caltech researcher Mike Brown, who spoke with reporters via a teleconference while monitoring the vote. The decision also means a Pluto-sized object that Brown discovered will not be called a planet.
"Pluto is not a planet," Brown said. "There are finally, officially, eight planets in the solar system."
The vote involved just 424 astronomers who remained for the last day of a meeting of the International Astronomical Union (IAU) in Prague.
"I'm embarassed for astornomy," said Alan Stern, leader of NASA's New Horizon's mission to Pluto and a scientist at the Southwest Research Institute. "Less than 5 percent of the world's astronomers voted."
"This definition stinks, for technical reasons," Stern told SPACE.com. He expects the astronomy community to overturn the decision. Other astronomers criticized the definition as ambiguous.
The resolution
The decision establishes three main categories of objects in our solar system.
Planets: The eight worlds from Mercuryto Neptune.
Dwarf Planets: Pluto and any other round object that "has not cleared the neighborhood around its orbit, and is not a satellite."
Small Solar System Bodies: All other objects orbiting the Sun.
Pluto and its moon Charon, which would both have been planets under the initial definition proposed Aug. 16, now get demoted because they are part of a sea of other objects that occupy the same region of space. Earth and the other eight large planets have, on the other hand, cleared broad swaths of space of any other large objects.
"Pluto is a dwarf planet by the ... definition and is recognized as the prototype of a new category of trans-Neptunian objects," states the approved resolution.
Years of debate
Astronomers have argued since the late 1990s, however, on whether to demote Pluto. Public support for Pluto has weighed heavily on the debate. Today's vote comes after a two-year effort by the IAU to develop a definition. An initial comittee of astronomers failed for a year to do so, leading to the formation of the second committee whose proposed definition was then redefined for today's vote.
Astronomers at the IAU meeting debated the proposals right up to the moment of the vote.
Caltech's Mike Brown loses out in one sense. The Pluto-sized object his team found, called 2003 UB313, will now be termed a dwarf planet.
"As of today I have no longer discovered a planet," he said. But Brown called the result scientifically a good decision.
"The public is not going to be excited by the fact that Pluto has been kicked out," Brown said. "But it's the right thing to do."
Textbooks will of course have to be rewritten.
"For astronomers this doesn't matter one bit. We'll go out and do exactly what we did," Brown said. "For teaching this is a very interesting moment. I think you can describe science much better now" by explaining why Pluto was once thought to be a planet and why it isn't now. "I'm actually very excited."
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