New rules mean Pluto is no longer a planet

New rules are announced all the time for things like professional sports, driving and politics. Astronomy is no different. In fact, new rules are the reason why Pluto, which had planet status from 1930 to 2006, was demoted to the lesser level of trans-Neptunian object.

“When scientists sharpened the definition of a planet, they realized that Pluto has a lot more in common with the ‘Trans-Neptunian objects' than the regular planets,” said Dr. Geza Gyuk, director of astronomy at Chicago's Adler Planetarium.

In the early 1900s, a miscalculation led scientists to believe there was something beyond the eight planets in our solar system, so they scoured the skies to locate and identify this mysterious planet. Dubbed Planet X, this space object was presumably the cause of discrepancies in the orbits of Neptune and Uranus, causing them to tug or weave outside of a predictable orbit.

Percival Lowell, the benefactor for the Lowell Observatory at the University of Arizona in Flagstaff, hoped the discovery of Planet X would resolve the unexplained variations.

New on the job in 1930, Clyde W. Tombaugh, a 23-year-old astronomer at the Lowell Observatory, examined photos taken over time and quickly realized that beyond Neptune, an oversized space object existed at the outer reaches of our universe. He named it Pluto.

“When they found Pluto, it was natural to consider it a planet,” Gyuk explained. “As it turns out, the measurements of Neptune's orbit were incorrect. There weren't any unexplained tugs after all, and Pluto turned out to be vastly less massive than originally thought.“

This and other data led scientists to develop new rules for defining planets.

“In the 1990s and early 2000s, astronomers started discovering a whole bunch of other objects out there that were a lot like Pluto — strange orbits, about the same size, etc.,” Gyuk said. “But all of these couldn't be planets, could they?”

The International Astronomical Union, or IAU, is the authority that names and identifies objects in the solar system. The IAU confirms three requirements for planet status: the object must orbit around the sun directly; it has to be big enough so its gravity squeezes into a ball; and it has to have cleared out most other objects in its orbit.

“The eight major planets fall into this definition,” Gyuk said. “The asteroids don't, even if some are large enough to the spherical. Pluto and its gang share similar orbits so they aren't planets. Instead, the IAU has classified them as dwarf planets.”

There's more information, such as NASA's exploration of the planets, online at voyager.jpl.nasa.gov. Or visit the Adler Planetarium's “Our Solar System” exhibit, which reveals facts about space objects such as planets, comets, asteroids and trans-Neptunian objects.

See http://www.adlerplanetarium.org for admission times and exhibit information.