Pluto’s Back on the Planet List


I was delighted to read in USAToday that Pluto, a celestial snowball with a surface of methane ice 3.6 billion miles from the sun, might be making its way back into the solar system fraternity.

First discovered and classified as planet in 1930, Pluto was relegated to “dwarf-planet” status by the International Astronomical Union in 2006. They booted it out because there appeared to be a bunch of other big rocks just like Pluto out beyond the eighth planet (Neptune), all considered too puny to be called a planet.

Now, some scientists say that Pluto should be back.

Harvard science historian Owen Gingerich, who chairs the IAU planet definition committee, argued at a forum last month that “a planet is a culturally defined word that changes over time,” and that Pluto is a planet.

Another expert, Gareth Williams, associate director of the IAU’s Minor Planet Center, said that Pluto is not a planet, citing the official definition, which states that a planet is a celestial body that:

• Is in orbit around the sun.

• Is round or nearly round.

• Has “cleared the neighborhood” around its orbit, meaning it is not surrounded by objects of similar size and characteristics.

Williams said Pluto failed on that third qualification, since it had several other “dwarf planets” near it and also overlaps Neptune’s orbit at times.

Pluto is indeed on the puny side, with a radius close to 750 miles — about one-fifth the size of Earth’s.

The debate among Gingerich, Williams and Dimitar Sasselov, director of the Harvard Origins of Life Initiative, took place Sept. 18 with scientists, teachers and civilians watching. Two of the three, Gingerich and Sasselov, said Pluto should be a planet. A vote among audience members agreed.