Lyon professor says designation of Pluto as a dwarf planet
gives a definition 'consistent with new discoveries'

September 5, 2006

Dr. David Thomas, Lyon College’s associate professor of microbiology, said Pluto’s recent demotion from planet to dwarf planet may have shrunk our solar system from nine to eight planets, but it expanded our definitions of what a planet is.

“In the planetary community, we’ve always seen Pluto was different,” Dr. Thomas said. “It’s chemical composition is more like a comet than a planet.”

The traditional definition of planet is an object orbiting a star that is not a brown dwarf, but bigger than an asteroid.

Dwarf planets, the newly defined classification, are objects that orbit the sun and are bound by self-gravity, yet have not cleared the neighborhood around their orbits. Since that definition includes Pluto, the official number of “classical” planets has fallen from the traditional nine to eight.

Asteroids are naturally formed solid bodies that orbit the sun, have no atmosphere and no signs of gas or dust coming from them. Most are found in orbit between the orbits of Mars and Jupiter, and are made of rocky, iron-nickel material.

Comets are small bodies of rock, iron and frozen water and gases that orbit the sun in elliptical orbits. As they get close to the sun the gas vaporizes leaving a tail of dust and debris.

Meteors are rock, iron and/or icy bodies entering Earth’s atmosphere, and a meteorite is any meteor that hits the ground.

“This has forced the planetary community into defining what a planet is,” Dr. Thomas said. “If we kept Pluto as a planet, we’d have a lot of other planets out there, because a lot of other (celestial bodies) qualify as planets.”

Planets have been demoted before, although not within living memory. Astronomers discovered the asteroid Ceres in 1801. Uranus had been discovered about 20 years earlier, so scientists designated Ceres as the eighth planet. They had no reason to suspect that they had discovered a new class of celestial object.

More asteroids were soon discovered and these, too were summarily designated as newly found planets. But when astronomers continued finding numerous other asteroids in the region, the astronomical community demoted Ceres and the others in the early 1850s and coined the new term “minor planet.”

Xena was discovered on January 8, 2005, at Palomar Observatory with the NASA-funded 48-inch Samuel Oschin Telescope. It’s a Kuiper-belt object like Pluto, but slightly less reddish-yellow in color, and is currently visible in the constellation Cetus.

California Institute of Technology planetary scientist Mike Brown and his colleagues in late September 2005 announced that Xena has at least one moon. This body has been nicknamed Gabrielle, after Xena’s blond sidekick on the television series.

Gabrielle is about 250 kilometers in diameter and reflects only about 1 percent of the sunlight that its parent reflects. Because of its small size, Gabrielle could be oddly shaped.

The designation of Pluto as a dwarf planet clears the way for proper identification and classification of other celestial objects in the future, Dr. Thomas said.

“This gives us a definition that will be consistent with new discoveries,” he said.