Scientist Proposes New Definition of a Planet

Greg Gerkens

An experienced special education teacher, Greg Gerkens taught science for five years in the Wyandanch Union Free School District in New York. There, Greg Gerkens helped children with different needs to understand concepts from many different areas of science, including astronomy.

For astronomy students at all levels, the definition of a planet is an increasingly complex concept. Common wisdom once held that planets were large, round bodies that orbited the Sun, but recent discoveries have revealed a number of other different bodies that fit that definition.

The first major shift in planetary definition to impact the public consciousness occurred in 2006, when the International Astronomical Union released an official statement, which said that to be a planet, a celestial body must orbit the sun, clear the area around the orbit, and be massive enough to maintain a nearly round shape. Under this definition, the former planet Pluto was demoted to dwarf planet status.

Now, a number of other celestial objects in orbit around the Sun and other Sun-like stars have caused astronomers to rethink that definition. In a new paper published in January 2018, astronomer Kevin Schalufman of Johns Hopkins University suggested that a planet should have a mass that is between 4 and 10 times that of Jupiter.

Schalufman made this suggestion after observing 146 solar systems, where he looked at both giant planets and celestial phenomena known as brown dwarfs, which tend to be larger than planets and smaller than stars. He noted that brown dwarfs get so big in size relative to planets because they are not composed of rocky elements from stars and other dense material. He suggested setting a limit on planet mass rather than size so as to more effectively identify brown dwarfs.