Ever since 2006, when the International Astronomical Union (IAU) officially defined the term planet — introducing the term ‘dwarf planet’ to classify Pluto, Eris, Ceres and others — the scientific community has been split in two. Only you have enough mass to pull yourself into a spheroid, orbit the Sun and no other body, and can clear your orbit within Solar System timescales, can you be classified as a planet.
On the one hand are astronomers, mostly planetary astronomers, who largely like the IAU’s definition, but want to extend it to more general cases, including exoplanetary systems. On the other hand are planetary scientists and planetary geologists, who look at intrinsic properties only, and argue that if you can pull yourself into a spheroidal shape, you deserve to be a planet. But to an astrophysicist, both definitions are insufficient. Here’s why.