Sorry, Pluto lovers. A recently published paper by Jean-Luc Margot of UCLA (as a Star Trek Next Generation fan, I had to give the author’s full name) proposes a mathematically rigorous way to define a planet. Pluto, for all its undeniably fascinating appeal—it just doesn’t make the cut.
The official body tasked with naming and defining astronomical objects is the International Astronomical Union (IAU), of which most people had never heard until 2006. That was when the IAU gave official sanction to what astronomers had known for years, that Pluto was qualitatively distinct from what we now think of as “classical” planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The decision prompted millions of people to mostly good-natured outrage. You mean my fourth-grade teacher lied to me? Why can’t those scientists get their story straight? It didn’t help that the proposed definition was both vague and confusing. Here is the original IAU definition.
“A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.”
Well, that first criterion is simple enough and explains why the moon (which orbits Earth) is not considered a planet. It does exclude any possible planets that orbit other stars, but we’ll get back to that.
The second criterion is hard to judge. How “nearly” round does the planet have to be? No one would argue against calling Jupiter a planet, but its fluid composition and rapid rotation flatten it at the poles and bulge it at the equator. It is certainly not perfectly round! The dashed line in the image below is a perfect circle.