“One should not need a teleportation device to decide whether a newly discovered object is a planet.” -Jean-Luc Margot
It was a harsh lesson in astronomy for all of us in 2006, when the International Astronomical Union released their official definition of a planet. While the innermost eight planets made the cut, Pluto did not. But given the discovery of large numbers of worlds in the Kuiper belt and beyond our Solar System, it became clear that we needed something even more than what the IAU gave us.
This diagram compares the sizes of the newly-discovered planets around the faint red star TRAPPIST-1 with the Galilean moons of Jupiter and the inner Solar System. All the planets found around TRAPPIST-1 are of similar size to the Earth, but the star is only approximately the size of Jupiter. Image credit: ESO / O. Furtak.
We needed a way to look at any orbiting worlds around any star and determine whether they met a set of objective criteria for reaching planetary status. Recently, Alan Stern spoke up and introduced a geophysical definition of a planet, which would admit more than 100 members in our Solar System alone. But how does this stand up to what astronomers need to know?
The orbits of the known Sednoids, along with the proposed Planet Nine. Image credit: K. Batygin and M. E. Brown Astronom. J. 151, 22 (2016), with modifications/additions by E. Siegel.
As it turns out, not very well. But the IAU definition needs improving, too, and modern science is more than up to the challenge. See who does and doesn’t make the cut into true planetary status, and whether Planet Nine — if real — will make it, too!