Planet Nine (and a reminder of what is and isn’t a planet)

The internet and news media are awash with news of “Planet Nine”. For the last few years astronomers have known that some of the objects orbiting the Sun beyond Pluto appear to be mysteriously aligned. One possible explanation for this is that there is a large body even further out and the gravity of that world is marshalling the objects inside it.

It is an exciting prospect, one that I wrote about in a magazine feature article several years ago. But while some media outlets are speaking as if Planet Nine has categorically been discovered, it hasn’t (for an excellent analysis of some of the problems with the idea, see here). But just as the prediction of a planet perturbing the orbit of Uranus led to the discovery of Neptune in the 19th century, this latest announcement is a call to arms for astronomers to scour the skies for a potential new world. We live in exciting times.

Pluto was reclassified as a dwarf planet in 2006 but not because of its size
Pluto was reclassified as a dwarf planet in 2006 but not because of its size

However, one thing I’ve noticed in the media reporting of the story is that they often refer to Pluto’s reclassification as a dwarf planet and explain that the reason for the change is that Pluto was too small. In turn they say that the potential Planet Nine – thought to be 10 times more massive than the Earth – would definitely be big enough to be classified as a planet.

This is such a common mistake that I hear it almost every week. Size has little to do with planets. Here’s a quick reminder of the resolution passed by the International Astronomical Union in 2006 on planets:

 

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

(c) has cleared the neighbourhood around its orbit.

 

 

A “dwarf 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

(c) has not cleared the neighbourhood around its orbit

(d)is not a satellite.

 
For condition (b) in both cases ignore all the scientific jargon and just read “round”. So to be a planet you must orbit the Sun, be round and be the boss of your orbit around the Sun.

It is because Pluto has not cleared the neighbourhood around its orbit that it was reclassified in 2006. Nothing to do with its size.