What is a Planet?

A last look back at Pluto from New Horizons.
Dwarf planet Pluto sends a breathtaking farewell to New Horizons. Backlit by the sun, Pluto’s atmosphere rings its silhouette like a luminous halo in this image taken by NASA’s New Horizons spacecraft around midnight EDT on July 15. This global portrait of the atmosphere was captured when the spacecraft was about 1.25 million miles (2 million kilometers) from Pluto and shows structures as small as 12 miles across. The image, delivered to Earth on July 23, is displayed with north at the top of the frame. NASA/JHUAPL/SWRI

What is a planet? Astronomers get this question a lot. It seems like a simple-enough thing to ask about, but in reality the definition of the term "planet" seems to be a moving target. It's also at the heart of a controversy that continues to spur debate in the astronomy community. To answer the question "what is a planet?" it helps to look back at the event that brought the issue to a head in 2006: the seeming demotion of Pluto from planet to "dwarf" planet.

Pluto: Planet or Not?

In 2006, the International Astronomical Union demoted a small ball of rock and ice out in the outer third of the outer solar system from planet status. Pluto became known as a dwarf planet. The outcry, both from within and outside of the scientific community, was astounding and the subject is still under intense discussions today. The planetary science community, which is probably the best equipped to help define the term, was overruled by astronomers (not many of them planetary scientists) at an IAU meeting where the issue came up for discussion and a vote. 

Why Define "Planet" At All?

The argument of course, is that our current clumping of all the round, non-star, non-moon objects in our solar system was not ideal. Clearly Mercury and Jupiter don't share much in common, yet they are classified as planets.

In 2000, the New York Hayden planetarium was renovated, and one of the exhibits grouped the planets by similar features.

This made them easier to study and understand. It also created a more coherent display with greater educational potential. However, it put Pluto as the odd world out.  That by itself didn't change the definition of "planet", however. The idea of planet was under discussion for a long time before that.

It continues to be an issue as scientists discover more and more worlds "out there". 

The 2006 decision by the IAU has been steeped in controversy among scientists, particularly those in planetary science who had not attended the meeting where a scant few astronomers voted on planetary status. However, beyond that gaffe, the greater point of contention is that the definition as arrived at by the IAU committee frankly doesn't even make sense.

What is the Definition of a Planet?

Let's look at what the IAU thinks a planet is. There are three requirements: 

  • a planet is a world that orbits the Sun and not another object;
  • a planet is an object with sufficient gravity to become round;
  • a planet is a world that has cleared its orbital path of debris.

This last one was thought to be a problem for Pluto, although recent discoveries by the New Horizons spacecraft show that there's not much to clear around Pluto, not even a ring! 

One could argue that Earth has not been able to completely clear its path of debris. However, no one is arguing with the classification of Earth as a planet. Effectively the IAU was placing a distance cap on how far a planet can be from its host star. And this just doesn't make sense.

So What Should the Definition Be?

OK, so the IAU's definition has problems, but it's still clear that the definition of "planet" needs more thought and work. It is important to classify objects, it's simply part of the scientific endeavour. Biologists classify life, while chemists classify compounds, and so on. But the means by which you classify the objects in a system needs to be cohesive and non-conditional.

So what about the planets, and Pluto specifically? What if we just took the first two conditions laid forth by the IAU and let it go at that: massive enough to be round, but not so much that it ignites nuclear burning? That would leave the eight objects that we already consider planets and add in the ones that we currently call dwarf planets.

It just so happens that Pluto is large enough that it formed itself into a sphere under the pressure of its own gravity.

 And, this fact is at the heart of the IAU's third condition for planet-hood. But that isn't the end of the debate either, and for now, officially, Pluto remains a dwarf planet. 

Edited and updated by Carolyn Collins Petersen.