Physical Sciences

In 2006, the International Astronomical Union demoted Pluto to "non-planet" status. Johns Hopkins University scientist Kirby Runyon led a group of six researchers to draft a new definition of "planet" — one that includes more than 100 other celestial bodies, including Pluto. The proposal was presented at the Lunar and Planetary Science Conference, hosted in Houston, Texas on March 21.

Runyon and his team work on the New Horizons mission to Pluto, operated for NASA by Johns Hopkins University Applied Physics Laboratory.

Why is the definition of “planet” so important – and why is it so important to consider Pluto a planet?

Kirby Runyon: Definitions — and words in general — don't just affect how people communicate, but also how people conceptualize, organize, synthesize, and contextualize information. The word "planet" carries a connotation of importance for worlds that have enough gravity to assume a spheroidal shape but that aren't stars.

By grouping tiny satellite planets like Enceladus with giant planets like Jupiter, we can better conceptualize the vast range of diversity of planets on a continuous, sliding scale that doesn't have an arbitrary cut-off for worlds that don't have enough gravity to affect the orbits of smaller worlds around them. Also, note that I use the word "world" for both round and non-round objects.

So, it's important to consider Pluto a planet because it better facilitates comparison among other types of larger and smaller planets rather than seeing Pluto as a completely different type of world, or "non-planet.”

What should the criteria be for a “planet?”

Runyon: Criteria for "planet" will differ by scientist, or at least by scientific discipline. To orbital dynamicists, perhaps the IAU definition is sufficient: that definition deals with "zone" clearing and gravitational effects on smaller, irregularly-shaped worlds. For me, as a planetary geologist, roundness is the most useful criterion for a world to be a planet. This roundness criterion better facilitates communication, conceptualization, organization, synthesis, and contextualization about round worlds, which exist on a continuous sliding scale of size from Enceladus up to Jupiter.

How many “dwarf planets” are in the galaxy, do you think?

Runyon: Assume our Milky Way Galaxy has a conservative 200 billion stars. Assume, conservatively, that each star is a double star (two stars orbiting each other), so there are 100 billion possible planetary systems. Assume a conservative 100 dwarf planets in each system. That's 10 trillion dwarf planets in just our galaxy. If you assume 5 giant planets per planetary system, that's only 500 billion giants in the galaxy compared to 10 trillion dwarfs. Thus dwarfs outnumber giants 20 to 1.

What does the quantity of dwarf planets tell us?

Runyon: That tells us something similar to the fact that there are many more dwarf stars in our galaxy than large or medium size stars. I think it's illuminating that most planets are small, and that large planets are the exception. The inside of dwarf planets can stay warm for about a billion years after formation, likely with lots of water-rock interactions, so perhaps microbial life could at least get started — before likely going extinct — on many of the galaxy's tiny planets.

What has been the reaction to your thoughts on Pluto?

Runyon: The majority of the reaction has been positive, both from scientists and the general public. However, a sizeable minority has been negative. What I'm thrilled about is that this discussion draws attention to dwarf planets and especially the discoveries from New Horizons about the beautiful double planets of Pluto-Charon and their four small moons.

While many planetary scientists both agree and disagree with our geophysical planet definition, we all agree that Pluto is a dwarf planet and that dwarf planets are a huge and interesting component of our solar system and likely other exoplanet systems around other stars. For the many dwarf planets in our solar system, I'd like to see more robotic missions to explore them! What glaciers, mountains, valleys, craters, and dry river channels exist on Eris, Haumea, Makemake, Quaoar, and others? These planets are further away than Pluto and Charon, so missions to them should get started now and use advances in propulsion to get there soon.

My hope is that this planet definition discussion stays positive, upbeat, and with a recognition of how much all planetary scientists have in common. This is a powerful teachable moment for understanding the power of words, for science, and for solar system exploration.

Weigh in on the debate. Pluto’s status was our most recent Question of the Week.

See how NASA’s ‘Ralph’ Camera shows Pluto in color.

Learn more about the Johns Hopkins scientists’ case to restore Pluto’s planetary status.

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