Dwarf planets, exoplanets, regular planets, moons. Why can’t we just have planets? NASA scientists led by Alan Stern, the principal investigator for NASA’s New Horizons Pluto mission, have proposed a new way to define planets.

First, let’s take a look at how the International Astronomical Union (IAU) defines a planet. Back in 2006, the IAU kicked Pluto out of the regular planet club with this definition.

“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.”

Understandably, Alan Stern didn’t appreciate his mission’s planet being kicked out. Stern and a group of planetary scientists are proposing a new geophysical definition. Here it is.

“A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters.”

This makes Pluto a planet again, while also keeping other round bodies like stars, white dwarfs and black holes out. The proposed definition also adds a bunch of new planets such as the dwarf planet Ceres and even our moon.

The scientists also suggest the new definition could be simplified for younger school students. Something along the line of “round objects in space that are smaller than stars.”

Stern and company aren’t slamming the IAU definition just because it kicked Pluto out. They believe the definition is “technically flawed.” First, it only recognizes planets orbiting our Sun. The new planets NASA plans to announce tomorrow? Those are ‘exoplanets.’

I’ll let their proposal explain the second reason.

“Second, it requires zone clearing, which no planet in our solar system can satisfy since new small bodies are constantly injected into planet-crossing orbits, like NEOs near Earth. Finally, and most severely, by requiring zone clearing the mathematics of the definition are distance-dependent, requiring progressively larger objects in each successive zone. For example, even an Earth-sized object in the Kuiper Belt would not clear its zone.”

The proposal also dives into how this would affect education. If this new definition is accepted, there would be at least 110 known planets in our solar system. Remember, moons would be included now.

The proposal points out memorizing 110 planets shouldn’t be expected from students. They use elements as one analogy. It’s not like we go around memorizing the entire periodic table. Instead, the scientists propose that students learn the zones of the solar system instead of the memorizing a certain number of planets.

“Teaching the zones of the Solar System from the Sun outward and the types of planets and small bodies in each is perhaps the best approach: The zone closest to the Sun consists of rocky planets; the middle zone consists of gaseous, rocky, and icy planets; and the third zone consists of icy planets. All zones also have small, non-round, asteroidal/cometary bodies.”

Today, Pluto is classified as a dwarf planet. Maybe that will change back to a regular planet. Maybe it won’t. But New Horizons shows us a stunning world worthy of exploration. Who knows what the tiny probe will find as it ventures deeper into the Kuiper Belt.

If calling Pluto a regular planet again would mean more people would care about, I say go for it. Images like this do enough for me.

Pluto stunning haze

Pluto heart

Image credits: NASA

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