Pluto's day

Not the pup but the planet[?] Pluto popped into the planet ensemble on March 13th.

"Happy Pluto Discovery Day"

by

Doug Cornelius

March 10th, 2010

Wired

The discovery of Pluto was announced on March 13. Pluto was labeled a planet when it was first discovered in the Percival Lowell Observatory by Clyde Tombaugh in 1930.

Like me, you probably grew up being taught that there were the nine planets in our solar system. Maybe you even used the old mnemonic: My Very Excellent Mother Just Sent Us Nine Pizzas.

It turns out that we were taught wrong. There are not nine planets in our solar system. I thought nine planets was just a pure scientific fact. Since I was first taught about our solar system astronomers have discovered new objects orbiting the Sun that make us have to question whether Pluto and some other celestial objects should be considered planets.

As parents, we will need to help our kids better understand our solar system. Maybe, as parents, we can learn some new things for ourselves along the way.

Blame Neil de Grasse Tyson?

The trouble started when technology improved and we could start seeing many more objects orbiting the sun. Neil de Grasse Tyson of the Hayden Planetarium was labeled as the first mainstream trouble-maker. He lumped together the four terrestrial planets: Mercury, Venus, Earth and Mars. He lumped together the four gas giants: Jupiter, Saturn, Uranus and Neptune.

That left Pluto out in the cold with the thousands of other Kuiper Belt objects. He pointed out that if Pluto were as close to the Sun as Earth, Pluto would have a tail and look like a jumbo comet.

How Many Planets Are There?

Even if you have a sentimental attachment to Pluto as a planet, there are still not nine planets. If you keep Pluto as a planet, then you will have to include some other celestial objects.

Eris was discovered in 2005 in an area further from Sun known as the Scattered Disc. It orbits the Sun, is bigger than Pluto and has a moon. If you are going to include Pluto as planet, you have to include Eris.

You also need to think about adding in the asteroid Ceres. It’s not much smaller than Pluto and has enough mass that its gravity has pulled it into a nearly round shape. Being massive enough that the objects body pulls it into a nearly round shape is a key attribute for a planet.

Then you also need to think about Haumea and Makemake, two other Kuiper belt objects that are massive enough to have been rounded by their own gravity. You only have to think about these two because they have not been subject to as much observation as Eris and Ceres.

Since there are thousands of objects in the Kuiper Belt and the Scattered Disc we may end up with more than the 13 objects that could be considered a planet. That leaves us with having to define a planet.

What is a Planet?

The International Astronomical Union decided on this definition of a “planet”:

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.

Since Pluto crosses the orbital path of Neptune, it has not cleared the neighborhood of its orbit, throwing it (and similarly Ceres, Eris, Haumea and Makemake) into the category of dwarf planet.

The International Astronomical Union decided on this definition of a “dwarf planet”:

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) shape2, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.

It is important to keep the moons out of the definition because there are seven moons in our solar system bigger than Pluto: Io, Europa, Ganymede, Callisto, Titan, Triton, and Earth’s Moon.

What went Wrong with Pluto?

You have to credit some of the Pluto problem to bad science.

For millenia, humans thought there were only six planets. As we got better with telescopes, Sir William Herschel announced the discovery of Uranus on March 13, 1781. That was the first planet discovered with a telescope. As astronomers measured the orbit of Uranus, they found irregularities in its orbit and reached the conclusion that there must be another unseen planet in our solar system exerting its gravitational force on Uranus.

After grinding out the calculations on the orbit of Uranus, astronomers were able to calculate the location of Neptune in the night sky. Sure enough, Johann Galle observe that “moving star” within a degree of the position predicted by Urbain Le Verrier.

Some astronomers also found an irregularity in the orbit of Neptune and concluded that there must be a Planet X beyond the orbit of Neptune. Percival Lowell took on that search. (Actually, Lowell’s obsession was trying to prove the existence of life and civilization on Mars.)

It turns out that there was a bad data point mixed in with the observations of Neptune’s orbit. They also had the wrong mass of Neptune. Planet X was a miscalculation.

Astronomers at the Lowell Observatory were so focused on finding Planet X that they assumed that Pluto must be a planet. The designation of Pluto as a planet sounds like it was an astronomical mistake.

Politicians Fight Back

In 2007, the New Mexico state legislature declared March 13 Pluto Planet Day. They also declared that when Pluto is overhead through new Mexico’s night skies, Pluto will have full planetary status. That’s got to be tough on teaching astronomy in that state. New Mexico also proclaimed February 18, 2009 as “Pluto is a Planet in New Mexico Day.”

In 2009, the Illinois General Assembly declared March 13, 2009 to be Pluto Day. They also declared that when Pluto is overhead through Illinois’ night skies, Pluto will have full planetary status. That’s got to be tough on teaching astronomy in that state.
New Horizons and the Exploration of Pluto

We will learn more about Pluto when the New Horizons spacecraft reaches it in 2015. It will tell us more about the composition of Pluto and its three moons: Charon, Nix and Hydra. It may even discover additional moons or rings.

But My Kid’s Favorite Planet is Pluto!

Neil de Grasse Tyson has received “endless hate mail from third graders” for demoting Pluto from planet status. It seems most kids are the most attached to Pluto. Clearly, this is because of sharing its namesake with the Disney character and not with the ancient Roman god.

This idea for this post came from a similar problem in my geeklet’s school. The kids were upset the Pluto was not going to be part of the solar system on the wall.
So is Pluto a Planet?

In the end, its a matter of battling definitions. I’ll resist my inner lawyer instincts and avoid the arguments over whether Pluto is a planet. I’ll let you reach your own conclusion.

But regardless of where you come out in that argument, it is clear that there are not nine planets orbiting our Sun.

The IAU has concluded that there are eight planets in our solar system, eventually that will make it into our kids’ text books. We need to keep an eye out for books that discuss nine planets. You will have to do some editorial reading in most astronomy books that are more than a few years old.

If you are stubborn and insist that Pluto is planet, you have to up the planet count to at least eleven planets. If you insist on keeping Pluto as a planet then you also have to let in Ceres and Eris. If you love Pluto as a planet then you have to embrace those eleven. That also leaves you with Haume and Makemake waiting in line for planetary status. As our technology improves and our searches of the solar system continue we will find more and more celestial bodies in our solar system.

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