Tonight’s show had a special guest star, Dr. Neil deGrasse Tyson, Director of the Hayden Planetarium in New York City. As we learned from Sheldon, Dr. Tyson was instrumental in defining a new class of solar system object, the “dwarf planet” — and then demoting Pluto into it.
Several objects nearly the same size as Pluto share approximately the same orbit, one even larger. If Pluto is a planet, then so must be the other five or so large round blobs orbiting our Sun (that aren’t moons). Up until a few years ago you could recall their names in order with a simple mnemonic: “My Very Elegant Mother Just Served us Nine Pizzas” (for Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto).
But as Dr. Tyson points out at the Hayden, let us not simply count planets. More important, the planets seem to fall into different classes. The first four are small and rocky with iron cores, typically called the terrestrial planets. The next four are the gas giants. Having two different types of planets is not accidental. As the solar system formed out of a blob of gas about 5 billion years ago, the innermost material was much hotter due to the proximity of the Sun. So small dusty and metallic grains could form and coalesce, but nothing that would be gaseous. Only out around the distance of Jupiter and beyond was it cool enough that methane and water would crystallize into ice and coalesce into the gas-giant planets.
At least so goes the theory. We can’t watch our own solar system being formed. But over the last two decades we started taking actual data, because we have the ability to observe planets around other stars. We are now on the verge of being able to measure the actual distribution of terrestrial and gas-giant planets around many different stars. We’ll see what the data tell us.
We would probably have a fifth rocky planet but the constant gravitational influence of Jupiter as it orbits the Sun keeps the material beyond Mars from coming together and instead we find the Asteroid Belt. Some are still large enough to forms spheres under the influence of their own gravity, such as the asteroid Ceres. Ultimately the definition of a planet included the new criterion that it have mostly cleared its orbit of other material. Ceres and Eris fail. And so too went Pluto.
Beyond Neptune we find a different kind of objects. Out near Pluto are thousands of objects composed of rock and ice. Their composition is similar to that of comets. And it is no surprise. That is where many of the short period comets (say 50-200 year orbits) originate from. These objects form a third type of object, beyond the terrestrial planets and the gas-giant planets and form what is called the Kuiper Belt, a large collection of objects that also orbit the Sun in roughly the same plane as all the planets. These are some of the objects of Raj’s research in previous seasons, “Trans-Neptunian Objects”.
I think having the public ruckus over Pluto was good for science. After all, we showed our benefactors that our ideas are not written in stone. We demonstrated a hallmark of science, that when a better idea comes along we are willing to change our definitions and theories.
What really amuses me though is that this happened with astronomers, who otherwise cling onto old nomenclature more than any other field I know:
Let’s start with “planetary nebulae”. A nebula is the word that astronomers give to any cloud-like object that astronomers find. So far so good. Astronomers long ago found some of them around of stars. At the time, astronomers thought that this was the gas and debris that lay in a planetary disk, hence the name. Astronomers now know, that these nebula are actually formed by stars in their death throes. Gas is thrown off as the star ends its life. It has nothing to do with forming planets. As long as astronomers are keen to fix up the definition of planet, why not fix up “planetary nebula” to something else as well?
Another pain left over from 2000-year old astronomy is the classification of the intensity of the stars in our sky, called apparent magnitude. Now you might think that a larger magnitude is brighter, but you’d be wrong. That’s backwards. Fine, we can live with a minus sign. The dimmest star you can see with your naked eye is 6. The star Vega was chosen as the calibration point, 0. Well actually Vega is magnitude -0.03. But why change the scale? Ever? Once per 2000 years? No. Was Vega chosen because it was the brightest star in the sky? That would be sensible. Well close. It is fifth brightest. It was the first star to be photographed, however, and that set the scale forever after.
To make matters worse, take a look at magnitude quantitatively. Out here in California, we are used to talking about Richter scale for magnitudes of earthquakes. And it is pretty simple. A “magnitude 6” earthquake is about 10 times the shaking as a magnitude 5 quake. A quake with magnitude 7 is 100 times more shaking than 5. That’s a good way to measure when the distributions vary so widely. It is called a logarithmic scale, or ratio scale. So is a magnitude=5 star 10 times as dim as a magnitude=4? No. What is it then? A factor of 2.5119. Why? Because that’s the fifth root of 100. Ask a silly question, you’ll get a silly answer. This scale is left to us from the Ancient Greeks. There’s nothing wrong with it, but it does make teaching it to non-science majors needlessly difficult.
Stars can classified by their surface temperature. From hottest to coolest they are tagged with letters. By now, you are not expecting A,B,C,D,…. Right. It is O,B,A,F,G,K,M. It is a relic of a previous attempt to classify stars based on the strength of the absorption of light by hydrogen. It is easy to remember though: “Oh, Be a Fine Girl (Guy), Kiss Me!”.
At least the reclassification of Pluto is progress. Astronomy will be around at least another 2000 years to slowly fix the others. From now on we only need concern ourselves with “My Very Elegant Mother Just Served Us Nachos”.