When I was growing up there were 9 planets. Then scientists decided that Pluto was a dwarf planet and there was only 8. So why did this happen?
To start out 200 years ago we had 11 planets and Pluto had not been discovered yet. Those 3 extra planet were the largest of the asteroids. They were Vesta, Juno and Ceres. Ceres has since been classified as a dwarf planet. As they discovered more and more asteroids they reclassified them all as asteroids as they were different than the planets.
When Pluto was discovered they thought it was much bigger than it really is. Thus they had no problem with considering it the ninth planet. When they found Pluto’s moon Charon they were able to calculate the mass of Pluto using the laws of physics. They found that is was tiny although still bigger than the asteroids.
In the 1990’s scientists started finding more object out in the area where Pluto is. This area is called called the Kuiper Belt. One of these, Eris, is actually bigger than Pluto. They had a problem. If Pluto was a planet then so was Eris and probably some of the others that they were finding. We were going to end up with 13 planets and it would have grown from there. They estimate that there could be 200 objects big enough to be called dwarf planet out in the Kuiper belt. We certainly didn’t want to have to memorize 200+ planets.
In 2006 they defined what is required in order to be a planet.
The object must orbit the sun. So the Earth meets this one since we go around the sun once a year. Our moon doesn’t because it orbits the Earth.
It must be nearly round due to its gravity pulling it into a sphere. Basically the force of gravity it is creating must be greater than the forces holding its shape.
It must have cleared its orbit of other objects or they are being controlled by the gravity of the planet. When the planets formed the bigger ones either vacuumed up or threw out object in their path. Pluto was unable to do this as was Ceres. Both are in orbits where there are other objects like them.
So if they pass 1 and 2 but not the third they are considered to be a dwarf planet. If they meet all three then they are a planet. This was a controversial decision with a number of scientists wanting Pluto to remain a planet but really it was the best solution. For science to go forward we must be willing to reexamine, redefine and change based on the facts.
Years ago I had the privilege of attending a seminar put on by the Shoemakers and David Levy talking about this new comet they had discovered. The cool thing about this comet was when they did the math to figure out its orbit they found that it was going to hit Jupiter. Needless to say that got my attention.
The comet was named Shoemaker-Levy 9. On its way into Jupiter it broke apart in to 21 pieces. The picture below was taken using the Hubble Space Telescope. They had to take six pictures to get all of the pieces. At this point it was spread out over 1.1 million km.
The largest of the fragments was about 2km across. When it hit the impact released the equivalent of 6,000,000 megatons of TNT. That is 600 times bigger that the worlds entire nuclear arsenal. It left a dark mark on Jupiter almost as big as the earth.Below is a series of images taken from the Galileo Space Probe of Fragment W as it impacts on the dark side of Jupiter.
In the picture below you can see the dark marks left by the impacts. Recently we saw a new dark mark that matched the characteristics of the ones from Shoemaker-Levy 9. It is probable that Jupiter got hit again but we didn’t see it, only the aftermath.
What is a comet?
Comets are ice, dust and rock held together by its own gravity. They travel through space in an elliptical orbit. These orbits can be as short as a few years or thousand of years. When we see them they have a thin atmosphere and a tail. These are caused by the solar wind and radiation as it comes closer to the sun creating off-gassing.
Where are they located?
They are mainly located in the Kuiper belt which if out beyond the orbit of Neptune. Pluto resides in the Kuiper belt. He picture shows the main belt in green. The gaps are areas that have not been observed rather than an actual gap. Plus there are a lot more that we have not found yet. The orange ones have been knocked lose by Neptune and are the ones that we sometimes see.
What is an ellipse?
When we talk about an elliptical orbit we are talking about an ellipse. There are two easy ways to see an ellipse. The first is to take a piece of paper and pin it to a cork-board using two pins somewhere near the middle of the page. The distance between the pins will determine the shape of the ellipse. Then take a piece of string and tie it into a loop so it fits over the pin with some slack. Then keep the loop taunt with a pencil and draw around the pins.
The second way is with a cone. If you cut a cone at an angle the resulting surface is an ellipse. I tried this with some play-dough. To get a nice shape I cut using a sharp serrated knife although you could demonstrate with something less sharp and little more mushing of the shape. The angle of the cut determine the shape of the ellipse.
An asteroid is a small body that orbits the sun. The size range from 10m across and up. The upper end isn’t well defined yet. Anything smaller than 10m across is considered to be a meteoroid. The upper end question is where do they go from being an asteroid to being dwarf planets.
When was the first one discovered? What is the largest asteroid?
The first asteroid to be discovered was Ceres in 1801. It is also the biggest with a diameter a 952km. When it was discovered they considered it to be a planet. As they found more and more of them they stopped calling them planets and renamed them asteroids. In 2006 scientist need to define what a planet is. In doing so they created a category called dwarf planet. Ceres meets the criteria for dwarf planet and is the only one in the inner solar system. It is also still considered to be the largest asteroid.
Where are they located?
Most asteroids are found in the asteroid belt between Mars and Jupiter. Some are found in Trojan points of a planet with Jupiter having the most. Trojan points are 60 degree in front of or trailing the planet. Thus these asteroids travel around the sun with their planet but are not orbiting the planet. The third type and the ones we want to keep an eye on are called Near Earth Asteroids. These have orbits that pass close to Earth.
How many are there?
Millions. They estimate that the belt has between 1.1 and 1.9 million asteroid over 1km in diameter. There are less Near Earth asteroid, about 7000 and only 500 to 1000 of those are over 1 km. Most of the Trojan asteroid are travelling with Jupiter. We think there are a lot of them and maybe as many as the asteroid belt. It is an area we don’t know a lot about yet.
What are they made of?
Another area that we don’t know a lot about. Various forms of rocks for most although Ceres is thought to have an icy crust. Some may be a solid single rock. A lot seem to be rubble piles of smaller rocks held together by gravity.
How do we find them?
The first ones were found by people looking through telescopes and keeping very detailed records. Things that moved compared to the background stars were planets and as stated earlier Ceres. They found a number this way. Later people used photographs and digital cameras through telescopes. A computer can watch an area of space and compare multiple photos for things moving.