We have eight planets in our solar system, but until a few years ago, everyone thought there were nine. For many years we considered the little planet at the edge of the galaxy, called Pluto, to be our ninth planet. Pluto was small, but it has its own moons that circle it and because of that, a lot of people considered it to be a planet. The more they tried to study Pluto, the more they realized that it wasn’t an actual planet, and so it was reclassified as a ‘dwarf planet’. That decision put many schools into an uproar, because they had to change everything in all of the science books. So if Pluto wasn’t the smallest planet, which planet would take its place?
Our Milky Way galaxy is very unique in a lot of ways. We have both gas giants and solid planets, planets with rings and some with a lot of moons. As we study other galaxies, we are learning that many solar systems have their largest planets closer to their sun. This is thought to be a kind of ‘get out of my way’ attitude. The bigger planets were drawn in by the gravity of the sun and either knocked the other planets away or absorbed them. This is not so in our solar system. The largest planets take their place in a variety of locations throughout the solar system.
If we put our planets in ‘size order’ they would be listed as the following, from large to small: Jupiter, Saturn, Uranus, Neptune, Earth, Venus, Mars, and Mercury. Since we lost Pluto as an official planet, it appears that Mercury is now considered the smallest planet in the solar system. But if you look at Mercury, it is quite sizable.
First, Mercury looks an awful lot like our own moon. It has the same kind of craters and rocky mountains as the moon. Pictures that have been sent back show Mercury might have had volcanic action at the surface when it first formed. The surface also shows a kind of ‘wrinkling’ and it is believed that it is caused from the intense pressures of the planet. Mercury is closest to the sun, and while it gets most of the heat, it doesn’t have an atmosphere to hold any of the heat and it expels it back out. Due to this fact, it cannot attain the title of the ‘hottest’ planet (that title goes to Venus). Mercury does hold the title of having the largest metallic core of all of the planets. It makes up 75% of the entire planet and when radar images were sent back, it appears that the core is molten.
There are a few scientific theories about Mercury, one of them is that the reason it has such a large molten core is that some of the surface was burned off when it was forming, due to Mercury being so close to the sun. Another theory is that Mercury was actually larger in the beginning but was hit by another planet, reducing it in size.