We have a solid understanding of how the universe and the Milky Way and the solar system all came to be, as well as a brief history of how humans started to understand the solar system as it truly is, we are ready to describe each object in the solar system in detail. We will be skipping forward in time a little bit from the last chapter, as much has happened in astronomy since the time of Newton, but later we will backtrack and discuss 20th century discoveries as well as the current forefront of the field and its implications. For now, let’s go one planet at a time and understand them as astronomers see them today.
We can divide the eight planets up into two groups of four. The inner four are the terrestrial planets, which are small and rocky, while the outer four are the gas giants, which are big and gaseous. So let’s start at the sun and work our way out. First up is Mercury, the innermost planet. It also happens to be the smallest of all the planets, with a radius one-third of the earth’s, and a mass one-twentieth. Mercury is a desolate place, gray and barren, covered with craters. With an average orbital radius of a little under sixty million kilometers, it’s so close to the sun that there is no chance of an atmosphere.
This also means that Mercury gets very hot during the day, while facing the sun, but very cold at night, with no atmosphere to trap the heat, so things go from a scorching 700 Kelvin, which is hot enough to melt lead, down to a frigid 100 Kelvin, which is colder than any place on Earth by a lot. This makes Mercury simultaneously one of the hottest and coldest places in the solar system. As we said, Mercury is closest to the sun, and with an orbital period of less than ninety days, it travels around the sun faster than any other object. This is why it was given the name Mercury, because observers noticed that it moved the fastest across the sky, and to the Romans, Mercury was the messenger of the gods, so they imagined Mercury swiftly delivering messages across the heavens.
In terms of Mercury’s interior, our knowledge is still somewhat speculative, as we have never sent any spacecraft to land on the planet, we simply have images from passing and orbiting crafts, first Mariner 10, which flew by in the 70s, and then MESSENGER, which orbited Mercury several thousand times earlier in this decade before running out of fuel and crashing into the planet. These missions yielded lots of images and data, and further inferences about the planet can be made by doing calculations with its density and gravitational field.
We believe that Mercury has an iron-nickel core beneath a silicate crust, with its core occupying much of the volume of the planet, around 55 percent, and we believe that this core is molten, which would explain the weak magnetic field, around one percent the strength of earth’s. Mercury also rotates on its own axis, but it does so very slowly, only once every 59 days. That means that for every two years on mercury, it rotates on its axis precisely three times. This is called a spin-orbit resonance. Combining this rotational motion with its orbital motion, a solar day, as experienced on Mercury, would be equal to 176 earth days. Mercury is an inferior planet, which doesn’t mean that it’s not very smart, it just means that it orbits more closely to the sun than earth does.
This fact impacts the location in the sky that Mercury can be found, and it also makes it quite difficult to see in general. Nevertheless, Mercury has a special place not just in astronomy, but in theoretical physics as well. When Newton used Kepler’s laws to explain with great precision all of the motion we can see in the solar system, there was one anomaly. Mercury’s orbit exhibited a slow precession, shifting over time and refusing to conform to predictions as well as the other planets. This perihelion precession was a problem for astronomers until the early 20th century, when Einstein updated our perception of gravity. With his general theory of relativity, we became aware of the warping of spacetime induced by the sun, and Mercury being so close to the sun, is affected the most by this phenomenon.