Chapter 2, Problem 1RQ

To determine

The reason why the astronomers consider the space between the planets to be a vacuum in comparison with the atmosphere near the sea level.

Answer to Problem 1RQ

The astronomers believe that the space between the planets is a vacuum that is related to the atmosphere near the sea level due to the very low density of matter there.

Explanation of Solution

The interstellar space in the solar system hardly consists of one atom per liter when compared to the air at the sea level which has around $2.7\times {10}^{22}\text{atoms}\text{per}\text{liter}$. The interplanetary space in the solar system contains about $5,000-100,000\text{atoms}\text{per}\text{liter}$ which is little denser when related to the interstellar space. However, this volume is very less when compared to the air at the sea level.

The density of atoms in the space between the planets or stars is very low when related to the air at the sea level and is very close to a vacuum condition. It is because of this reason that the astronomers consider the space between the planets as a region of vacuum.