It is thought that Pluto's tenuous atmosphere may become even thinner as the planet moves toward aphelion (which it will reach in 2113), then regain its present density as it again moves toward perihelion. Why should this be?

Understanding the terms 'aphelion' and 'perihelion' is crucial when discussing the dynamics of planetary orbits. Aphelion is the point in the orbit of a planet, such as Pluto, where it is furthest from the sun. Conversely, perihelion is the point where the planet is nearest to the sun. Due to the elliptical nature of planetary orbits, the distances from the sun at these points can vary significantly, affecting the planet's environment and atmospheric conditions.

For Pluto, as it journeys away from the sun towards its aphelion, it experiences a decrease in temperature due to the reduced solar heat. This change in temperature influences the planet's atmospheric density, which leads to atmospheric changes that can be predicted as Pluto progresses on its orbital path.

Planetary orbits are not perfect circles but elliptical paths around the sun, with varying distances from the sun at different points. The shape of these orbits influences the amount of solar energy that a planet receives, which is particularly important for bodies like Pluto with extreme variations in their distance from the sun over time.

In layman's terms, the path that Pluto follows around the sun causes it to experience significant environmental shifts. The elliptical orbit brings about cyclical changes in temperature, affecting the planet's atmospheric conditions as it moves from perihelion to aphelion and back again.

Sublimation is the process by which a substance changes from a solid state directly into a gas without passing through a liquid phase. On Pluto, this occurs when its surface ice, composed of nitrogen, methane, and carbon monoxide, absorbs enough solar energy to transform directly into a gas.

This process is vital in creating Pluto's atmosphere, particularly as the planet approaches perihelion. The warming effect of the sun causes the sublimation of the icy surface, contributing to the density of the atmosphere. However, as Pluto moves away from the sun, towards aphelion, the decrease in temperature leads to the reverse process—deposition—where gaseous substances condense back into solid ice on the surface.

The density of an atmosphere on a planetary body like Pluto is closely related to the temperature of its surface. As Pluto approaches perihelion, the increased solar energy results in a warmer surface and a higher rate of sublimation from ice to gas, leading to a denser atmosphere.

On the other hand, moving towards aphelion results in a cooler surface as it receives less solar energy, causing the gaseous components of the atmosphere to freeze and fall back onto the surface as ice. Thus, the atmospheric density decreases. This intricate dance between temperature and atmospheric density presents a cyclical pattern that dictates the thinning and thickening of Pluto's tenuous atmosphere over its long orbital period.