Here's why:

* Altitude: The thermosphere extends from about 80-1000 km (50-620 miles) above Earth's surface.

* Density: While the thermosphere is characterized by very high temperatures (up to 1500°C or 2732°F), the air is extremely thin, meaning the density is incredibly low.

* Other Layers: The troposphere (where we live), stratosphere, and mesosphere all have higher densities than the thermosphere, despite being at lower altitudes.

Important Note: Even though the thermosphere has the lowest density, it is still a part of the atmosphere and plays a crucial role in absorbing high-energy radiation from the sun.