Here's why it's considered the outermost layer:

* Very thin air: The exosphere has extremely low density, with very few gas molecules.

* Gradually fades into space: There's no defined boundary between the exosphere and outer space. It gradually thins out until it merges with the interplanetary medium.

* High altitudes: The exosphere extends from about 700 km (435 miles) above Earth's surface and stretches outwards for thousands of kilometers.

Let me know if you have more questions about the atmosphere!