The exosphere is not considered the coldest layer of the Earth’s atmosphere. In fact, it is often regarded as a transitional space between the atmosphere and outer space.
As you ascend through the layers of the atmosphere, temperatures generally decrease up to the mesosphere, where the temperature can drop to around -90 degrees Celsius (-130 degrees Fahrenheit). However, when you reach the exosphere, temperatures can actually rise due to the sparse presence of particles and direct exposure to solar radiation.
In the exosphere, which starts around 600 kilometers (about 370 miles) above the Earth’s surface and extends up to about 10,000 kilometers (6,200 miles), particles are so far apart that they rarely collide. This means that while temperatures can be measured as high as 2000 degrees Celsius (3632 degrees Fahrenheit) or more, there are very few particles present to transfer that heat. Consequently, the concept of temperature in this layer is quite different from what we experience in the lower atmospheric layers.
In summary, while the exosphere can have high temperature readings, it is not the coldest layer of the atmosphere; that title goes to the mesosphere.