Well, that depends on who you ask! According to NASA the upper layer of Earth's atmosphere — the exosphere — extends up to 6,200 miles (10,000 km), above which the atmosphere and space blend.

This is the thickest layer of the atmosphere and can reach extreme temperatures. Also, a cool fact, there aren’t enough molecules for sound waves to travel.

Located between about 700 and 10,000 kilometers (440 and 6,200 miles) above Earth’s surface, the exosphere is the highest layer of Earth’s atmosphere and, at its top, merges with the solar wind.