The White House said Wednesday it was skeptical of North Korea's claim that it successfully detonated a thermonuclear device.
"The initial analysis is not consistent with the North Korean claims," White House spokesman Josh Earnest said.
But the Pentagon has a way to be more certain whether a hydrogen bomb was tested.
A U.S. official told CNN that a nuclear test of any type would emit distinctive elements into the air, and collected samples can be analyzed to determine exactly what occurred. That's exactly what the WC-135W jet, dubbed the "Constant Phoenix," does.
The four-engine Boeing jets are equipped with external devices that collect radioactive material from the atmosphere on filter paper. The planes also have "a compressor system for whole air samples collected in holding spheres," according to an Air Force fact sheet
The Air Force has two of the WC-135 jets that operate out of Offutt Air Force Base in Nebraska.
Officials said the United States also has ground stations in the area that will also be taking samples to verify or debunk North Korea's claim.
The Constant Phoenix program originated with Gen. Dwight D. Eisenhower in 1947. The then-Army Air Forces, which would later become the Air Force, used WB-29s, variants of the B-29 bomber model, to try to detect evidence of Soviet nuclear tests, according to the Air Force.
The WB-29s were replaced by WB-50s beginning in 1950, with the current WB-135s coming on line in 1965.
The radiation-sniffing planes have been used to monitor compliance with nuclear weapons treaties, and also monitored effects of the 1986 Chernobyl nuclear power plant disaster in the Soviet Union, the Air Force says.