Scientists exploring the farthest reaches of our solar system have detected an eerie 'hum.'
Instruments on board NASA's Voyager 1 spacecraft, which nine years ago left our solar system's outer reaches, have picked up the faint, monotonous sound.
Experts said it was caused by the constant vibrations from small amounts of gas found in the near-emptiness of interstellar space.
It represents the background noise present in the vast expanse between star systems, according to a study published this week in the journal Nature Astronomy.
These vibrations, called persistent plasma waves, were identified at radio frequencies in a narrow bandwidth during a three-year period as Voyager 1 travels interstellar space.
Stella Koch Ocker, a Cornell University doctoral student in astronomy and lead author of the study, said: "The persistent plasma waves that we've just discovered are far too weak to actually hear with the human ear.
"If we could hear it, it would sound like a single steady note, playing constantly but changing very slightly over time."
The Voyager 1 spacecraft, launched in September 1977, is currently located about 14.1 billion miles from Earth - roughly 152 times the distance between our planet and the sun, and is still obtaining and transmitting data.
Having decades ago visited the huge planets Jupiter and Saturn, Voyager 1 is now providing insight into interstellar space.
Voyager 1 previously detected disturbances in the gas in interstellar space triggered by occasional flares from our sun.
The new study reveals steady vibrations unrelated to solar activity that could be a constant feature in interstellar space.
This hum has a frequency of about 3 kilohertz (kHz).
"When the plasma oscillations are converted to an audio signal, it sounds like a tone that varies. It's a bit eerie," said Cornell University astronomy professor and study co-author James Cordes.
"Voyager 1 will keep going but its power supply will run out most likely this decade after up to 50 years of service.
"There are conceptual designs being made for future probes whose intended purpose is to reach further than the Voyager spacecraft.
"That is the message I find appealing: our reach is expanding into interstellar space."