A Candidate from the Cosmos
In March 2003, the SETI@home project, a distributed computing experiment that used volunteers' personal computers to analyze radio telescope data, identified a curious signal. Labeled SHGb02+14a, the transmission originated from a point between the constellations Pisces and Aries. The now-decommissioned Arecibo Observatory in Puerto Rico captured the signal on three separate occasions, with a total observation time of about one minute.
The signal's most significant feature was its frequency: approximately 1420 MHz. This value is significant in the search for extraterrestrial intelligence. It corresponds to the hydrogen line, a natural emission frequency of neutral hydrogen, the most abundant element in the universe. Scientists have long theorized that any civilization attempting interstellar communication might use this fundamental frequency as a universal sign. The source of SHGb02+14a is in a direction where no stars are observed within 1000 light-years of Earth.
An Unexplained Drift
The characteristics of SHGb02+14a presented a puzzle for astronomers. The signal exhibited a rapid frequency drift, changing at a rate of 8 to 37 hertz per second. Natural Doppler shifts caused by the Earth's movement are accounted for in SETI@home's software. If the observed drift were caused by the rotation of a planet, that world would need to be spinning nearly 40 times faster than Earth.
Another strange attribute was the signal's behavior upon each detection. Every time it was observed, it first appeared at 1420 MHz before beginning its rapid frequency shift. This "resetting" behavior is not consistent with typical astrophysical phenomena. Despite these unusual properties, the signal remains an unconfirmed candidate. Explanations range from a previously unknown type of astronomical object to instrumental noise or random chance. Follow-up observations have not re-detected the signal, leaving its origin a persistent mystery.