A radio signal from a nearby star system appeared once. What made it?

The team could distinguish the positions of GJ 725A and its companion star, GJ 725B. The detection lined up with A. The measurement could not separate the star from a planet orbiting it, so the researchers considered what each could contribute.

GJ 725A already has a known rocky planet, GJ 725Ab, that completes an orbit in about 11 days. The paper argues that the planet is too small to generate the necessary magnetic field on its own. It could still play a part if its motion through the star’s magnetic environment triggers emission closer to the star. Repeated bursts at a consistent phase of the planet’s orbit would give that idea a test.

Another possibility is an aurora on a giant planet farther out. Jupiter produces low-frequency radio emission through a similar process. An unseen planet would need a strong enough magnetic field and a supply of energetic electrons. At the distances the authors consider, the star’s wind would struggle to provide those electrons. They suggest an internal source, such as a volcanically active moon, as one way to supply them. The paper reports no detection of that planet or moon.

The star could also produce the burst through its own magnetic activity. Monitoring for changes in X-rays, ultraviolet light and a hydrogen emission line during future radio observations could help test that idea. Catching a burst in more than one kind of measurement would let astronomers compare what the star was doing at the same time.

What one observation can establish

The detection window was an hour long. That tells us how long the telescope looked, not how long the emission lasted. Other archival observations yielded no repeat detection. The team has a candidate mechanism and three natural explanations to test, but it still has only one detection from which to infer a pattern. A longer radio campaign could show whether the signal returns in step with the known planet or turns up alongside a change in the star.

For more investigations that follow a puzzling observation back to the evidence, browse Unexplained History. This is a general series recommendation.