Nine Transients, One Plate — April 12, 1950

Before digital cameras, astronomers recorded the sky on glass plates coated with light-sensitive chemicals. A telescope tracked one region of sky while the plate collected light, sometimes for nearly an hour. When developed, stars appeared as small points on the glass.

The first Palomar Observatory Sky Survey used this method to photograph large parts of the northern sky. Because its plates survive, researchers can scan them at high resolution and compare the recorded positions with other Palomar exposures and with modern sky surveys.

On April 12, 1950, one fifty-minute Palomar exposure contained nine star-like points that were not present on a plate taken about thirty minutes earlier. They were also absent six days later and have never been recovered in modern observations. All nine appeared on the same piece of glass during the same exposure.

Beatriz Villarroel and an international team identified the group while visually examining POSS-I transient candidates. They published the analysis in the peer-reviewed journal Scientific Reports in 2021. The researchers checked the shapes and positions of the points and compared the plate with exposures taken before and after it. Deep follow-up with the 10.4-meter Gran Telescopio Canarias covered six of the nine positions and found no convincing persistent counterpart linked to the original transients.

The plate therefore records nine features that looked like stars for one exposure and were absent before, afterward, and seventy years later. The observation does not by itself reveal whether the features were brief events in the sky or marks produced within the photographic material. It establishes the event that any explanation must account for.