While the search for exoplanets has only comparatively recently picked up steam, a chance re-examination of an old astronomical glass plate has shown that the very first evidence of an exoplanetary system was actually recorded almost 100 years ago. The data on the plate doesn't outright confirm exoplanets in the system, but astronomers are confident that it's only a matter of time before their existence in such systems is confirmed.

While modern astronomers use digital tools to image stars, that hasn't always been the case. Turn the clock back a century, and stargazers were using glass photographic plates to record stellar spectra, showing a spread of all of the component colors of the light from the distant objects. That information can be used to ascertain a lot of information about a star, including its chemical composition.

The new discovery was made when University College London's Jay Farhi contacted the Carnegie Institute of Science looking for a plate in its archive that showed a star discovered by Dutch-American astronomer Adriaan van Maanen in 1917, known as van Maanen's star.

The plate was made by former Observatories Director Walter Adams at Mount Wilson Obseravtory, at the time part of the Carnegie Institute. Back then, not much out of the ordinary was noticed about the distant object, with an attached note commenting only that the star appeared to be a little warmer than our own Sun.

The astronomical plate was found in this paper sleeve, with handwritten notes made by observer Walter Adams(Credit: Carnegie Institution)

However, examining the plate using modern technology revealed much more, confirming the presence of absorption lines on the spectrum, which are essentially gaps in the data where the light coming from the star was absorbed by a substance that it passed through.

Studying these lines tells us about the chemical makeup of the obscuring object, in this case highlighting the presence of heavier elements such as calcium, magnesium and iron. These elements would usually have long since sunk into the interior of the star due to their weight.

Research over the last 12 years has shown that the presence of such elements indicates the existence of a type of planetary system incorporating huge rings of rocky planetary remnant material around the star. These are known as "polluted white dwarfs," and while actual planets are yet to be detected in such a system, astronomers believe that it's only a matter of time until the discovery in made.

"The mechanism that creates the rings of planetary debris, and the deposition onto the stellar atmosphere, requires the gravitational influence of full-fledged planets," said Farihi. "The process couldn't occur unless there were planets there."

The chance discovery is an astonishing one, and considering that there are some 250,000 plates sitting in the Carnegie archive, it's likely that there's plenty more information stored away that we could only fully appreciate with the benefits of modern technology.

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