Astronomers have discovered thousands of planets in star systems other than our own. They’ve also found evidence of planets around many other stars — including planets that no longer exist.

The first possible example of such an “ex”-planet was recorded a century ago. But nobody realized what the discovery really was until recently.

On October 24th, 1917, Walter Adams, an astronomer at Mount Wilson Observatory, took the spectrum of a newly discovered star, known as Van Maanen’s Star. The star is a white dwarf — the hot, dead core of a once-normal star like the Sun.

A spectrum shows the intensity of the star’s light at different wavelengths. Each chemical element imprints a unique “barcode” in the spectrum, revealing its presence at or above the star’s surface.

Van Maanen’s Star showed the presence of iron, calcium, and other “heavy” elements — elements that should have sunk deep into the star. Their presence at the surface was odd, and astronomers of the day couldn’t explain it.

A couple of years ago, another astronomer looked at the spectrum, which was recorded on a glass plate. He recognized the heavy elements as the likely remnants of a planet. As the star was becoming a white dwarf, its inner planets should have been pulverized. Their rocky debris then could have fallen onto the star — “polluting” its surface with heavy elements.

So a century-old glass plate contains what may be the first recorded evidence of planets beyond our own solar system.

Script by Damond Benningfield