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This 1917 image is the earliest proof of exoplanets

Unearthed at Carnegie Observatories, the image on a glass plate is the earliest evidence we have for planets outside the solar system.

The Kepler mission has found 1,963 confirmed planets outside the solar system, and probably billions more are out there.

Yet there was a time in the not-so-distant past when we had no evidence of exoplanets. In fact, a researcher has found the earliest documentation of an exoplanet, and it is less than 100 years old.

The image on a glass plate -- made in 1917 by then-director Walter Adams of Carnegie Observatories based in Pasadena, California -- was sitting in the institution's archives. It was retrieved last year at the request of University College London researcher Jay Farihi, who was studying van Maanen's star, the white dwarf discovered by Adriaan van Maanen in 1917.

The plate in question shows the absorption lines that caught Farihi's eye.

Carnegie Institution for Science

Adams' plate captured the star's spectrum, or the light a star emits. Spectrum data has been used for many years to learn more about stars. The light a star emits can be used to discover the elemental makeup of a star, and what the light passes through on its way to Earth.

When the light is blocked by something, it shows up on the spectrum as an absorption line, where the light blocked indicates the chemical makeup of the object blocking the light. It was one of these that caught Farihi's attention. It indicated the presence of heavier elements such as calcium, magnesium and iron.

These heavier elements are often present in stars, but only in their cores. For it to show up in the star's spectrum indicates the presence of something else entirely: a planetary system complete with enormous asteroid belts made up of the material left over from planet formation.

This is known as a polluted white dwarf, and its discovery is only recent, which explains why nobody noticed the significance of the plate until now.

"The unexpected realization that this 1917 plate from our archive contains the earliest recorded evidence of a polluted white dwarf system is just incredible," Carnegie Observatories director John Mulchaey said Tuesday in a statement.

It is important to note that although the spectrum indicates the presence of an asteroid field, which usually means planets, direct evidence of planets themselves has yet to be found. However, Farihi thinks it's just a matter of time.

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