This 1917 image is the earliest proof of exoplanets - CNET

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

Yet there was a time in the not-so-distant past when we didn't have any evidence of exoplanets at all. In fact, a researcher has just found the very earliest evidence of an exoplanet we have, and it is less than 100 years old.

The plate in question, made in 1917 by the then-director of the Caegie Observatories Water Adams at Mount Wilson Observatory, was in the Institutions archives. It was retrieved at the request of University College London researcher Jay Farihi, who was researching a particular star, van Maanen's star, a white dwarf discovered by Adriaan van Maanen in 1917.

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Adams' plate captured the spectrum of this star, a picture of the light the star emits. These have been used for many years to lea more about stars. The light a star emits can be used to discovery the elemental makeup of that 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.

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"The unexpected realisation that this 1917 plate from our archive contains the earliest recorded evidence of a polluted white dwarf system is just incredible," said Caegie Observatories director John Mulchaey 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 now.

"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 explained. "The process couldn't occur unless there were planets there."