The search for exoplanets has been ongoing for decades. To date, thousands of them have been catalogued, ranging from the potentially habitable to the decidedly strange and hostile.

The discovery of the very first exoplanets, two worlds orbiting around a violent pulsar, was announced in 1992, with the first exoplanet orbiting a Sun-like star being announced in 1995. However, as a curious tale uploaded to the arXiv server reveals, the very first hints of planets outside our Solar System could have accidentally been discovered as far back as 1917 – astronomers at the time just didn’t realize it.

Back when the First World War was nearing its end, a Dutch-American astronomer named Adriaan van Maanen spent his time diligently photographing the stars at the Mount Wilson Observatory near Los Angeles. He serendipitously discovered one of the first white dwarfs, a remnant of a star like our own and the nearest known to us, lingering in the cosmos 14 light-years away. This stellar remnant was named “van Maanen 2.”

“With 20–20 hindsight, it is now possible to say that the first observational indication – by any means – of the existence of an extrasolar planetary system came almost a century ago when van Maanen discovered and noted the spectrum of the nearest single white dwarf to Earth,” wrote Ben Zuckerman, a professor of physics and astronomy at the University of California, Los Angeles (UCLA), and the paper’s author. A recent review of debris interacting with white dwarfs confirms this notion.

^{The 1917 spectrum of van Maanen 2, pictured here as the thin, dark line in the middle of the zoom-in box. The small gaps represent absorption lines for heavier elements. Carnegie Institution for Science}

Maanen used a glass plate to capture the white dwarf’s emission spectrum, which is its escaping radiation. These spectra allow astronomers not only to discover the chemical composition of the star, but they also reveal anything that gets in its way – a dust cloud, for example, or even a planet. These obstacles in the line of sight show up as “absorption lines” on the spectral image.

At the time, a few mysterious absorption lines went unnoticed by the astronomer. The plate was stored away, and Maanen continued his work until he died in 1946. Fast forward to 2014, and Zuckerman was spending time looking for the plate, hoping to use it as part of a scientific symposium.

While examining it, Zuckerman noticed these previously unseen absorption lines, which seemed to indicate that van Maanen 2 contained heavy elements, like calcium, magnesium, and iron. Although these can be observed in other star types, white dwarfs should only contain hydrogen and helium. So where were these additional elements coming from?

^{The spectrum of van Maanen 2 reassessed by the Very Large Telescope (VLT). These peaks and troughs reveal the previously unexpected presence of heavier elements. Ben Zuckerman}

In the last 12 years, astronomers have realized that some white dwarfs can be “polluted” by nearby rings of rocky debris, much of which inevitably tumbles into the star and contaminates it. This debris originally comes from the remains of planetary objects, which would contain these heavier elements.

Putting two and two together, Zuckerman had his own eureka moment: van Maanen 2 was a white dwarf surrounded by rocky planetary debris that was “polluting” it. This, by most accounts, means that the first planetary system outside of our own Solar System was discovered not in 1992, but 1917 – 75 years earlier.

Although whole exoplanets themselves had not been detected, they may have once been there in order to create planetary debris in the first place.