Back in 1911, a young Dutch astronomer came to the US looking for work. He found unpaid work at the Yerkes Observatory in Wisconsin but eventually got full time employment at the Mount Wilson Observatory near Los Angeles.

His name was Adriaan van Maanen and he has become one of the more infamous characters in the history of astronomy. Now a reinterpretation of his early work has revealed that van Maanen found the first evidence of an extrasolar planetary system way back in 1917.

Today, astronomers have a number of different ways of spotting exoplanets. Astrophysicists had been using many of these techniques for decades before they spotted evidence for the first extrasolar planets in the early 90s.

One of the great frustrations of modern astronomy is that exoplanets could have been discovered decades earlier. Indeed, various astronomers have picked through earlier observations to show that the first observation probably took place in 1988.

Now Zuckerman, one of the world’s leading authorities on white dwarfs, says the first evidence dates back almost a century, although van Maanen couldn’t have known what he had found. This evidence comes from van Maanen’s discovery in 1917 of one of the first white dwarfs, and the closest to the Sun at distance of only 14 light years. it is now known as “van Maanen 2".

The photosphere of a white dwarf should contain only hydrogen and helium, which is what the spectra of a standard white dwarf shows. Anything else that falls into the star should rapidly sink beneath the surface and so be unlikely to show up.

But the spectrum of van Maanen 2 contains evidence of all kinds of heavier elements.

In recent years, Zuckerman and other astronomers have shown that these elements can only come from rocky debris orbiting the star. In other words, these elements come from asteroids regularly falling into their parental white dwarf and burning up.

These elements show up in the spectra of lots of white dwarfs. Indeed, various studies of the spectral characteristics of this debris have revealed the make-up of asteroids orbiting other stars for the first time.

One question that Zuckerman and others have puzzled over is why asteroids are regularly falling into white dwarfs. And this has led them to a fascinating discovery.

It turns out that all of these white dwarfs are surrounded by rocky debris and at least one large planet. It is the gravitational perturbations from this planet that cause the asteroids to collide with each other and then spiral into their parent star.

This evidence is now so strong that Zuckerman and others have been able to turn it on its head to find white dwarfs with exoplanets. When they find white dwarfs with heavy elements in their spectra, they now consider this good evidence of an extrasolar planet.

So looking back, van Maanen’s observations were the first evidence that other planets must be orbiting other stars. “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,” says Zuckerman.

Curiously, van Maanen’s best known legacy is his infamous involvement in the debate over whether the fuzzy nebulas that astronomers could see were nearby and therefore part of the Milky Way or extremely distant and therefore “island universes” of their own.

Hubble famously argued that Andromeda and other nebulas were distant galaxies in their own right. But others argued that the observational evidence proved they were much closer.

This evidence was unfortunately gathered by van Maanen who somehow made catastrophic errors in his measurements. His evidence was later discredited by astronomers who have puzzled ever since over how he could have got the data so badly wrong.

The new interpretation of his observations of van Maanen 2 are unlikely to rehabilitate him. On the contrary, the most likely outcome is that his unfortunate story will now become even more widely known.

Ref: arxiv.org/abs/1410.2575 : Recognition of the First Observational Evidence of an Extrasolar Planetary System