Hidden in the archives: Finding the first-ever evidence of exoplanetary system

You never know what hidden treasures can be uncovered in the archives.

And this was certainly the case at Carnegie Observatories’ collection when research for an article led to the unexpected discovery of a 1917 glass plate showing the first-ever evidence of a planetary system beyond our own Sun.

It all started last year when UCL astrophysicist Dr Jay Farihi contacted Carnegie Observatories’ Director, John Mulchaey, whilst researching an article on planetary systems surrounding white dwarf stars. Farihi was searching for a glass plate that contained a stellar spectrum of van Maanen’s star – a white dwarf discovered by Dutch-American astronomer Adriaan van Maanen.

“The historical significance of this 1917 observation was first called to my attention by my PhD advisor, Professor Ben Zuckerman of UCLA at a meeting in Cambridge two years ago. And it struck me then that someone needed to track down the actual spectrum and show it to the world — being able to confirm the metal absorption features present in 1917 was a real treat for me as a scientist,” said Dr Farihi.

Stellar spectra are recordings of the light emitted by distant stars and exhibit all the component colours of light interspersed with absorption lines. Astronomers can use these to determine the chemical composition of a star and how emitted light is affected by the other objects it passes through before reaching Earth.

These stellar spectra images allowed 19^th century astronomers to develop a classification system for stars that is still in use today. Although modern astronomers now use digital tools to image stars, glass photographic plates were used for decades to record stellar spectra and take images of the sky.

As requested, the van Maanen’s stellar spectrum plate was located from the archives and sent to Farihi. Other than a notation on the plate’s sleeve indicating that the star looked a bit warmer than our own Sun, everything seemed very ordinary.

However, when Farihi examined the spectrum, he found something quite extraordinary.

The clue was in an absorption line on the spectrum. Absorption lines indicate areas where the light coming from a star passed through something and had a particular color of light absorbed by that substance. These lines indicate the chemical makeup of the interfering object.

“It was instantly clear that Carnegie’s 1917 spectrum of van Maanen’s star revealed two strong lines of calcium, which should have long since disappeared into the star’s interior due to its weight. We only know from research within the last twelve years that van Maanen’s star, and other white dwarfs with heavy elements in their spectra, represent a type of planetary system featuring vast rings of rocky planetary remnants that deposit debris into the stellar atmosphere. Yet the evidence for this has existed since 1917 and the plate spectrum has almost certainly not been looked at in nearly a century,” added Dr Farihi.

“The unexpected realization that this 1917 plate from our archive contains the earliest recorded evidence of a polluted white dwarf system is just incredible,” Mulchaey said. “And the fact that it was made by such a prominent astronomer in our history as Walter Adams enhances the excitement.”

Planets themselves have not yet been detected orbiting van Maanen’s star, nor around similar systems, but Farihi has said that he is confident it is only 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 explained. “The process couldn’t occur unless there were planets there.”

With one of the world’s largest collections of astronomical plates, who knows what other historical discoveries may be found hidden in Carnegie’s archives.