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Century-Old Glass Photographic Plate Reveals Earliest Evidence of an Exoplanet

Photographic plate showing the spectrum of the star discovered by Adriaan van Maanen (1917) (courtesy Carnegie Institution for Science)
Photographic plate showing the spectrum of the star discovered by Adriaan van Maanen (1917) (courtesy Carnegie Institution for Science)

According to NASA’s Jet Propulsion Laboratory, the “first true extrasolar planet” was discovered in 1994. But it turns out that a far earlier observer inadvertently captured evidence of an exoplanet, or a planet not circling our Sun.

The Carnegie Institution for Science announced this week that one researcher’s dive into a collection of glass photographic plates turned up an unexpected image from 1917 that indicates the presence of an exoplanetary system. When Jay Farihi of University College London was working on a paper about a year ago, he got in touch with the Carnegie Observatories in Pasadena, California, to track down a plate showing the “stellar spectra” of a white dwarf star found by astronomer Adriaan van Maanen in 1917. The imaging of these light emissions is invaluable for astronomers seeking to learn about the chemical compositions of distant stars and the objects their light passes through on its way to Earth.

When the plate was pulled from the archives — with handwritten notes by observer Walter Adams, then director of Mount Wilson Observatory, clearly marking the date as October 24, 1917 — Farihi noticed something unusual: the presence of an “absorption line,” where it seemed the star’s light went through something made of heavier elements than should have been visible. The Carnegie Institution explains the significance of this:

Only within the last 12 years has it become clear to astronomers 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. These recently discovered systems are called “polluted white dwarfs.” They were a surprise to astronomers, because white dwarfs are stars like our own Sun at the end of their lifetimes, so it was not at all expected that they would have leftover planetary material around them at that stage.

Handwritten notes by Walter Adams that accompanied the glass plate (courtesy Carnegie Institution for Science)
Handwritten notes by Walter Adams that accompanied the glass plate (courtesy Carnegie Institution for Science)

Farihi’s article,”Circumstellar Debris and Pollution at White Dwarf Stars,” came out this month in New Astronomy Reviews, and you can read it online as a freely accessible PDF.

Glass photographic plates were a vital tool of record and research for 19th- and early-20th-century astronomers, but their archives often remain under-examined, especially since digital imaging rendered them obsolete. For example, this January Hyperallergic reported on over 150 glass plate photographs of the moon, stars, and solar eclipses that were rediscovered in the basement of the Niels Bohr Institute in Copenhagen; they’d been totally forgotten, until a scientist happened to make tea in the subterranean storage space.

Carnegie Observatories Director John Mulchaey stated in the announcement that the institution has around 250,000 plates from three observatories in its collections. “We have a ton of history sitting in our basement,” he said, “and who knows what other finds we might unearth in the future?”

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