In a classic case of “one man’s trash is another man’s treasure,” an astronomical glass plate from the Carnegie Observatories’ collection was inquired about in order to complete research in a whole different field when the researcher realized that the plate theorized a a planetary system beyond the Sun. It provides the earliest known evidence of exoplanets.
Random understanding of exoplanets from a strange source
Jay Farihi of University College London, was researching an article about planetary systems that exist in the vicinity of white dwarf stars (recently published in New Astronomy Reviews), when he contacted the director of the Carnegie Observatories’ archive, John Mulchaey. Farihi was interested in plate that he knew to show the spectrum of van Maanen’s star, which was discovered by Adriaan van Maanen in 1917.
Stellar spectra images were first used by astronomers in the 19th century to build a system for classifying stars that is nominally still used today. Clearly, astronomers today have a wealth of digital tools at their disposal, but they are still looking for the light emitted by stars as component colors of light, in a manner similar to a rainbow through a prism. These colors help astronomers determine the chemical composition of stars. Stellar spectra also show how light from the star captured affects that which it passes through on the way towards observation from Earth.
Once Farihi got his hands on the plate, he realized that the plate showed the earliest evidence of a system of exoplanets beyond our Sun. The plate was made by former Observatories Director Walter Adams at Mount Wilson Observatory, which, at the time, was part of the Carnegie.
What he found
In the “absorption line” on the spectrum, Farihi noticed that the light emitted from the star clearly passed through something and parts of the spectrum were absorbed by something. The plate showed the presence of calcium, iron and magnesium, heavier elements that had no business on the plate as their weight should have seen them “fall” into the interior of the star.
It is only in the last 12 years that astronomers have realized that white dwarfs such as van Maanen’s star with heavy elements in their spectra are actually a kind of planetary system. They have rings of rocky remnants that then leave debris in the stellar atmosphere, and are known as “polluted white dwarfs.”
Scientists were surprised by the findings because white dwarfs are stars like our own Sun that have reached the end of their life. It was not predicted that planetary material would be left over at that stage.
Exoplanets first discovered in 1917
“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.”
Although there have not been any planets detected in orbit around van Maanen’s star at this point, Farihi believes that there will be in the future.
“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.”
“Carnegie has one of the world’s largest collections of astronomical plates with an archive that includes about 250,000 plates from three different observatories—Mount Wilson, Palomar, and Las Campanas,” concluded Mulchaey. “We have a ton of history sitting in our basement and who knows what other finds we might unearth in the future?”