Century-old data holds earliest evidence for existence of exoplanets
Exoplanets, or planets orbiting stars other than the Sun, were first discovered in the 1990s, but old photographic plates taken nearly 100 years ago and recently found in storerooms at Carnegie Observatories in Pasadena, California, contain the first evidence of their existence.
That evidence exists in the form of spectral data on the atmospheres of white dwarfs – stellar remnants of Sun-like stars that have exhausted their fuel, shed their outer layers, and left behind hot cores.
Ben Zuckerman, now a professor emeritus of astronomy at the University of California, Los Angeles, was asked by his former student Jay Farihi in July 2014 to address a symposium on the subject of “polluted white dwarfs” – white dwarfs with outer atmospheres that contain heavy elements.
The presence of these elements in white dwarfs’ atmospheres puzzled scientists because these should have been pulled into the dead stars by their powerful gravity.
Exactly 100 years ago, the first polluted white dwarf, designated van Maanen 2 (“van Maanen’s Star”) and located 14 light-years from Earth, was discovered by Adrian van Maanen, who has spent three years studying its motion in relation to other stars.
Two years later, Walter Sydney Adams, who would one day become director of California’s Mount Wilson Observatory, took a spectrum of the star using the observatory’s 60-inch telescope using a small glass plate.
The spectrum of van Maanen 2 showed strong absorption lines for calcium and other heavy elements, which Adams had presumed that the object fitted the characteristics of an F-type star – a main-sequence, hydrogen-fusing star that is slightly hotter and more luminous than the Sun.
From 1917 onward, more of the polluted white dwarfs with heavy elements in their atmospheres were found, leading scientists to suspect that these elements originated in interstellar space.
Determined to find the spectrum of van Maanen’s star, Farihi contacted Carnegie Observatories, which own the telescopes at Mount Wilson and store archives of observations made with them.
With the assistance of Carnegie’s director and a volunteer, that spectrum was found in just two days. When the glass plate was displayed on a viewer by a Carnegie astronomer, the spectrum measured only one-sixth of an inch.
Farihi immediately noticed two dips in the spectrum – absorption lines of a calcium ion – which confirmed van Maanen 2’s atmosphere contains heavy elements, meaning that it probably has at least one orbiting planet.
“I can’t say I was shocked, frankly, but I was pleasantly blown out of my seat to see that the signature was there, and could be seen even with the human eye,” he reported.
A yellow sticky note reading “Possibly first record of an exoplanet” has since been stuck on the card on which the spectrum was initially recorded on October 24, 1917.
LEFT: The envelope containing the spectrum of van Maanen’s Star, taken Oct. 24, 1917. Photo Credit: NASA / JPL-Caltech. RIGHT: The spectrum of van Maanen’s star. Image Credit: Carnegie Institution for Science
In 2016, Farihi published his findings in the journal New Astronomy Reviews.
Zuckerman, along with colleague Eric Becklen, had studied another white dwarf back in 1987 and found it surrounded by infrared light suspected to be coming from a brown dwarf, an object more massive than the heaviest giant planets but less massive than the smallest stars.
A second study of the star conducted in 1990 attributed the infrared light not to a brown dwarf but to a dusty, hot disk orbiting the dead star.
With the discovery of the first exoplanets, scientists developed a new theory to explain polluted white dwarfs, specifically that planets orbiting the dead stars were pushing smaller planets and asteroids into the stars, disintegrating the former and leaving behind dust containing the planets’ heavy elements, which then fell back onto the star.
While ground-based telescopes cannot measure the exact amount of infrared light coming from a white dwarf, the launch of NASA’s Spitzer Space Telescope brought the study of these objects to a whole new level because, even though the stars are faint, they can be detected in infrared light.
Spitzer has found some 40 more of these polluted white dwarfs while NASA’s Wide-field Infrared Survey Explorer (WISE) discovered several more.
In 2007, Zuckerman and a team of researchers discovered a white dwarf whose atmosphere contains 17 heavy elements, many of which are similar to those found on the Earth and Moon.
He considered the presence of these elements evidence that the dead star had swallowed a small rocky planet or several smaller rocky bodies.
To date, no planet has been discovered orbiting van Maanen 2 or any white dwarfs. However, Zuckerman, along with colleagues, used Hawaii’s W.M. Keck Observatory recently and found evidence of a white dwarf having swallowed an object similar to those in the Solar System’s Kuiper Belt.
While approximately 30 percent of known white dwarfs are “polluted”, their debris disks are difficult to find. Collisions between objects pushed into the stars could transform the dust into gas, which would not exhibit the same infrared signals.
“This star is an icon,” Farihi said of van Maanen 2. “It is the first of its type. It’s really the proto-prototype.”
He recommends exoplanet searches specifically concentrate on white dwarfs.
Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.