Early evidence for extrasolar planets in century-old photographic plates

Mount Wilson is where some of the founding discoveries in our knowledge of the galaxy and the universe were made at the beginning of the 20th century. But there is also a lesser-known discovery, a discovery from a century ago, a discovery that was not recognized or appreciated until recently - the first evidence for the existence of planets outside the solar system, if you know where to look for them 

Beneath an elegant office with a Spanish-style red-tiled roof in Pasadena, California, sit three weather-beaten warehouses that hold well-preserved findings from more than a century of astronomy. Whoever descends the stairs and turns to the right, finds himself in a magical basement. Countless wooden boxes and drawers, arranged in piles from floor to ceiling, with photographic plates of telescopes, paintings of sunspots, and many different records. The smell of ammonia fills the air, a smell reminiscent of old photographic film.

One of the storage rooms is protected by a black door with a sign that says "This door should remain closed".

The Carnegie Observatory houses more than 250,000 photographic plates taken at the Mount Wilson, Palomar, and Las Campanas observatories. In their heyday, the Mount Wilson telescopes—one 60 inches (about 1.5 m) in diameter and the other 100 inches (about 2.5 m) in diameter, the larger of which saw starlight for the first time on November 1, 1917—were the most advanced telescopes most in the world. The observations made with them changed our understanding of our place in the universe. But these marvels of technology were ahead of their time - and in one case, picked up signs of distant worlds about a century before they could be detected.

Mount Wilson was the site where some of the most important discoveries about our galaxy and the universe were made in the early 20th century. It was there that Edwin Hubble realized that the Milky Way galaxy could not be our entire universe, as Andromeda (or M31) is further away from the farthest reaches of our galaxy. The photographic plate from the 100 Hooker telescope (1923-inch Hooker), which preserved the same seminal understanding, was enlarged into a giant poster displayed outside Carnegie's warehouses.

Hubble and Milton Humason (Milton Humason) who began his career at Mount Wilson as a server, worked together in the study of the universe and the study of the expansion of the universe. Using the mythical telescopes, combined with data from the Lovell Observatory in Flagstaff, Arizona, they detected that clusters of galaxies are moving away from each other - and the more distant galaxies are moving away from each other at a faster rate.

But there is a much lesser-known discovery, also from the Mount Wilson archives, a discovery that took a century to only recently be properly understood: the first evidence of planets outside the solar system.

A detective story

The story began with Ben Zuckerman, Professor Emeritus of Astronomy at the University of California, Los Angeles, California. Prof. Zuckerman prepared a lecture on the material composition of planets and smaller rocky bodies outside our solar system for a symposium held in July, 2014. He was invited by Jay Farihi - Zuckerman was Farihi's supervisor when Farihi studied at the University of California. Freehy suggested that Zuckerman lecture on the contamination of white dwarfs - dead stars that shine with a weak intensity and are composed mostly of hydrogen and helium. The meaning of "pollution" in astronomy is the invasion of heavier elements into the photospheres - the outer atmospheres - of these stars. Those additional heavy elements are not supposed to be there - the strong gravity of a white dwarf is supposed to pull the elements into the star, and out of our ability to see them.

The first contaminated dwarf detected was called Van Maanen's star - or "van Maanen 2" in the scientific literature. It is named after the star discoverer Adrian Penn Mannen. Penn Mannen discovered it in 1917 by detecting a slight movement relative to other stars between 1914 and 1917. Astronomer Walter Sidney Adams, who was later appointed director of the Mount Wilson Observatory, captured the spectrum—the chemical fingerprint—of Penn Mannen's star on a small photographic plate using Mount Wilson's 60-inch telescope. Adams interpreted the spectrum as characteristic of an F-type star, apparently based on the significant presence of calcium and other light-absorbing properties characteristic of heavy elements, but with a temperature slightly higher than that of our Sun. In 1919 Pen Manen called it "a star with a very weak light".

Today we know that Pen Manan, which is 14 light-years away, is the closest white dwarf to Earth that is not part of a binary system.

"This star is a symbol," Freehi said recently. "He is the first of his kind. In fact, he is the prototype.”

While Zuckerman was preparing the lecture, something happened to him that in retrospect he called "his moment of enlightenment". The Penn Manan star is the first observation that has evidence for the existence of planets outside the solar system - something that was unknown to the astronomers who studied it in 1917, and was unknown to astronomers who pondered it for decades after it was discovered.

What does this have to do with planets outside the solar system?

Heavy elements in the outer layer of the star could not form inside the star, as they would immediately sink due to the strong gravity of the white dwarf. With the discoveries of more white dwarfs with heavy elements in their photospheres, scientists have developed an explanation that the origin of the exotic materials must be in the interstellar medium - that is, elements that move in the space between the stars.

In 1983, more than 70 years after the Penn Manan spectrum was preserved, Zuckerman and his research partner Eric Becklin reported an excess of infrared light around a white dwarf. They believed that it originated from a "failed star" with low intensity, a brown dwarf. In 1990 their interpretation was that it was a hot disk of dust surrounding a white dwarf. In the early 2000s, a new theory was developed about white dwarf contamination: planets can pull small rocky bodies towards the star, and the star's strong gravity crushes the small bodies into dust. The dust, containing heavy elements from the crushed body, falls on the star.

"The important principle is: if you're an asteroid or a comet, you can't just change your address. You need someone to move you," explains Frihy. "The best candidates to be the ones who move you are planets."

The Spitzer Space Telescope has played a vital role in expanding the study of the polluted heart dwarfs surrounded by hot dust disks. Since its launch in 2004, Spitzer has confirmed observations of about 40 such stars. Another NASA space telescope, WISE (Wide-field Infrared Survey Explorer), also discovered a handful of similar stars, bringing the number of known stars to about four dozen. Since these celestial bodies are of low luminous intensity, infra-red light is needed to detect them.

According to Freehi, it is not possible to accurately measure the amount of infrared light that comes from the objects using telescopes on the ground. And the Spitzer telescope broke the field wide open.

In 2007, Zuckerman and his colleagues published observations of a white dwarf's atmosphere with 17 elements - substances similar to those found in the terrestrial-lunar system. The observations supported the "dust disk" theory around white dwarfs. (The late Professor Michael Jura of the University of California, who made vital contributions to the study of contaminated white dwarfs, was part of the research team.)

It was further evidence that at least one rocky body – perhaps even a planet – is being crushed by the gravity of a white dwarf. Scientists today tend to agree that a white dwarf with heavy elements in its spectrum likely has at least one other rocky ejecta belt—remnants of bodies that collided violently and never formed planets—and at least one planet.

Heavy elements that happened to be moving in the interstellar medium cannot provide an explanation for the observations. According to Zuckerman, about 90 years after Pen Manen's discovery, astronomers said "Wait - the growth model from interstellar elements cannot be correct."

Chasing the spectrum

Inspired by Zuckerman, Freehy was captivated by the idea that someone had photographed a spectrum with the first evidence of extrasolar planets as early as 1917, and that there must be some record of that observation. "I grabbed the question with both hands, and I wasn't ready to let it go." Freehi said.

Freehy approached the Carnegie Observatory, which owns the Mount Wilson telescopes and is also responsible for their archives. Carnegie administrator John Malkahy gave the task to volunteer Dan Cohen. Cohen dug into the archives and after two days from your Leahy to Freahy a picture of the spectrum.

Freehi described his reaction: "I can't honestly say I was shocked, but I jumped out of my chair with joy when I saw that the fingerprint was visible there, and you can even see it with the eye."

The spectrum of the planet Pen Manen that Frihi requested is currently in a small archive sleeve, with a date ending in handwriting "October 24, 1917" and a contemporary yellow sticky note "apparently - the first record for a planet outside the solar system".

Cynthia Hunt, an astronomer who chairs the Carnegie History Committee, took the photographic plate out of the envelope and placed it on a display device that illuminated it. The size of the spectrum itself is a little more than 0.4 cm.

Although at first glance you don't see anything special about the board, Freehi saw two prominent "fangs" that represent dips in the spectrum. For him, this was the smoking gun: two absorption lines from the same calcium ion, meaning two heavy elements in the white dwarf's photosphere - indicating that it likely has at least one planet. He wrote about it in 2016 in Astronomy Reviews.

Extrasolar planets and ejecta disks

It has long been widely believed among scientists that the gravitational pull of giant planets holds the accretion belts in place. A recent study published in The Astrophysical Journal showed that young stars with dust rims are more likely to have giant planets at a greater distance from the star than stars without dust belts.

A white dwarf is not a young star - on the contrary, it was formed when a low- to medium-mass star burned up all its fuel. But the principle is the same: the gravitational force of the giant planets can throw small rocky bodies towards the white dwarf.

Our Sun will become a red giant in about 5 billion years, and it will grow to the point where it will engulf the Earth as well before it sheds its outer layers and becomes a white dwarf. At this point, Jupiter's gravitational pull on the asteroid belt may increase, throwing asteroids toward the waning Sun. This scenario may explain the heavy elements in the planet Penn Manan.

Spitzer's observations of the Penn Manan star have so far found no planets there. In fact, to date, no planets have been found orbiting white dwarfs, although one of them has an object that is said to be a giant planet. More compelling evidence has been discovered in recent years. Zuckerman and other scientists, after observations made at the WM Keck Observatory in Hawaii, announced that they had discovered evidence of a Kuiper Belt-like object swallowed by a white dwarf.

Scientists are still studying polluted white dwarfs and looking for planets that may orbit them. About 30% of all white dwarfs known to us are contaminated white dwarfs, but their accretion discs are hard to find. Prof. Jura raised the possibility that since many asteroids collide in the eruption, it is possible that the dust turns into gas, and it is more difficult to detect gas in infrared because it does not have the same infrared signal that dust has.

Freehy was very excited that his detective work in the Mount Wilson archives had yielded such results. In 2016, he described the historic discovery in the context of an article on contaminated white dwarfs, and claimed that white dwarfs are "compelling targets for systematic research on planets outside the solar system."

Who knows what other treasures are waiting to be discovered in the archives of the growing observatories - records of observations of the sky of the cosmos are highly detailed. The curiosity of people who ask the right questions may lead them to more clues.

Frihy said: "The personal interaction with the data can spur us to be committed to the questions we ask."

translated

Overlooked Treasure: The First Evidence of Exoplanets