The first exoplanet – a planet orbiting a star other than our sun – was discovered by Michael Mayor and Didier Queloz in 1995, a finding for which they were awarded the 2019 Nobel Prize in Physics. Thus, it was to my great surprise that, while reading a collection of nonfiction by Arthur C. Clarke, I came upon the following sentence:

“The first discovery of planets revolving around other suns, which was made in the United States in 1942, has changed all ideas of the plurality of worlds”

– “The Challenge of the Spaceship”, Arthur C. Clarke 1946

A deep dive into Google led me to the scientific paper “61 Cygni as a Triple System”  by astronomer K. A. Strand. This short paper, published in 1943, shows that the measured position of the stars in the 61 Cygni system were changing slightly over time, in a predictable cycle. Small changes in position like this indicate that there is an unseen orbiting companion to the star, whose gravitational pull is slightly shifting the movement of the primary star.

By measuring the size of the cyclical changes in position, the mass of the object creating them can be determined. And this paper did just that! Strand found that the companion (61 Cygni c) had a mass of about 16 times the mass of Jupiter. Strand asserts:

“With a mass considerably smaller than the smallest known stellar mass … the dark companion must have an intrinsic luminosity so extremely low that we may consider it a planet rather than a star. Thus planetary motion has been found outside the solar system.”

Excellent! So why then isn’t 61 Cygni C lauded today as the first discovery of an exoplanet?

There are several problems with the discovery, the most significant of which deals with the accuracy of the position measurements used. In the 1930s and 40s, when these measurements were taken, astronomers did not have access to the more precise and accurate methods and tools that we use today to measure position and velocity. Instead, they used glass photographic plates and used a ruler to measure the distances between the stars. By repeating this with hundreds of observations (each on its own 8” by 8” glass plate), they could see how the positions changed over time.

An example of a photographic plate from one of the telescopes used in the 1943 61 Cygni study.
An example of a photographic plate from one of the telescopes used in the 1943 61 Cygni study. The plate is a negative, showing stars as black dots, and empty space in white. Brighter stars appear as larger dots. Written at the bottom of the plate are notes indicating when the image was taken (Nov. 10, 1963), and what part of the sky it shows.

This method had problems though, and it was later shown that changes in the telescope lens or a change in the chemicals used to develop the photographic plates produced small changes in the position of stars. In the 1980s a number of papers came out, which, using more precise methods of measurement, found no evidence for any orbiting companion in the system. This debunked not only 61 Cygni C as the “first exoplanet” but also several other purported exoplanet discoveries that had been made using the same techniques in the 40 years following.

The “discovery” was later eclipsed by the first true exoplanets in the 1990’s and by the thousands more  (4,521 and counting) that have been confirmed since. When I brought it up at a lab meeting, no one in the room had ever heard of the 1942 61 Cygni c exoplanet discovery. And thus one sentence, written 75 years ago, sent me off down a week-long rabbit hole into early “planet” discoveries.

Peer Editor: Alayna Mackiewicz

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