Some things just run in families. If you look into the life of Otto Struve, you’ll find that the Russian-born astronomer was the great grandson of Friedrich Georg Wilhelm von Struve, who was himself an astronomer known for his work on binary stars in the 19th Century. Otto’s father was an astronomer as well, as was his grandfather. That’s a lot of familial energy packed into the study of the stars, and the Struve of most recent fame (Otto died in 1963) drew on that energy to produce hundreds of scientific papers. Interestingly, the man who was director at Yerkes and the NRAO observatories was also an early SETI advocate who thought intelligence was rife in the Milky Way.

Of Baltic-German descent, Otto Struve might well have become the first person to discover an exoplanet, and therein hangs a tale. Poking around in the history of these matters, I ran into a paper that ran in 1952 in a publication called The Observatory titled “Proposal for a Project of High-Resolution Stellar Radial Velocity Work.” Then at UC Berkeley, Struve had written his PhD thesis on the spectroscopy of double star systems at the University of Chicago, so his paper might have carried more clout than it did. On the other hand, Struve was truly pushing the limits.

Image: Astronomer Otto Struve (1897-1963). Credit: Institute of Astronomy, Kharkiv National University.

For Struve was arguing that Doppler measurements – measuring the wavelength of light as a star moves toward and then away from the observer – might detect exoplanets, if they existed, a subject that was wildly speculative in that era. He was also saying that the kind of planet that could be detected this way would be as massive as Jupiter but in a tight orbit. I can’t call this a prediction of the existence of ‘hot Jupiters’ as much as a recognition that only that kind of planet would be available to the apparatus of the time. And in 1952, the idea of a Jupiter-class planet in that kind of orbit must have seemed like pure science fiction. And yet here was Struve:

…our hypothetical planet would have a velocity of roughly 200 km/sec. If the mass of this planet were equal to that of Jupiter, it would cause the observed radial velocity of the parent star to oscillate with a range of ± 0.2 km/sec—a quantity that might be just detectable with the most powerful Coudé spectrographs in existence. A planet ten times the mass of Jupiter would be very easy to detect, since it would cause the observed radial velocity of the star to oscillate with ± 2 km/sec. This is correct only for those orbits whose inclinations are 90°. But even for more moderate inclinations it should be possible, without much difficulty, to discover planets of 10 times the mass of Jupiter by the Doppler effect.

Struve suggested that binary stars would be a fertile hunting ground, for the radial velocity of the companion star would provide a “reliable standard of velocity.”

Imagine what would have happened if the discovery of 51 Pegasi (the work of Michel Mayor and Didier Queloz in 1995) had occurred in the early 1960s, when it was surely technically possible. Joshua Winn (Princeton University) speculates about this in his book The Little Book of Exoplanets (Princeton University Press, 2023). And if you start going down that road, you quickly run into another name that I only recently discovered, that of Kaj Aage Gunnar Strand (1907-2000). Working at Sproul Observatory (Swarthmore College) Strand announced that he had actually discovered a planet orbiting 61 Cygni in 1943. Struve considered this a confirmed exoplanet.

Now we’re getting deep into the weeds. Strand was using photometry, as reported in his paper “61 Cygni as a Triple System.” In other words, he was comparing the positions of the stars in the 61 Cygni binary system to demonstrate that they were changing over time in a cycle that showed the presence of an unseen companion. Here I’m dipping into the excellent Pipettepen site at the University of North Carolina, where Mackenna Wood has written up Strand’s work. And as Wood notes, Strand was limited to using glass photographic plates and a ruler to make measurements between the stars. Here’s the illustration Wood ran showing how tricky this would have been:

Image: 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. Credit: Mackenna Wood.

Strand’s detection is no longer considered valid because more recent papers using more precise astrometry have found no evidence for a companion in this system. And that was a disappointment for readers of Arthur C. Clarke, who in his hugely exciting The Challenge of the Spaceship (1946) had made this statement in reference to Strand: “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.”

Can you imagine the thrill that would have run up the spine of a science fiction fan in the late 1940s when he or she read that? Someone steeped in Heinlein, Asimov and van Vogt, with copies of Astounding available every month on the newsstand and the great 1950s era of science fiction about to begin, now reading about an actual planet around another star? I have a lot of issues of Astounding from the late 1930s in my collection though few from the late ‘40s, but I plan to check on Strand’s work to see if it appeared in any fashion in John Campbell’s great magazine in the following decade. Surely there would have been a buzz at least in the letter columns.

Image: Kaj Aage Gunnar Strand (1907-2000) was director of the U.S. Naval Observatory from 1963 to 1977. He specialized in astrometry, especially work on double stars and stellar distances. Credit: Wikimedia Commons / US Navy.

We’re not through with early exoplanet detection yet, though, and we’re staying at the same Sproul Observatory where Strand did the 61 Cygni work. It was in 1960 that another Sproul astronomer, Sarah Lippincott, published work arguing that Lalande 21185 (Gliese 411) had an unseen companion, a gas giant of ten Jupiter masses. A red dwarf at 8.3 light years out, this star is actually bright enough to be seen with even a small telescope. And in fact it does have two known planets and another candidate world, the innermost orbiting in a scant twelve days with a mass close to three times that of Earth, and the second on a 2800-day orbit and a mass fourteen times that of Earth. The candidate planet, if confirmed, would orbit between these two.

Image: Swarthmore College’s Sarah Lippincott, whose work on astrometry is highly regarded, although her exoplanet finds were compromised by faulty equipment. Credit: Swarthmore College.

The work on Lalande 21185 in exoplanet terms goes back to Peter van de Kamp, who proposed a massive gas giant there in 1945. Lippincott was actually one of van de Kamp’s students, and the duo used astrometrical techniques to study photographic plates taken at Sproul. It turns out that Sproul photographic plates taken at the same time as those Lippincott used in her later paper on the star were later used by van de Kamp in his claim of a planetary system at Barnard’s Star. It was demonstrated later that the photographic plates deployed in both studies were flawed. Systematic errors in the calibration of the telescope were the culprit in the mistaken identifications.

Image: Astronomer Peter van de Kamp (1901-1995). Credit: Rochester Institute of Technology newsletter.

We always knew that exoplanet hunting would push us to the limits, and today’s bounty of thousands of new worlds should remind us of how the landscape looked 75 years ago when Otto Struve delved into detection techniques using the Doppler method. At that time, as far as he knew, there was only one detected exoplanet, and that was Strand’s detection, which as we saw turned out to be false. But Struve had the method down if hot Jupiters existed, and of course they do. He also reminded us of something else, that a large enough planet seen at the right angle to its star should throw a signal:

There would, of course, also be eclipses. Assuming that the mean density of the planet is five times that of the star (which may be optimistic for such a large planet) the projected eclipsed area is about 1/50th of that of the star, and the loss of light in stellar magnitudes is about 0.02. This, too, should be ascertainable by modern photoelectric methods, though the spectrographic test would probably be more accurate. The advantage of the photometric procedure would be its fainter limiting magnitude compared to that of the high-dispersion spectrographic technique.

There, of course, is the transit method which has proven so critical in fleshing out our catalogs of exoplanets. Both radial velocity and transit techniques would prove far more amenable to early exoplanet detection than astrometry of the sort that van de Kamp and Lippincott used, though astrometry definitely has its place in the modern pantheon of detection methods. Back in 1963, when van de Kamp announced the discovery of what he thought were planets at Barnard’s Star, he relied on almost half a century of telescope observations to build his case. No one could fault his effort, and what a shame it is that the astronomer died just months before the discovery of 51 Pegasi b. It would be fascinating to have his take on all that has happened since.