A Very Useful Exoplanet

 

by Edward M. Wysocki, Jr.

 

Published in September/October 2022 issue of Analog Science Fiction & Fact. Copyright © 2022. All rights reserved. No part may be reproduced in any form without the explicit permission of the author.

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Planets that orbit a star other than our Sun, which we now call exoplanets, have long been a feature of works of science fiction. It is only in the past few decades that the existence of exoplanets has been confirmed. Claims had been made, however, for many years before the first confirmation of an exoplanet around a main-sequence star in 1995.

 

One particular early claim resulted in two articles and a novel that appeared in Astounding Science-Fiction. The article that reported the claim was “The World of 61 Cygni C” by Robert S. Richardson, which appeared in the July 1943 issue.

 

Robert Shirley Richardson was born in Kokomo, Indiana on 22 April 1902. He received his bachelor’s degree in astronomy from UCLA in 1926. After working for two years at Mt. Wilson Observatory, he went to Berkeley to obtain a doctorate in astronomy. He obtained his Ph.D. in 1931, with a dissertation titled “An Investigation of Molecular Spectra in Sun-Spots.” He returned to Mt. Wilson as an observational astronomer until 1958. He then became associate director of the Griffith Observatory in Los Angeles, a position he retained until 1962. Richardson died on 12 November 1981.

 

In addition to technical papers, articles, and books on astronomy, he wrote for Astounding, Analog, and other science fiction magazines. His fiction appeared as by Phillip Latham.

 

Richardson was very prolific when it came to non-fiction in Astounding and Analog. He had 50 articles between September 1939 and February 1971. One that was a bit different was “Turn on the Moon – Make it Hotter” in the November 1943 issue of Astounding. It related his experiences as a technical advisor to the 1944 romantic comedy film “The Heavenly Body.” The plot focused on a professional astronomer, played by William Powell, and his wife, played by Hedy Lamarr.

 

Before looking at his article, let us consider the various ways that exoplanets have been discovered. Practically all of the confirmed exoplanets were discovered using either of two techniques: radial velocity and transit photometry.

 

The radial velocity method measures small variations in the velocity of a star toward or away from Earth. Periodic variations will be due to the gravitational pull of a planet upon the star during its orbit. Very sensitive spectrometers determine the velocity changes by the shift in spectral lines due to the Doppler effect. The size of the shift is dependent upon the mass of the planet and its distance from the star. The ideal situation is when the plane of the planet’s orbit is closely aligned with the line of sight to Earth.

 

The transit method is a bit easier to understand. If a planet passes between the star and an observer, the light from the star is reduced by some amount. One condition is similar to that of the radial velocity technique – the plane of the orbit must be aligned with the line of sight to Earth. The amount of reduction in the light depends on the relative sizes of the star and the planet. All of the exoplanets discovered by the Kepler Space Telescope were found using the transit method.

 

The oldest technique used in the search of exoplanets is called astrometry. It involves precise measurements of the positions and velocities of celestial bodies. It is the method described in “The World of 61 Cygni C.”

 

Richardson began by clearly associating the discovery with the work done by double star astronomers. The data used in the study of double stars consists of measurements of the distance and direction of the fainter star (B) from the brighter star (A). These measurements were originally made visually using a micrometer.

 

One sees the same objects that are visible through a normal eyepiece, but there are also two parallel lines illuminated from the side. One line is fixed and the other is movable. This pair of lines may be rotated to any desired orientation. By moving one line, the width of a single object or the relationship between two objects may be determined.

 

Richardson described the delicate steps that are involved in making a measurement with the micrometer. He emphasized that an important factor was that the observer should be in a comfortable position. As observations are being made, the telescope drive mechanism is continually moving it to track the stars. Eventually it will be oriented so that the observer is unable to place himself in a position to make the observation.

 

The other problem is one that affects all earthbound observations – the atmosphere. Depending on the amount of atmospheric turbulence, it might be impossible on some night to place the star image on one of the micrometer lines.

 

Assume that you are a very diligent and experienced observer. You have obtained a number of measurements for a particular double star over several years. Depending on the size of the orbit of the double star chosen, it might take several decades or even longer of such observations. You must combine your observations with those published by others. What result might you get?

 

In the article, Richardson displayed a plot for a different double star, OΣ 278. The points plotted were obtained by observations from 1843 to 1927. Each point represented the distance and direction of component B, the fainter star, from component A. The distribution of the points was such that it would be impossible to plot any kind of smooth curve. How this related to the problem of locating a planet was clearly stated by Richardson:

 

If we are unable to trace the orbit of B with respect to A, how then can we expect to distinguish tiny sinuosities in the motion of B caused by an invisible companion C?

 

What most will consider the obvious solution is to make use of photography. The results are not obtained by a fleeting measurement by the observer, but are preserved for later careful examination and measurement. But, as Richardson pointed out, photography is not free of problems.  Although some portion of his article was concerned with these problems and how they were solved, that discussion has no direct bearing on the main sense of this article and I will not consider it here.

 

Our real interest is in the results obtained by Kaj Aage Gunnar Strand (1907 – 2000), a Danish astronomer who worked both in Denmark and the United States. His work on 61 Cygni was performed from 1938 to 1942 at the Sproul Observatory of Swarthmore College where he worked as a Research Associate under Peter van de Kamp (1901 – 1995).

 

In the case of 61 Cygni, Strand took five hundred plates with the 24-inch refractor at the Sproul Observatory. As Richardson pointed out, those few years would have provided too little data to determine the orbit of a double star with a period that he said was estimated at 700 years (A good estimate – the recent value I found was about 659 years).

 

Fortunately, other astronomers had also photographed 61 Cygni. There were 19 plates taken from 1870 to 1874 by Lewis Rutherfurd (given as Rutherford in the article), who was a pioneer in celestial photography. To this were added plates taken by Ejnar Hertsprung from 1914 to 1918. Richardson said that these plates were combined with the most reliable of the micrometer results, but did not explain how they were determined to be reliable.

 

The first step in locating a possible planet was the determination of the orbit of B around A. After that had been accomplished, it was then found that there were small irregularities in the orbit. The question facing Strand and other planet hunters was given by Richardson as:

 

Are these real deviations from the orbit arising from the disturbing effect of an invisible component or merely the last traces of some systematic error still only partially eliminated?

 

In January 1942, some degree of uncertainty still existed for Strand. After taking additional plates throughout 1942, he then felt that he had the data to support the conclusion that a third component existed in the 61 Cygni system and that its mass was 16 times that of Jupiter. This was announced at the end of 1942.

 

Both of the stellar components of 61 Cygni are dwarf stars. Richardson’s article gives the masses of these as 0.58 and 0.55 of that of our Sun. The modern values are 0.70 and 0.63. Unlike some double star systems, there is not a component which is obviously much larger and brighter. The usual convention is that the slightly more massive component is to be called A.

 

The mass of B and the perturbations of B caused by C allowed the mass of C to be determined as 16 times the mass of Jupiter. The article stated that the orbit of C around B was an ellipse with an eccentricity of 0.7. This is a higher eccentricity than any planet or dwarf planet in our system. Such an eccentricity is more typical of a comet.

 

The first question to be considered is what a body of 16 Jupiter masses would be like. Would it just be a much larger Jupiter? To address that question, Richardson presented the results of two Indian physicists, Kothari and Majumdar. Their results had been presented in Nature and the Monthly Notices of the Royal Astronomical Society. They showed that planetary radius increases as the mass increases until a certain limit is reached. Past that limit the radius will begin to decrease as the mass continues to increase. This is due to an increase in the density of the planet.

 

Their results were shown in the article by a rough curve that plotted mass versus radius. On one side of the maximum radius point were the planets with which we are familiar.  On the far side of the curve were objects containing degenerate matter, such as white dwarfs. 61 Cygni C, as a result of its mass, was on the same side as the white dwarfs. It just so happened that Jupiter occurred at the maximum point on the curve.

 

Proceeding along these lines, it was stated that 61 Cygni C would be smaller than Jupiter, about the size of Uranus. In such a case, the average density of the planet would be 410 times that of water or 74 times that of Earth. The surface gravity would be on the order of 300 g.

 

Richardson’s view of the situation was:

 

Thus it is seen that we have just enough theoretical knowledge to make guesses at the physical state of the new planet without being able to make any definite statements about it or give an emphatic NO to even the wildest proposals. An ideal world for exploration via Science-Fiction!

 

The article concluded with an Addendum that began by discussing details of the binary star system 70 Ophiuchi. Astronomers at the Leander McCormick Observatory at the University of Virginia claimed to have found a third body in the system with a mass of only 10.5 Jupiter masses, smaller than 61 Cygni C. Also mentioned were comments by Henry Norris Russell of Princeton University. These comments were aimed at the problem of whether 61 Cygni C should be considered a planet or a star.

 

The last sentence of Richardson’s comment indicates the next direction to follow. We must jump nearly 10 years to the June 1953 issue of Astounding. It is there that we encounter the article “Whirligig World” by Hal Clement. This article presented the background behind his novel Mission of Gravity, then appearing in serial form in Astounding from April to July.

 

There may be some of you who have never read Mission of Gravity. The action takes place on a planet named Mesklin. The planet is highly oblate and spins at such a rate that while the gravity at the poles is 700 g, it is only 3 g at the equator. A probe launched by humans has become stranded at the pole and the high gravity prevents any direct human effort to recover it. The plot concerns the efforts of intelligent inhabitants of the planet, capable of withstanding the polar gravity, to recover the probe.

 

Clement began by describing the game of finding as many items in a science fiction story that conflict with current scientific knowledge. What the author has to do is minimize such conflicts. Certain exceptions are allowed, the most usual is finding some way around the prohibition of travelling faster than light. Whatever exceptions to known science are used, Clement maintained that they must be presented in way that is fair to the reader. No last-minute surprise solutions. He said that in creating “Mission of Gravity,” he was playing as fairly as he could.

 

The first diagram of the article was a plot of the orbit of 61 Cygni C around one of the stars. Clement stated as did Richardson that the nature of the measurements made it impossible to say whether the planet was orbiting around star A or star B. Richardson assumed star B. Clement assumed the slightly more massive of the two, star A. In any case, measurements of Clement’s diagram and some simple calculations gave orbital parameters consistent with what Richardson had provided.

 

Without referring to the Indian physicists mentioned in Richardson article, Clement repeated the claim that Jupiter represented a special case as mass increased. As the mass increases beyond the “Jupiter point,” the density increases and the radius decreases. He arrived, as did Richardson, with a planet the size of Uranus and Neptune and a surface gravity of 300 g.

 

While Clement clearly stated that the background he developed was based on Strand and his discovery of 61 Cygni C, there is no mention at all of Richardson.  In Richardson’s article, it is clear that he had two sources: one for information on the 61 Cygni results and another for the work of the Indian physicists. Either Clement accessed both of Richardson’s sources or he got all of what he needed from Richardson’s article. In the absence of any other information, I will go with the latter case.

 

To repeat my earlier statement, we have the case of two articles and a novel appearing in Astounding on the basis of one scientific discovery. Someone will no doubt point out that were additional stories that took place on Mesklin, but the important work of creating the planet was performed for Mission of Gravity.

 

It was at this point that Clement began his development of what became the planet Mesklin. The first question is, how does one obtain a more reasonable value of surface gravity, at least on some part of the planet. The answer is to spin the planet at such a rate as to generate a great deal of centrifugal force. You can then make gravity at the equator be as low as you like.

 

Clement chose a value of 3 g at the equator. One consequence was that the world would be an oblate spheroid with considerable flattening. The equatorial diameter worked out to 48,000 miles, but the measurement from pole to pole was only 19,740 miles. To accomplish this, the speed of rotation is slightly more than 20 degrees per minute. This makes a Mesklin “day” approximately 17 ¾ minutes long.

 

Clement then added a ring system and some moons. The diagram of the flattened planet and its rings in the article does, as Clement admitted, look like a fried egg.

 

With the size and shape of the planet established, Clement then had to worry about the conditions that would permit life to exist. It is here that he diverged slightly from information presented by Richardson, who gave the temperature on the equator as -30° C at perihelion and -170° C at aphelion. Clement gave values, based on his own calculations, of -50° C and -180° C, respectively.

 

With such a temperature range, the problem then became the selection of a substance that would take the place of water. After considering various substances, in conjunction with Isaac Asimov, he arrived at the choice of methane. This selection, however, was not without its problems.

 

The biggest problem is that methane boils at what Clement called a “inconveniently low temperature” of -164° C at normal atmospheric pressure. Why not just increase the atmospheric pressure? Methane’s critical temperature is about – 82° C. This means that it will always be a gas above that temperature, no matter how great the pressure.

 

Some additional calculations by Clement gave a limit of eight times normal atmospheric pressure for Mesklin. At that pressure, the boiling point of methane is -143° C. With a highly eccentric orbit, the methane would remain liquid for about 5/6 of the year of 1800 days. But for the remaining 300 days, you would have Mesklin’s oceans of methane boiling.

 

His solution was to incline the axis of rotation by 28 degrees, with the northern hemisphere’s midsummer at the perihelion. Much of the northern hemisphere would get no sunlight for three quarters of the year. This would cause a massive buildup of frozen methane in the northern hemisphere. As the planet approached perihelion, the southern hemisphere would be pointed away from its sun. The effect of the increased temperature in the northern hemisphere would be the melting and boiling of the large methane ice cap.

 

Would all of this actually work? Probably not. But it did provide Clement with the basis for his story, and he was then able to develop the additional details he needed.

 

There is just one small problem with Richardson’s article on 61 Cygni C and Clement developing the planet Mesklin with 61 Cygni C as a starting point:

 

THE PLANET 61 CYGNI C DOES NOT EXIST!!

 

To understand this situation, we must not only look at 61 Cygni C, but at additional claims that were made by the Sproul Observatory. Before proceeding with those claims, I must dispose of the 70 Ophiuchi claim at the end of Richardson’s article. This claim was not associated with the Sproul Observatory. The paper in which these results were presented was considered vague, and additional analysis finally led to the conclusion that there was nothing there. For the moment, that left only 61 Cygni as a planet orbiting another star. But more were to follow.

 

Although Richardson’s article only mentioned Dr. Strand, we must also consider Dr. van de Kamp, the person who brought Strand to Swarthmore. Peter van de Kamp studied at the University of Utrecht. Following that, he was at the Kapteyn Astronomical Institute at Groningen. He came to the United States in 1923. As a result of work done during a one-year Fellowship at Lick Observatory, he received a Ph.D. from the University of California in June 1925. He received a second Ph.D. from the University of Groningen in 1926. He became director of the Sproul Observatory in 1937.

 

As early as 1951, the data on star Lalande 21185 appeared to indicate that there was a companion object. Unlike the cases of 61 Cygni and 70 Ophiuchi, this star was not a member of a binary system. During the next nine years, Sarah Lee Lippincott, working with van de Kamp, gathered additional data. In August 1960, it was possible to announce the presence of an object of 10.4 Jupiter masses.

 

In 1963, van de Kamp made the announcement that there was a planet around Barnard’s Star. It is the second closest star system to the Sun, only 6 light years away. Like Lalande 21185, it is a single star rather than a member of a binary system. The planet was much smaller than previous claims, only 1.6 Jupiter masses.

 

This was followed by another announcement in 1966. This one concerned Luyten’s Star, at a distance of a bit over 12 light years from the Sun. Van de Kamp’s analysis led to a claim that the planet orbiting Luyten’s Star had a mass of only twice that of Jupiter.

 

Finally, we return to Barnard’s Star. Additional analysis indicated the possible presence of a second planet. The explanation proposed by van de Kamp in 1969 was that there were actually two planets. One was only 1.1 Jupiter masses and orbited in 26 years; the other just 0.8 Jupiter masses and orbited every 12 years.

 

This is where matters stood until the early 1970s.

 

The first planets to go were the ones around Barnard’s Star. This came about as a result of work done by George Gatewood at the Alleghany Observatory at the University of Pittsburgh. In addition to plates from the Alleghany Observatory covering more than 50 years, he had 80 plates from another observatory. When all of this data was analyzed, there was no wobble in the motion of Barnard’s Star that would indicate the presence of planets.

 

When Gatewood analyzed 143 plates taken of Lalande 21185 using the telescope at the Alleghany Observatory, there was nothing there to indicate the presence of a planet. 

 

There were also doubts about 61 Cygni C. Wulff-Dieter Heintz came to the Sproul Observatory in 1969. He became director of the observatory after van de Kamp’s retirement. In a paper published in 1978, he said that the evidence for a planet in 61 Cygni was very weak.

 

I have not been able to locate any specific mention of problems associated with Luyten’s Star. It would be reasonable, however, to assume that it had the same difficulties as the other claims that have been discussed.

 

What had gone wrong at the Sproul Observatory? One source of trouble was revealed by examination of plates showing a star used as a reference for measurements. This star, Gliese 793, should not have shown any deviations from its path, but deviations were indicated on some plates. They occurred in 1949 and 1957-58, which is when maintenance had been done on the refractor.

 

These particular maintenance problems have no direct effect on 61 Cygni C, since they occurred later than the original claim. But they did indicate the possibility of other problems. It was then reported in 1975 that there were thermal problems with the Sproul refractor. The cooling of the objective lens occurred in a manner that introduced deformation. This would have caused errors in the astrometric measurements. Most likely, these thermal problems were of long standing and could have affected many measurements, including the 61 Cygni data. We can only speculate about other sources of error.

 

It must be remembered that the original discoveries, such as for 61 Cygni, came from the study of double star systems. Errors of the type described would not seriously affect basic astrometry work done with double stars. But the attempted detection of a planet by astrometric methods requires measurements at the very limits of a telescope such as the Sproul Observatory refractor. Any source of error, no matter how small, could have been misinterpreted as a true motion.

 

What is the current situation of planets in the systems I have discussed? All subsequent discoveries have been made using the radial velocity method, rather than astrometry.

 

There is still no evidence for a planet around 70 Ophiuchi. There are two confirmed planets in Lalande 21185, plus another that has not been confirmed. Luyten’s Star has two confirmed planets, plus two that have not been confirmed. The most recent claim of a planet orbiting Barnard’s Star was made in 2018, but this claim has been disputed.

 

Finally, there is still no definite evidence of a planet around either star in the 61 Cygni system.

 

As this short history of such discoveries has shown, Clement did his work in developing the planet Mesklin at a time when the existence of 61 Cygni C was accepted. It was only 20 years later that doubts about its existence began to occur. Mission of Gravity as a work of fiction has lost nothing by the elimination of the planet on which it was based. It was probably not the first work of science fiction based a scientific “fact” later found not to be true. And it will probably not be the last.

 

 

SOURCES:

 

Other than the works that appeared in Astounding, my primary reference was The Lost Planets: Peter van de Kamp and the Vanishing Exoplanets around Barnard’s Star by John Wenz. One criticism of Wenz’s book is that its subtitle is misleading. It implies that its scope is restricted to the work done by van de Kamp and others using astrometry at the Sproul Observatory. Actually, the book follows the search for exoplanets up to very recent discoveries made using a variety of techniques.