Otto Struve & Exoplanets, 1952

It’s too bad the remarkable Russian-born American astronomer Otto Struve (1897-1963) never lived to see the discovery of the first exoplanets, especially considering how he was probably the first to suggest the two main techniques by which they are now discovered.

The first discovery of something that could be called an exoplanet was announced in 1992 by the Polish astronomer Aleksander Wolszczan (1946-) and Canadian astronomer Dale Frail (1961-). They found two planets orbiting a neutron star 2,300 light years away in the constellation Virgo. This neutron star is the pulsar PSR 1257+12, which had only recently been discovered by Wolszczan (1990). The pulsar planets were detected using a variant of the Doppler (radial velocity) method, and a third planet was discovered by the same team in 1994. These planets likely formed from the debris disk formed when two white dwarf stars merged, so they could be considered “exotic” planets, quite unlike anything found in our solar system.

In 1995, the first exoplanet orbiting a “normal” star was announced by Swiss astronomers Michel Mayor (1942-) and Didier Queloz (1966-). Using the Doppler (radial velocity) method, they found a “hot Jupiter” orbiting the star 51 Pegasi at a distance of 51 light years (nice coincidence!).

In 1999, independent teams led by Canadian-American astronomer David Charbonneau (1974-) and American astronomer Gregory W. Henry (1972-) were the first to use the transit method to detect an exoplanet. They confirmed a hot Jupiter orbiting the star HD 209458 (also in Pegasus, another nice coincidence) 157 light years distant that had been discovered using the Doppler (radial velocity) technique only weeks earlier.

As you can see, the 1990s was the decade when exoplanetary science got its start!

Getting back to the prescience of Otto Struve—40 years prior to the discovery of the first exoplanets—Joshua Winn (1972-) in his newly-published The Little Book of Exoplanets writes:

Although the discovery of hot Jupiters came as a surprise, it’s not quite true that nobody foresaw them. In 1952, Otto Struve, an astronomer at the University of California at Berkeley, published a short paper pointing out that the precision of Doppler measurements had become good enough to detect planets—but only if there existed planets at least as massive as Jupiter with orbital periods as short as a few days. Setting aside the question of how such a planet might have formed, he realized there is no law of physics that forbids such planets from existing. In an alternate history, Struve’s paper inspired astronomers to launch a thousand ships and explore nearby stars for hot Jupiters. In fact, his paper languished in obscurity. None of the pioneers—neither Walker, Latham, Mayor, nor Queloz—were influenced by Struve’s paper. The planet around 51 Pegasi probably could have been discovered in the early 1960s, or surely by Walker in the 1980s, had the Telescope Time Allocation Committee allowed him to observe a larger number of stars.

Here is Otto Struve’s 1952 paper in its entirety (references omitted), published in the October 1952 issue of The Observatory.


By Otto Struve

With the completion of the great radial-velocity programmes of the major observatories, the impression seems to have gained ground that the measurement of Doppler displacements in stellar spectra is less important at the present time than it was prior to the completion of R. E. Wilson’s new radial-velocity catalogue.

I believe that this impression is incorrect, and I should like to support my contention by presenting a proposal for the solution of a characteristic astrophysical problem.

One of the burning questions of astronomy deals with the frequency of planet-like bodies in the galaxy which belong to stars other than the Sun. K. A. Strand’s discovery of a planet-like companion in the system of 61 Cygni, which was recently confirmed by A. N. Deitch at Poulkovo, and similar results announced for other stars by P. Van de Kamp and D. Reuyl and E. Holmberg have stimulated interest in this problem. I have suggested elsewhere that the absence of rapid axial rotation in all normal solar-type stars (the only rapidly-rotating G and K stars are either W Ursae Majoris binaries or T Tauri nebular variables, or they possess peculiar spectra) suggests that these stars have somehow converted their angular momentum of axial rotation into angular momentum of orbital motion of planets. Hence, there may be many objects of planet-like character in the galaxy.

But how should we proceed to detect them? The method of direct photography used by Strand is, of course, excellent for nearby binary systems, but it is quite limited in scope. There seems to be at present no way to discover objects of the mass and size of Jupiter; nor is there much hope that we could discover objects ten times as large in mass as Jupiter, if they are at distances of one or more astronomical units from their parent stars.

But there seems to be no compelling reason why the hypothetical stellar planets should not, in some instances, be much closer to their parent stars than is the case in the solar system. It would be of interest to test whether there are any such objects.

We know that stellar companions can exist at very small distances. It is not unreasonable that a planet might exist at a distance of 1/50 astronomical unit, or about 3,000,000 km. Its period around a star of solar mass would then be about 1 day.

We can write Kepler’s third law in the form V^{3} \sim \frac{1}{P}. Since the orbital velocity of the Earth is 30 km/sec, 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.

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.

Perhaps one way to attack the problem would be to start the spectrographic search among members of relatively wide visual binary systems, where the radial velocity of the companion can be used as a convenient and reliable standard of velocity, and should help in establishing at once whether one (or both) members are spectroscopic binaries of the type here considered.

Berkeley Astronomical Department, University of California.
1952 July 24.

Nearest Neutron Star

So far as we know, RX J1856.5-3754 is the neutron star closest to our solar system.  This radio-quiet isolated neutron star can be found between 352 and 437 ly from our solar system, with its most likely distance being 401 ly.  Directionally, it is located within the constellation Corona Australis, near the topside of the CrA circlet, just below the constellation Sagittarius.  Its coordinates are:

α2000 = 18h 56m 35.11s, δ2000 = -37° 54′ 30.5″.

RX J1856.5-3754 was formed in a supernova explosion about 420,000 years ago.  Today, this tiny 1.5 M star about 15 miles across has a surface temperature of 1.6 million K and shines in visible light very feebly with an apparent visual magnitude of only 25.5.  Its surface is so hot that its thermal emission is brightest in the soft X-ray part of the electromagnetic spectrum; this is how it was discovered in 1992.

Like all neutron stars, RX J1856.5-3754 has a very intense surface magnetic field (B ≈ 1013 G) which causes the electromagnetic radiation leaving it to exhibit a strong linear polarization.  In the presence of such a strong magnetic field, the “empty” space through which the light travels behaves like a prism, linearly polarizing the outgoing light through a process known as vacuum birefringence.

An active area of neutron star research currently is a precise determination of their diameters.  We do not yet know whether the extremely dense central regions of these stars contain neutrons, or an exotic form of matter such as a quark soup, hyperons, a Bose-Einstein condensate, or something else.  Knowing the exact size and mass of a neutron star will allow us to infer what type of matter must exist in its interior.  The majority of neutron stars are pulsars with active magnetospheres that make it difficult for us to see down to the surface.  More “quiet” neutron stars such as RX J1856.5-3754 are the best candidates for precise size measurements of the neutron star itself.  An accuracy of at least ± 1 mile is needed to begin to distinguish between the various models.

Mignani, R.P., Testa V., González Caniulef, D., et al. 2017, MNRAS 465, 1, 1
Özel, F., Sky & Telescope, July 2017, pp. 16-21
Yoneyama, T., Hayashida, K., Nakajima, H., Inoue, S., Tsunemi, H. 2017