Reflecting Back (The Mirror is Foggy)

For the entirety of human history, up until just under two decades ago, it was genuinely unknown whether or not other stars possessed planets. There was no reason to suspect they didn’t, but by the 1980’s, there was some evidence to suggest they did. Among the two most important observations were that 1) The most well studied star to date (an unremarkable G dwarf by the name of “Sol”) was known to harbour nine eight planetary companions, and most importantly, 2) Young stars were observed to be hosts to circumstellar material suggestive of an ongoing planet-formation process, much as the limited understanding of planet formation predicted should exist.

As to the properties of these so-called “extrasolar planets” (or “exoplanets”), nothing was known, but one might infer some of their basic properties of their planetary systems from observations of our solar system. The basic rules one can derive are

  1. Gas giant planets form far from their stars.
  2. Terrestrial planets form close to their stars.

The reasoning behind this can also be understood from the context of planetary formation, as was understood at the time. A gas giant planet (that is, a planet for which the majority of its mass are “gases,” like H and He) will more likely form where mass is abundant in the disk. After a large core coalesces together and reaches a critical mass, it begins runaway accretion of gases around it, accreting several times its mass worth of H and He. But gas does not last terribly long in a planet-forming disk. Radiation pressure from the new star will drive out the light gases in a time-scale on the order of a few million years (Myr). So gas near the star is likely to be short lived, preventing gas giant formation in the inner regions of a planetary system. Solid cores, however, could form there and go on to be terrestrial planets that is, planets made dominantly of solids or “metals”).

The challenge of detecting extrasolar planets at the time was therefore contemplated in the context of the difficulty of detecting solar planets if they orbited nearby stars. Of all of them, Jupiter would be the easiest to find through the same techniques used for discovering stellar companions to known stars, but the instrumentation lacked the required sensitivity.

In 1952, Otto von Struve proposed an idea that did not get much attention at the time. He proposed that gas giant planets might occasionally be found so close to their stars that they might be commonly detectable in transit, where the planet blocks light from the star. I quote brief snippets here, but the page-and-a-half summary of the problem is a very interesting read from the perspective of hindsight.

“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. … But how should we proceed to detect them? … 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 AU from their parent stars. … But there seems to be no compelling reason why the hypothetical stellar planets should not, in some cases, 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 AU, or about 3,000,000 km. Its period around a star of solar mass would then be about 1 day.”

He then goes on to argue how such planets might actually be comparatively easy to detect with the instrumentation of the late 1950’s. The ‘burning question’, however, would remain unanswered for another 40 years.

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