The Nebular Hypothesis

One of the most amazing theoretical advances in astronomy was Immanuel Kant’s conception of the nebular hypothesis, which was that the solar system, galaxy, and the rings of Saturn formed in a disk around a central large mass. After Kant proposed it in the mid-18th century, this hypothesis was forgotten, revived, rejected, and finally resurrected in its final form approximately 200 years later as Von Wiezsacker's protoplanetary disk hypothesis. Telescopes confirmed it in the late 20th century.

Prior to the 18th century, nobody ever considered that the solar system formed naturally. In the 18th century Descartes and Swedenborg proposed natural solar system formation models, but those efforts were based on incorrect physics. Immanuel Kant (1724-1804) observed that the Milky Way Galaxy, the solar system, and the rings of Saturn all had the shape of a disk, and he reasoned that they all must have formed by similar natural processes. He reasoned that gravity would cause cloud collapse and a massive core, and that centrifugal force due to rotational motion would stretch out galaxies, solar systems, and planetary rings. Kant described his theories of natural formation in “Universal natural history and theory of the heavens or essay on the constitution and the mechanical origin of the whole universe according to Newtonian principles.” Kant joined the concept of natural formation of the solar system with the correct physics and arrived at the nebular hypothesis.

Kant was in an isolated town in Germany, but he was a voracious reader. In 1750, Thomas Wright wrote that the Milky Way was "an optical effect due to our immersion in what locally approximates to a flat layer of stars." Kant reasoned that the stars in the Milky Way might be orbiting like the solar system, but traveling so slowly that we would not notice their movement.[1]

“Herr Wright of Durham, with whose treatise I became acquainted through the Hamburg Freie Urteile of the year 1751 first gave me cause to regard the fixed stars not as a scattered milling mass without any visible order, but rather as a system with the greatest similarity to a planetary one, so that, just as in the latter the planets are very close to a common plane, so also the fixed stars in their position relate as closely as possible to a certain plane, which has to be thought of as extending through the entire heavens, and where they are most densely massed, they form the bright band that is called the Milky Way. I have become convinced that, because this zone, illuminated by countless suns, has very exactly the direction of a very large circle, our sun must also be very close to this large plane of reference. While pursuing the causes of this feature, I have found the following to be very probable: that the fixed stars could actually be slowly moving planets of a higher order.” [2]

Based on Newtonian physics, Maupertius thought that rotating bodies would flatten out due to centrifugal force, and he showed that the diameter of the equator was greater than the distance between the poles due to Earth’s rotation. According to the following quote from Kant, Maupertius also attributed the shape of elliptical nebulae in space to the same flattening process due to centrifugal force. Kant thus reasoned that disk shaped solar systems and galaxies must have formed by this same process, which was the nebular hypothesis.

“I observed the kind of nebulous stars that Herr von Maupertius considers in his Treatise on the Figure of the Stars and which have the figure of more or less open ellipses, and readily assured myself that they could be nothing other than an accumulation of many fixed stars. The roundness of these figures that is measured at all times taught me that an inconceivably numerous mass of stars must be arranged here around a common centre point, because otherwise their free positions in relation to one another would present irregular shapes but not measured figures. I also realized that in the system in which they are united, they must be mainly limited to one plane, because they do not present circular but elliptical figures and that, because of their pale light, they must be incomprehensibly distant from us.” [3]

Isaac Newton had argued that God must have directly put the planets in their orbits. Newton’s rationale was that there was no material between the planets; thus, how could they form naturally? However, Kant argued that the solar system pointed to a natural formation process since all of the planets moved in the same direction of the sun’s rotation, and that there was a pattern in the distribution of the planets. The fact that there is no dispersed matter now does not mean that there was never dispersed matter if the dispersed matter was incorporated into planets. Kant argued that the universe consisted of randomly dispersed matter, as in a cloud, which then gathered and formed stars and planets, beginning with gravitational attraction toward cores of matter in small spherical regions of higher density. This was really an amazing insight that proved to be correct.

“I assume that when all matter of which the spheres that constitute our solar system, all the planets and comets, consist, was dissolved into its elementary basic material at the beginning of all things, it occupied the entire space of the universe in which these formed bodies now orbit. This state of nature, even if one considers it in and for itself without regard to any system, appears to be the simplest that could follow upon nothingness. At that time, nothing had formed yet.

The universal tranquility in space replete in this way lasts only for an instant. The elements have essential forces which set each other in motion and are, indeed, themselves an origin of life. The material is under an immediate impulse to develop. The denser type of scattered materials, thanks to the power of attraction, collect from a spherical area around them all the material with a lesser specific weight. But they themselves, together with the material which they have united with them, converge in the points where the small pieces of an even denser type are located, and these again to even denser points, and so on. When we think about this idea of a self-developing nature throughout the entire extent of chaos, we will easily see that all the consequences of this process will finally consist of the assembling of different clusters, which, after the completion of their development, would be calm and eternally motionless because of the equality in the force of attraction.” [4]

Kant combined the concepts of collapse toward gravitationally denser regions of matter, the disk shape of galaxies and solar systems, the stretching out of disks based on centrifugal force into the nebular hypothesis. He stated that moons naturally form around planets as matter around a planet aggregates together. He described how Saturn’s rings formed as the material around the planet moved toward the equatorial plane of the planet and formed a disk of material that rotated around the planet.

Kant realized that if the sun was but one star in the disk of the Milky Way and countless other galactic disks were out in space, then the Universe must be enormous and the earth must be extremely small in comparison. He saw the natural formation of stars and planets as a natural process initiated by the divine architect of the Universe.

“The theory we have put forward opens a perspective onto the infinite field of creation for us and presents some inkling of God’s work that is appropriate to the infinitude of the great architect. If the magnitude of a planetary system in which the Earth is as a grain of sand and scarcely noticeable puts our reason into a state of wonderment, then with what amazement are we delighted when we contemplate the infinite multitude of worlds and systems that constitute the sum total of the Milky Way; but how much does this amazement increase when one becomes aware that all these immeasurable orders of stars in turn are the unit of a number whose end we do not know, and which is perhaps just as inconceivably great as these and yet is in turn only the unit of a new combination of numbers. We see the first members of a progressive relationship of worlds and systems, and the first part of this infinite progression already gives us to understand what we can suppose about the whole. There is no end here but rather an abyss of a true immeasurability into which all capacity of human concepts sinks even if it is raised with the help of mathematics. The wisdom, the goodness, the power that has revealed itself, is infinite and in the same measure fruitful and industrious; the plan of its revelation must for that reason be as infinite and without limits as it is.” [5]

Kant published this and other concepts of natural evolution of the universe in his book, Universal Natural History and Theory of the Heavens, which was not widely circulated. Almost nobody heard of this book or his nebular hypothesis. As with Copernicus, he might have been reticent to publicize these concepts since they were controversial.

William Herschel (1738-1822) observed nebular clouds that thought that they might have collapsed and formed stars. (Figure 4‑3). In 1789, he proposed that nebulae were in different states of condensation. He thought that some nebulae had not separated into stars but were one glowing mass.

Most scientists in the 19th century thought that the planets and moon were initially small suns that then burned out. Scientists at the time did not know about nuclear fusion and had no idea how the sun produced its energy. There were many theories about the sun’s source of energy, most of which sound crazy to us since we know about nuclear fusion, but imagine trying to figure this out without knowledge of nuclear fusion.

The Laplace-Herschel nebular hypothesis lost support among astronomers at the end of the 19th century because it did not explain how almost all of the angular momentum in the solar system was in the planets even though the sun accounts for almost all the mass of the solar system. This means that the sun’s rotation rate is slow compared to the planets. For the next several decades, scientists thought that a nearby passing star pulled the planets out of the sun.

Carl Von Weizsacker revived the nebular hypothesis during World War II. He was supposedly developing an atomic bomb for Hitler when he developed his protoplanetary disk hypothesis in 1944. Von Weizsacker proposed that a gas cloud contracted and formed the sun, that a protoplanetary disk (Figure 4‑5) formed, and that the planets formed within the disk. The protoplanetary disk that formed our solar system is called the circumsolar disk. Disks that form other planetary systems are called circumstellar disks. Modern telescopes observe these disks (see Section 4-3) and thus confirm Von Weizsacker’s protoplanetary disk hypothesis.

As with the argument against the Laplace-Herschel nebular hypothesis, scientists argued against Von Weizsacker’s protoplanetary disk model because the sun’s angular momentum is low in comparison to the planets. Von Weizsacker argued that energy from the sun caused vortices in the disk, which led to accretion of the planets and slowed down the rotation of the sun. The most popular current explanation for the low angular momentum in the sun is that the magnetic field from the protosun interacted with the magnetic field of the circumsolar disk, which slowed down the rotation of the sun.

The first observation of a disk was in 1976 with a spectrograph of the star HL Tauri. Spectral analysis in the 2- to 4-micron range showed evidence of water ice in 1976. Spectral analysis of HL Tauri also showed evidence of carbon monoxide gas in 1986. The near-infrared camera (NICMOS) on the Hubble Space Telescope collected the first visual evidence of disks, which were side views of disks that blocked out the near-infrared light emitted by their protostar (Figure 4‑6). The protostar continues to illuminate the disk atmospheres above and below the disk midplane. This was a revolutionary set of images in the 1980s because there had not been evidence of protoplanetary disks prior to this time.

References

[1] Immanuel Kant, Universal Natural History and Theory of the Heavens, 1755, p. 202.

[2] Kant, Universal

[3] Kant, Universal

[4] Kant, Universal

[5] Kant, Universal

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Protoplanetary disk. Credit: NASA/Lynette Cook