4-1 Introduction - A Disk Bisects the Cloud

One of the most exciting recent developments in astronomy is the observations of protoplanetary disks in space, such as the disk that formed our solar system. The formation of disks in space has long fascinated astronomers (Section 4-2). Once astronomers settled on the protoplanetary disk hypothesis during World War II, they went to work on disk formation models (Section 4-3). Scientists recently developed telescopes to directly observe disks in space. Section 4-4 describes the current work on classifying telescope images of disks. In this introduction, I combined sections 4-3 and 4-6 in a discussion of the odd architecture of the inner solar system. Section 4-5 focuses on the question everyone wants to know. How common are solar systems and extraterrestrials? Finally, the chapter ends with a discussion of philosophical and theological perspectives on the formation of the solar system.

At the time of the Galileo controversy, Rene Descartes supported the Copernican model of the solar system, which is different from the modern Copernican Astrobiological Principle discussed later in this chapter. The Copernican model of the solar system is that the planets revolve around the sun. In 1629, he was the first to propose that the solar system formed naturally. He proposed that vortices in the universe contracted and formed the sun and planets. About a century later, Emmanuel Swedenborg proposed that the Sun had a shell that broke apart and formed the planets. In 1755, Kant proposed the nebular hypothesis, which is that clouds contracted by gravity and formed flat disks with a large mass at the center such as the solar system, Saturn, and the Milky Way. Although Kant is often given credit for proposing that the Milky Way is a disk, Thomas Wright had already noted in 1750 that the Milky Way appears as a line in space, that it was possibly shaped like a disk, and we are within disk. Pierre LaPlace had not heard of Kant’s proposed nebular hypothesis. In 1796, Laplace hypothesized that the solar system began as an incandescent gas cloud as large as the solar system. He reasoned that as the cloud contracted, it left disks behind from which the planets formed.

William Herschel had a great telescope, and he observed distant nebulae in space. He observed that the nebulae in space could be resolved into individual stars and were the result of the condensation of the clouds. Herschel’s observations led to the island universe theory, which was that there are other galaxies of stars in the universe. Combining LaPlace’s and Herschel’s work, the conception of the nebular hypothesis in the late 19th century was that a central light formed from the contraction of a cloud. The light then spread out and formed the Milky Way galaxy and then the solar system contracted within the Milky Way Galaxy, as proposed by LaPlace.  Scientists thought that everything was initially composed of light particles and that the earth and other planets were initially small suns, which then cooled off and became planets.

Primarily because of the difference in angular momentum between the planets and the sun, the nebular hypothesis was rejected in the early 20th century. They did not think that objects that formed in the same cloud would have such different angular momentum. In 1905, geologist Thomas Chamberlin and astronomer Forest Moulton proposed that a passing star pulled spiral arms out of the sun, which then cooled and formed planets. They might have thought that spiral nebula in space, such as the Whirlpool Galaxy, were spiral arms coming out of a star. This theory was embraced by the scientific community until 1939, when Lyman Spitzer showed that any material pulled out of the sun would just keep going out into space.

Carl Von Wieszacker had been researching nuclear fusion in the 1930s. In 1938, during the time when the planetesimal hypothesis was still in vogue, he realized that the sun was 99% hydrogen and helium and 1% heavier elements, but the planets were primarily heavier elements. He proposed that the solar system formed from a single cloud with 99% hydrogen and helium. He thought that 90% of the cloud collapsed and formed the sun, but that 10% part of the cloud was left behind, which would have had sufficient heavy material to form the planets.  He did not precisely define how the planets formed in this initial nebular hypothesis proposal. In 1944, he proposed the protoplanetary disk hypothesis, where the remaining cloud formed a disk, and particles gathered in collisions in vortices in the disk. He thought that planetesimals formed from aggregated particles in whirlpools. This is still regarded as the first proposal of the protoplanetary disk hypothesis.

The protoplanetary disk hypothesis was finally confirmed 50 years later by Hubble Telescope images in 1994. In this historic set of images, the Hubble Telescope showed side views of disks that were blocking out the remaining clouds around the protostars.

The ALMA (Atacama Large Millimeter Array) array is a set of radio telescopes that can be deployed in various configurations in the Atacama Plateau in Chile. It can collect direct images of disks in microwave wavelengths. Astronomers can classify the ALMA images by disk age and status. The disks on the left are 0.2 to 0.5 million years old. The next two columns are 0.5 to 1.0 million years old, and the right columns are 1 to 2 million years. The line in each image is the distance from Pluto to the sun.  The disks are hotter near the star. In a few disks there is a hole in the center, which probably means that the disk near the star has been blown away. In general, the planet forming phase in protoplanetary disks is finished by 3 million years after the formation of the protostar, but sometimes much sooner than that, depending on the temperature and radiation of the star.

The following video describes the ALMA array and its observations of disks. 

The first step in the process of star and disk formation is the breaking of the larger molecular cloud into cloud cores and contraction of the cloud core. There is high rotational momentum in the cores. When a core collapses during the ionization phase and forms a star and disk, the rotational energy is confined to a much smaller volume. This causes the smaller cloud to spin much faster than the larger core, just as when a figure skater draws in her arms and spins faster. The rotational energy is transferred to the disk, which causes the disk to stretch out.

The protoplanetary disk that formed around our sun is called the circumsolar disk. The circumsolar disk formed at the midplane of the remaining cloud around the protosun (Figure 4-2). The solid water ice and dust in the cloud fell to the midplane dust layer, which is where the planets formed. The disk atmosphere is above and below the midplane and contains mostly gas. High energy radiation from the star blows away the disk atmosphere, beginning near the star and on the outer edges of the disk atmosphere. The region near the midplane and far from the star stays cold and frozen for an extended period, which extends the period of planet formation.

Our solar system has an odd architecture (Sections 4-3 and 4-6), which refers to the distribution of planets. Our solar system has four small inner terrestrial planets and four large outer gas giants that are 20 to 300 times larger than the inner terrestrial planets. In the graph at the bottom of the page, the inner terrestrial planets have a size less than 1 kg x 10^24; however, the outer planets are much larger. Jupiter is almost 2,000 x 10^24 kg, and the other other gas giants are in the range 100 to 600. All observed exoplanet systems have evenly sized planets. With current telescope technology, the earth and other terrestrial planets are undetectable because they are too small so we will have to wait and see whether there are inner earth planets in exoplanet systems.

The progression of distances of planets from the sun is also a bit odd. There should be a progression of distances of planets from the sun; however, there is a gap in the progression between Mars and Jupiter.

In addition to the odd architecture of the entire solar system, the inner terrestrial planets by themselves have an odd architecture. Venus and Earth are large, and Mercury and Mars are relatively small. It is as if the materials that formed the four inner planets spread out from the region between Venus and Earth and no material entered from outside of Mars. In 2009, Brad Hansen simulated the formation of the inner terrestrial planets by starting with a set of small planetesimals between Venus and earth and no planetesimals between Earth and beyond Mars.  His simulation resulted in the formation of a large Venus and Earth, and a small Mercury and Mars. Based on his work and other evaluations, most scientists think that there was a gap in the disk from Earth to somewhere between Mars and Jupiter. There have been various proposals on the cause of the gap such as an inward and subsequent outward migration of Jupiter.

Based on the distribution of the planets, most planetary scientists think that the solar system is unusual. Batygin, Laughlin, and Morbidelli stated, “The solar system's configuration of small inner rocky worlds and large outer giants is anomalous in comparison with most other planetary systems, which have different architectures.” [1] Sean Raymond stated, “Typical exoplanet systems have large planets close to the star, and the planets in each exoplanet system usually are of similar sizes. We have dramatically different sized planets, which may be the oddest thing of all.” [2] Kevin Walsh stated, “We have no idea why our solar system doesn’t look like these others, and we would love an answer,” [3]

How did the discovery that the solar system is unusual affect the estimates of the number of intelligent civilizations in the universe? In 2020, Westby and Conselice estimated that there are 30 other intelligent civilizations in the Milky Way Galaxy. They followed the “Copernican” Astrobiological Principle in their analysis. As they stated,

“This principle asserts that the properties and evolutionary mechanisms in operation in our solar system are not unusual in any important way, and so we may feel justified in assuming that life, and even communicative intelligence, should stand an equal chance of evolving in any such system, given the requisite amount of time and raw materials.” 

Westby and Conselice did not consider that the solar system is unusual in their analysis. They stuck with the Copernican Principle, which is that the earth and solar system are normal. Their failure to address the discovery of the solar system's uniqueness is not surprising. There is major disagreement in the scientific community about the uniqueness of the Earth and solar system. The pronouncements that the solar system is unusual were earthshaking. The mediocre earth vs. rare earth debate had been sparked by the book, “Rare Earth Hypothesis,” published by Ward and Brownlee in the year 2000. The book generated an enormous debate because it argued against the popular Copernican Principle. Another name for the Copernican Principle is Mediocre Earth, which means that the Earth is not special but is a typical planet in the universe. The exoplanet data that indicates that the solar system and earth are unusual has swung the debate in favor of the rare Earth hypothesis. Many scientists from all philosophical backgrounds have adjusted their viewpoint and now think that the solar system is somewhat unique and that there are fewer intelligent civilizations in the universe than was previously thought. This does not mean that they have changed their philosophical perspective. Atheists still think that the solar system is a random chance event and that there was no intent in the universe to form our solar system or any other solar systems.

Theistic evolutionists believe that God set up the universe to form solar systems. They do not think that any divine intervention was needed to form our solar system or intelligent life. They might or might not be associated with a religious group. For example, Kant was a theistic evolutionist. Most theistic evolutionists strongly supported to the Copernican principle prior to the exoplanet data. As with atheists, many have adjusted their perspective and now think that the solar system is unusual; however, they do not think that God specifically intervened in the formation of the solar system other than to set up the system of the universe that forms solar systems.

Progressive creationists believe in evolutionary processes and an ancient universe, but they are not opposed to the concept that God might have periodically intervened in certain processes. They would tend to think that if the solar system is unique, then it is possible that God intervened in the process of solar system formation.

It is interesting that young earth creationists are overjoyed with the data that the solar system is unique. This is because they think that God specially formed the solar system and earth as a habitation for humans. Of course, they do not believe that the formation occurred through a protoplanetary disk.

Let’s revisit Arnold Guyot and see how his 19th century interpretation is faring with the updated nebular hypothesis. He aligned the expanse in Moses' second day of creation with the formation of the galaxy and solar system. 

Genesis 1:6 And God said, “Let there be an expanse (rāqîa) in the midst of the waters, and let it separate the waters from the waters.” Verse 7. God made the expanse and separated the waters below from the waters above.

Guyot interpreted the expanse as the spreading out of the Milky Way Galaxy, which is the upper image, and the formation of the solar system, which is the lower image in Figure 4E-1, upper. As with the LaPlace-Herschel nebular hypothesis of the 19th century, Guyot thought that the solar system formed by the contraction of part of the Milky Way, and the formation of disks and a central sun. Guyot mentioned that the Milky Way and the solar system had the shape of a disk, and he argued that the shape of a disk correlated with the meaning of the expanse of the second day. Theologians Charles Hodge and Augustus Hopkins Strong also supported Guyot’s interpretation; however, there were some complaints by theologians and scientists at the end of the 19th century. They didn’t think that the spreading out of light particles in the Milky Way could possibly include the waters of v. 6. They also did not think that the expanse had the shape of a disk.

The modern protoplanetary disk theory provides adjustments to Guyot’s interpretation that directly answer the 19th century complaints against his interpretation. The theologians of the 19th century would have been happy to hear that there were waters and earth in the dark molecular cloud, that the light of the first day was the sun and not a distant light in the Milky Way, and that the molecular cloud and disk contained earth and waters.

There are also several other correlations between Moses’ description of the expanse in v. 6 and the protoplanetary disk that Guyot and the 19th century theologians and scientists could not have imagined. First, Moses stated that the expanse was in the midst of the waters. Theologians agree that the word translated as “in the midst” means bisector, which is the position of a protoplanetary disk, in the remaining cloud around the protostar, as shown in the header.

The Hebrew meaning of “let it separate the waters from the waters” is the separation of useful waters from useless waters. Similarly, a protoplanetary disk attracts solid water and dust particles to the midplane, where they will form planets, as shown with the arrows in Figure 4-2. The useless gases remain in the disk atmosphere.

With respect to the 19th century argument that the expanse was a dome, Gorg stated in the Theological Dictionary of the Old Testament,[4] that the most likely shape of the expanse was a heavenly plate, which is a disk shape. There are many similar words in Akkadian and Hebrew and almost all described flat and thin objects.

With verse 7, we come back to the question of whether God intervened in the formation of our peculiar solar system. In verse 7, Moses stated that God made the expanse and separated the waters below from the waters above. The word translated as made does not mean create, it means that God modified the expanse.

The question is, which direction is below. The word translated as below normally refers to a position at the foot of, for example at the foot of Mt Sinai. It does not mean under. The word translated as above is used a few other times in the Bible. In one case, it refers to a position at the top of a wall. In another case, it refers to a plant that has grown above Jonah.  In both cases, it refers to a position that it gravitationally high. In the protosolar system, the primary direction of gravity was in the direction of the sun. Thus, the gravitational low point was toward the sun. From this perspective, below refers to the inner part of the disk closer to the sun, and above is the outer part of the disk further from the sun. A wall has the same flat shape as the disk, so the top of a wall is like the outer part of the disk.

Moses stated the God separated the lower part of the expanse from the upper part of the expanse, which would mean that God separated the inner part of the disk from the outer part of the disk. This corresponds with the likely gap between Earth and outside of Mars, as shown in this image.

How would God accomplish the formation of this gap? Morbidelli proposed that Jupiter moved inward and outward in the solar system and cleared out this gap in the solar system. Could God have formed this gap by triggering the migration of Jupiter. It is possible. Scientists have proposed other scenarios associated with Jupiter to explain this gap; however, there is new isotopic evidence, recently published in Nature Journal, that the earth and other inner terrestrial planets, formed prior to Jupiter, which would eliminate Jupiter as an explanation for the gap. We will look at this scenario in the next chapter.

In summary, Moses’ description of the expanse has several similarities with the protoplanetary disk that formed our solar system: The shape of the expanse as a disk. the position of the disk or expanse as a bisector between waters, the separation of useful waters for planet formation, and the unexplained presence of a gap in the protoplanetary disk that formed our solar system. Section 4E (optional excursus) is an extended and detailed look at the correlation of Moses’ second day and the formation of the protoplanetary disk and midplane dust layer.

References

[1] Batygin, Konstantin, Gregory Laughlin, and Alessandro Morbidelli. "Born of Chaos." Scientific American 314, no. 5 (2016): 28-37.

[2] Raymond, Sean N., Andre Izidoro, and Alessandro Morbidelli. "Solar System Formation in the Context of Extra-Solar Planets." arXiv preprint arXiv:1812.01033 (2018).

[3] NASA/Kepler Mission Mystery --"We Have No Idea Why Our Solar System is So Unusual" Daily Galaxy. Accessed on August 14, 2016 at <http://www.dailygalaxy.com/my_weblog/2016/08/nasakepler-mission-we-have-no-idea-why-our-solar-system-is-so-unusual.html>

[4] Gorg. Theological Dictionary of the Old Testament., vol. 10, ed. G. Johannes Botterweck and Helmer Ringgren (Stuttgart: Wm. B. Eerdmans Publishing Co., 1974), 156

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A Hubble image of disks around young protostars. Credit: NASA.