The Nebular Hypothesis for the Origin of the Solar System

It’s always fun to read an older science book and read about the cast of theories scientists have considered through the centuries to describe Earth’s beginnings. I love natural history, and many natural history—or even physical geography—books open with a chapter on Earth system history.

1755, Germany: Last semester I took an ethics class, which was required for graduation. The very first philosopher we covered was Immanuel Kant and his Kantian theories. They revolved around rationalization and a sense of duty, and the “right thing” is not so much the action itself but whether the intent is rational and follows a duty.

Well, this guy had his own theory on the origin of the Earth, and it wasn’t particularly far off from what astronomistsers believe today: before the Sun, planets, moons and whatnot, there were cold swirling clouds of dust and gas.

1769, France: Mathematician Pierre Simon LaPlace built upon the concept of astronomical clouds and theorized that the Sun, planets, moons and whatnot formed from such clouds. He thought that the swirling became consistent spinning and the centripetal force—which I like to think of in terms of winding our childhood tire swing as my big brother’s childhood arms could and letting it loose, with me sitting inside the tire hanging on tight!—induced a centralized source of gravity. The gravity pulled in more particles until they condensed. Some eddies within the spinning became planets and the core of the spinning became the Sun.

However, LaPlace’s idea had to be scrapped. Apparently the smaller the object of gravitational force the faster it spins, and our Sun does not spin nearly as fast as this theory would indicate if you did the calculations.

Scientists from various disciplines such as chemistry, geology, physics, mathematics and astronomy current turn to Kant’s theory, but with modification. This adapted theory was known as the nebular theory or proto-planet theory…and apparently modern sources prefer to call it the nebular hypothesis.

Fayetteville night sky
The night sky can always make one wonder.


The nebular hypothesis: the proto-Sun

It is assumed that the first atoms formed in space were roughly 90 percent hydrogen, 9.7 percent helium, and the other 0.3 percent was a cocktail of heavier elements such as carbon, oxygen and iron. The cosmic cloud, or nebula, of these elements started to spin, largely speculated to be due to some kind of shockwave from a supernova or a similar disturbance. The core of the spinning had the largest mass, but the turbulence caused peripheral eddies to form and breakdown. The center condensed into what would be the sun and the smaller, surviving eddies became what would be the planets—a proto-Sun and proto-planets.

With the cloud in a compacted form, the elements merged into light-weight compounds such as water and ammonia, and heavier crystals such as silicates. As gravity increased within the condensed clouds, the continual spinning tightened at the ends and broadened at the waist, into the familiar disc-shape we associate with the solar system now. The proto-Sun was the largest disc and varying sizes of smaller discs were the proto-planets.

This was before thermonuclear fusion. To this point, all the clouds remained cold. As a cloud, the proto-Sun was about 100 times larger than the proto-planets, and that large size was necessary to possess enough gravity to trap light hydrogen atoms to induce thermonuclear fusion and light up—big clouds become stars and small clouds become planets.

NASA image of LH 95 stellar nursery in the Large Magellanic Cloud–nebulae.


The nebular hypothesis: the proto-Earth

Proto-Earth was a ball of icy particles that orbited the proto-Sun, collecting more materials as it moved. It was able to form into a solid ball due to adhesion between water drops and ice crystals—a cosmic slush. Other proto-planets built mass in the same fashion, though not necessarily grabbing a high proportion of H2O.

Like how the proto-Sun trapped hydrogen at its core, proto-Earth trapped radioactive materials deep inside and the fusion reactions released heat, making the new Earth something of a warm-blooded planet. The reactions melted the material at its core.

The liquefaction allowed heavy metals such as iron and nickel to sink and become magma. Inevitably, volcanoes facilitated the transport of lighter elements in the core. The escaped vapor gradually accumulated near the outer layer to form an atmosphere, but only briefly. The heat from the sun was especially intense at the time and broke the chemical bonds of lighter molecules, which element remains were lost to space.

Larger, heavier molecules remained, but in this situation the Earth became a miniscule resemblance of proto-Earth. It stopped packing space junk. It cooled enough for the outer membrane to solidify into a crust. The remaining atmosphere was methane and ammonia, but eventually enough water vapor ejected from the core allowed oxygen gas to linger while the Sun’s heat stole more hydrogen.

By the time the sun compressed and cooled into a smaller star, Earth had a chance to cool enough for the water vapor to condense into liquid. The first rains would evaporate by the time they touched the ground, but the more the planet cooled the more water accumulated on its surface.

4.5 billion years ago: proto-Earth transitioned into the much smaller Earth.

Next 1.5 billion years: torrents of severe weather until a balance was found.


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