Nebular Hypothesis-The story of how the solar system formed

Updated: Jan 20, 2021

The sun plus its eight planets, their satellites, asteroids and comets make up the solar system. This post is the first of the many articles that will explore the properties, distances, comparisons, when and how things formed in the solar system.

In future posts, we will discover what the Solar System has undergone since its violent beginning. And, finally, we shall see what will befall them in the distant future, four or five billion years from now, as the yellow star we call the sun passes into old age. These and other matters are all part of a marvellous story; the story of the solar system.

The solar system- an overview:

Does our solar system have a shape? Where are its celestial bodies located? How do the planets and their satellites move relative to each other? These questions are paramount when we learn about our star system. When I started the research for this series of articles, I found what I already knew was just a drop in the vast ocean. And there is much more to learn, so let us begin.

The heliocentric model:

Let's first get this simple thing straight; the sun lies at the centre, and all the objects within its gravitational influence go around it. It is now a well-known, well-established fact, but before the Copernican era, people believed otherwise.

The human civilization believed that the earth was at the centre of the solar system for at least 1500 years. In 1543, when Nicolaus Copernicus proposed that the sun was at the centre, he faced severe religious opposition. He had deferred from publishing his data and research till the year of his death.

While it was revolutionary putting the sun in the centre, his planetary orbits were incorrect. Decades later, the German astronomer Johannes Kepler found the planets do not move in circular orbits, but their orbits are slightly elliptical.

Now we have a more refined understanding of the solar system. Let me summarize below what we know so far.

  1. The sun is at the centre, and all the planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and other dwarf planets) move around it in an anti-clockwise direction. The sun itself spins anti-clockwise.

  2. Only Neptune and Venus rotate in a clockwise direction.

  3. All of them go about in elliptical or near-circular orbits around the sun.

  4. The moons have the same orbital and spin directions as the planets.

  5. The four planets closest to the sun known as the terrestrial planets; they are rocky and metallic.

  6. The next four planets are collectively known as the Jovian planets. Jupiter and Saturn are gas giants; Uranus and Neptune are ice giants. These giant planets possess orbits ten times larger than the sizes and trajectories of the terrestrial planets.

  7. The solar system is flat, for instance, its flatter than a dinner plate. It looks almost like a flattened disc.

  8. Asteroids are also a part of the solar system. These are unevenly shaped chunks of metal and rock, found mainly between Mars and Jupiter forming a region known as the asteroid belt.

  9. The comets are small icy bodies, have two homes. Some creep beyond Neptune in a disc called the Kuiper belt.

  10. Trillions of comets exist a thousand times farther from the Sun than Pluto. They surround our galaxy in an enormous spherical structure known as the Oort cloud that envelops our solar system.


infographic about The distance between the sun and the planets
The distance between the sun and the planets

Theories of the origin of the solar system:

The theory about the origin of the Solar System was first proposed in 1755 by the German philosopher Immanuel Kant (1724–1804). He worked out that, the sun and the planets formed from an immense rotating disc of gas and dust that appeared from a cloud of interstellar material.

Pierre-Simon, marquis de Laplace (1749–1827), famous for his demon, independently came up with the same idea 54 years later and perfected it. He interjected that the rotation would produce an angular momentum in the cloud, causing it to flatten out. The sun would emerge at the centre, while the planets would form further out in the disc, condensing from concentric rings of material shed by the central star. This theory of origin became known as the nebular hypothesis.

The widely recognized contemporary modification of the nebular theory, the solar nebular disk model (SNDM) offers explanations for a variety of properties of the Solar System.

The nebular hypothesis clearly explains the disc-like heliocentric system with planets in neat orbits, but the theory has its imperfections.


Video credits: Simulating Solar System Formation | California Academy of Sciences

Drawbacks in the Nebular hypothesis:

The sun, like all the planets in the solar system, rotates in its own axis. For the Nebular hypothesis to be correct, the sun should spin 400 times faster than the current rate. Astronomers call this the angular momentum problem.

How planetesimals form is the most significant unanswered puzzle in the nebular disk model. How 1 km sized planetesimals form from 1cm sized specks remains a mystery. Understanding this mechanism might also point out why some stars are gifted with planets, while others have nothing.

The Nebular hypothesis does not account for extra-solar planets, also known as exoplanets, their formation remains mysterious.

The nebular hypothesis does not apply to other planetary systems other than ours. Other star systems seem to have Jupiter like planets known as hot Jupiters close to the star, and they have short orbital periods (only a few hours).

The bottom line is, there is nevertheless a long way to go before we have a model that can rightly reproduce the observed characteristics of every known planetary system, including ours. Until then, the nebular hypothesis is the best we have.

What we have learnt so far is only the beginning, in the next part of this solar system series we look inside our local star and try to understand everything about it! Stay tuned and subscribe to our weekly newsletter to stay updated.


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