Solar System Formation – Nebular Theory of Laplace

Welcome to DataFlair Physics Tutorial series. Let’s discuss about the formation of the solar system around each star. This topic has attracted many scientists from the early times and hence, several theories have been proposed.

Most popular among them is the Nebular Theory proposed by Pierre Simon de Laplace in 1796.

What Is The Solar System?

The Solar System is a gravitationally bound system of a star and astronomical objects orbiting around the star either directly or indirectly. In our very own galaxy, astronomers have found more than 500 solar systems.

The number for the entire universe is estimated to reach a figure of about tens of billions, or perhaps as many as 100 billion.

Talking about the solar system of the Sun, it consists of the Sun, eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune), five dwarf planets (three universally accepted – Ceres, Pluto, Eris; and two more expected – Haumea, Makemake), 796354 minor planets and several smaller objects including satellites, asteroids, etc.

How was the Solar System Formed?

Many of the earliest theories were developed based on the pieces of evidence observed by the astrologers. Consider, for example, the theory developed by William Whitson. He proposed his theory on the basis of gravitational tides caused by a passing comet.

Whitson and Buffon proposed that when a comet passed close enough to the Sun, it drew out a small fraction of the Sun’s mass. This material while rotating around the Sun cooled down and eventually condensed into planets.

This model was rejected because a comet doesn’t have enough mass to accomplish the task.

Formation of Sun

Let us now discuss in particular how the Sun came into being. The nebula (cloud of dust and gas) began to collapse gravitationally after becoming gravitationally unstable. The instability was probably caused by shock waves due to a nearby supernova.

The gas and dust then coalesced to the centre of the nebular cloud due to gravity. The centre got hotter and denser as more and more particles got pulled in. This attracted more particles inward causing a snowball effect.

About 99.9% of the matter fell in the centre, which was then known as protosun. Protosun had no sunlight. Nuclear fission was then triggered by high temperature and the Sun was born. This is how all-stars came into being.

Formation of Planets

The 0.1% matter left in the nebula orbited around the Sun and formed a flat disc shape. This disc, known as protoplanetary disc, is the site where planets were formed.

The dust particles in the nebula collided and clumped together, leading to the formation of planetesimals (infant stage of the planet). These had a size of about a few kilometers. This process is known as accretion.

As disc cooled down and more particles got attracted towards planetesimals, planetesimals grew in size forming protoplanets and eventually planets. In the outer region, ice was the dominant component whereas, in the inner region, silicates and metals were the dominant components.

Therefore, inner planets (Mercury, Venus, Earth, and Mars) have metalcore and outer planets (Jupiter, Saturn, Neptune, and Uranus) are giants made of ice and gas. The rocks that escaped the gravitational pull of planets were named as asteroids.

They were scattered in the entire solar system, but today they can be found in between Mars and Jupiter (in the Asteroid belt).

Nebular Hypothesis

The most widely accepted view about the formation of the solar system is the Nebular Hypothesis proposed by Laplace. It has the following key points-

1. A rotating nebula (cloud of dust and gas) forms from a collapsing interstellar gas cloud. The gas cloud spins faster to conserve angular momentum, thus it flattens.

The gas cloud can either break into several lumps to form a binary or multi-star system or it can form a central concentration (protosun) surrounded by a rotating disk of gas and dust.

The rotating disc later condenses into small pieces (like asteroids, protoplanets). These small pieces capture neighboring pieces of material and form full-scale planets.

It explains why planets contain 98% of total angular momentum and the Sun contains 99.9% of the total mass of the solar system. It also explains why the orbits of the planets are nearly coplanar and aligned.

2. The inner portion of the solar nebula has a higher temperature than the outer layers. At first Jovian planets (large and rarefied) came into being in the cooler outer region of the nebula.

Due to their large mass, they gravitationally gather much of the gas around them thus explaining their giant size and large spacing between the planets.

Terrestrial (small and dense) planets got a late start and couldn’t acquire much of cosmic matter before solar wind cleared the gas and small solar particles from the system.

3. Satellites and ring systems are expected to have formed around the planets in a similar way.

Drawbacks of Laplace Model

1. The nebular model could not explain angular momentum distribution between the Sun and the planets.
2. Rings pictured by Laplace would never condense into planets because of different rotational velocities of inner and outer parts of the ring.
3. If satellites were formed in a similar way, the moon must have its own atmosphere. This is obviously untrue.

Further Developments

In the 20th century, the nebular hypothesis was rejected. The planetesimal hypothesis took its place. According to this hypothesis, planets were formed from material drawn from the Sun. This theory also had many drawbacks and was therefore rejected.

In the 1970s, a modern and most widely accepted variant of Nebular hypothesis – the Solar Nebular Disk Model (SNDM) emerged. The SNDM explains almost every drawback of its predecessors including the origin of terrestrial planets.

Conclusion

In this article, we discussed about the Solar System Formation and the Nebular theory proposed by Laplace, how it came into being, and what were the reasons for its rejection.

We also discussed, in brief, the theories that followed the nebular theory. At last, we discussed how the Sun and planets came into being.