The Solar System’s History

From the year 1930 to 2006, nine objects in the solar system were considered planets. The planets Mercury, Venus, Earth, and Mars are known as terrestrial or rocky planets because they have a solid surface like that of the Earth. The planets Jupiter, Saturn, Uranus, and Neptune are known as Jovian planets. They are giant gaseous objects. Not much was known about Pluto. Even the mass of this planet was not known until the end of the last century. Now we know that Pluto is a rocky object and it is much smaller than even our Moon.

Terrestrial vs Jovian Planets

The terrestrial planets are also known as the inner planets and the Jovian planets are known as the outer planets because an asteroid belt between Mars and Jupiter separates them. This asteroid belt has many small objects orbiting around the Sun and some of them are as large as Pluto. All the planets except Pluto are orbiting in the same plane.

Pluto and Dwarf Planets

In 2006, the general assembly of the International Astronomical Union, the "Parliament" or "Senate" of astronomers, decided a definition of the planets and resolved that Pluto should no longer be considered a planet. So, if we go by that definition, then the number of planets reduces to eight. Pluto and other objects similar to the size or mass of Pluto are now considered Dwarf Planets.

The Solar System as "Our World"

If one is born and brought up in a small town, has never traveled outside it, and doesn’t know much about the people outside the town, then the small town becomes one’s very own world irrespective of one’s knowledge that there exist other towns and cities and even countries. So, in this book, we consider the solar system as "our world" because we know the parent of this planetary system, the Sun, and the neighbors of the Earth the best.

Gravitational Influence in the Solar System

All the planets and the Sun have mutual influence on each other gravitationally. Also, the formation and evolution of the Earth are coupled with the formation and evolution of all the other solar objects. The chemical composition or the elemental abundance of all the solar system objects is also the same because they all were born out of the same molecular cloud called the solar nebula.

Formation of the Solar System

It is believed that all the solar system objects—the Sun, the planets, the Moon, asteroids, comets, etc.—were formed at the same time and out of the same nebula or interstellar cloud. Therefore, the solar planets and their parent star, the Sun, have almost the same age—about 4.6 billion years.

How Stars Like the Sun Form

A star is formed by the gravitational collapse of a huge cloud of molecules and dust in the interstellar medium. The cloud is so large that light takes about 3–4 years or more to travel from one end to the other. It is spherical in shape and is spinning slowly around its own axis of rotation.

The outward pressure caused by the heat of the cloud exactly balances the gravitational pull towards its center. This is called hydrostatic equilibrium or hydrostatic balance. Imagine that a slight disturbance is given by compressing a certain part of the cloud. This would cause a perturbation in the cloud, and the disturbance will travel through the cloud at the speed of sound.

Jeans Length and Gravitational Collapse

The phenomenon where a disturbance in the molecular cloud causes it to collapse under its own gravity is called the Jeans instability. The minimum size of a region where collapse is possible is called the Jeans Length, and the mass of that region is called the Jeans Mass. If a region of a cloud has a mass greater than or equal to the Jeans Mass, it will collapse, leading to the formation of stars.

Protostellar Disk Formation

As the cloud collapses, the material spins faster due to conservation of angular momentum. This causes the formation of an equatorial bulge and eventually leads to the creation of a protostellar disk—a disk of material surrounding a new protostar.

The protostar at the center becomes denser and hotter, eventually reaching temperatures that allow hydrogen fusion. If the mass of the protostar is sufficient, nuclear fusion ignites, and the star begins its life as a main-sequence star.

Formation of Planets

The disk surrounding a new star, known as the protoplanetary disk, is where planets form. Dust particles in the disk collide and stick together, forming planetesimals. These planetesimals collide further, eventually leading to the formation of planetary embryos.

The Birth of Rocky and Gaseous Planets

The rocky planets (like Mercury, Venus, Earth, and Mars) form closer to the star, where the heat causes lighter elements like hydrogen and helium to evaporate. The outer, cooler planets (like Jupiter, Saturn, Uranus, and Neptune) retain these gases and become gaseous giants.

The Future of Planetary Systems

As we continue to observe other planetary systems, we can further understand how planets like Earth—and systems like our own—formed. New discoveries of exoplanets continue to challenge and expand our knowledge of the universe.