We all are well-aware of what country we live in and what surrounds our house.  But little do we know about our human home and the environment of which our home lies.

As it is always fascinating to think about what our neighborhood around our house is like, why not we try to know better about the one around our human home, earth. Our earth is in an interesting neighborhood, called ‘Solar System’.

In the following, we will discuss what a solar system is, the planets within the solar system, and how they are different from one another.

Solar system is a system our earth lives in

A solar system is a star and everything that travels around it in space. In our Earth’s solar system, all the planets including earth itself, asteroids and many other things in orbit one star, called the Sun. There are billions more stars grouped together in a galaxy, called the Milky Way. Billions of galaxies are scattered in space and this very broad spread of galaxies is known as the Universe. So in short note, Earth, the planet we live in, is in the solar system in the Milky Way, which is one of the billions of galaxies in the Universe. The following diagram will help understand the position of our earth, the Solar system, the Milky Way and the Universe more clearly.

Earth, the Solar system, the Milky Way, and the Universe in space

History of the Solar system

The formation of Solar system

It is easier to find out when and how exactly we were born. But how our earth was born is a more difficult story. There are many theories that suggest the origin of the universe or how the solar system formed. In the earlier days, scientists believed that the solar system was built up from contraction and condensation of the vortices of particles, stars and planets in the universe. Then many earlier theories suggested that the earth was stationary in the center of the universe, and the sun and other planets orbited around it. Astronomers and scientists have worked out many more theories to explain the formation of our solar system since then. The most scientific up-to-date model is the nebular hypothesis which was developed in the 18th century by Emanuel Swedenborg, Immanuel Kant, and Pierre-Simon Laplace.

The nebula hypothesis states that the solar system was formed about 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud, known as nebula. Majority of such collapsing mass gathered in the center and formed the Sun. The rest flattened into a protoplanetary disk out of which the solar system bodies such as planets, moon, asteroids, etc. were born.

 

The formation of our solar system timeline

As above, the solar system was formed about 4.6 billion years ago after the gravitational collapse and then the Sun, planets, moon, and asteroids and comets formed in order. Below is the chronology of the formation and evolution of the solar system cited from Wikipedia, “Formation and evolution of the Solar System”, https://en.wikipedia.org/wiki/Formation_and_evolution_of_the_Solar_System

Phase Time since formation of the Sun Time from present (approximate) Event
Formation of Sun 0–100,000 years 4.6 bya Pre-solar nebula forms and begins to collapse. Sun begins to form.[30]
Formation of Sun 100,000 – 50 million years 4.6 bya Sun is a T Tauri protostar.[9]
Formation of Sun 100,000 – 10 million years 4.6 bya By 10 million years, gas in the protoplanetary disc has been blown away, and outer planet formation is likely complete.[30]
Formation of Sun 10 million – 100 million years 4.5–4.6 bya Terrestrial planets and the Moon form. Giant impacts occur. Water delivered to Earth.[2]
Main sequence 50 million years 4.5 bya Sun becomes a main-sequence star.[26]
Main sequence 200 million years 4.4 bya Oldest known rocks on the Earth formed.[128][130]
Main sequence 500 million – 600 million years 4.0–4.1 bya Resonance in Jupiter and Saturn’s orbits moves Neptune out into the Kuiper belt. Late Heavy Bombardment occurs in the inner Solar System.[2]
Main sequence 800 million years 3.8 bya Oldest known life on Earth.[72][130] Oort cloud reaches maximum mass.[75]
Main sequence 4.6 billion years Today Sun remains a main-sequence star.[106]
Main sequence 6 billion years 1.4 billion years in the future Sun’s habitable zone moves outside of the Earth’s orbit, possibly shifting onto Mars’s orbit.[109]
Main sequence 7 billion years 2.4 billion years in the future The Milky Way and Andromeda Galaxy begin to collide. Slight chance the Solar System could be captured by Andromeda before the two galaxies fuse completely.[125]
Post–main sequence 10 billion – 12 billion years 5–7 billion years in the future Sun has fused all of the hydrogen in the core and starts to burn hydrogen in a shell surrounding its core, thus ending its main sequence life. Sun begins to ascend the red giant branch of the Hertzsprung–Russell diagram, growing dramatically more luminous (by a factor of up to 2,700), larger (by a factor of up to 250 in radius), and cooler (down to 2600 K): Sun is now a red giant. Mercury, Venus and possibly Earth are swallowed.[107][112] During this time Saturn’s moon Titan may become habitable.[114]
Post–main sequence ~ 12 billion years ~ 7 billion years in the future Sun passes through helium-burning horizontal-branch and asymptotic-giant-branch phases, losing a total of ~30% of its mass in all post-main-sequence phases. The asymptotic-giant-branch phase ends with the ejection of its outer layers as a planetary nebula, leaving the dense core of the Sun behind as a white dwarf.[107][117]
Pre-Solar System Billions of years before the formation of the Solar System Over 4.6 billion years ago (bya) Previous generations of stars live and die, injecting heavy elements into the interstellar medium out of which the Solar System formed.[14]
Pre-Solar System ~ 50 million years before formation of the Solar System 4.6 bya If the Solar System formed in an Orion nebula-like star-forming region, the most massive stars are formed, live their lives, die, and explode in supernova. One particular supernova, called the primal supernova, possibly triggers the formation of the Solar System.[16][17]
Remnant Sun ~ 1 quadrillion years (1015 years) ~ 1 quadrillion years in the future Sun cools to 5 K.[131] Gravity of passing stars detaches planets from orbits. Solar System ceases to exist.[3]

Planets in the solar system

Before we talk about the planets in our solar system, we need to take a look at the definition of planet first. So, what is a planet? According to the International Astronomical Union, a planet is a celestial body that is in orbit around the sun, has a nearly round shape, and has mostly cleared its orbital neighborhood of debris. So, there are eight planets that fit the IAU’s definition. They are – in the order from the closet to the furthest from the sun – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.  In order to remember better, the order of these planets from the sun goes in the rhythm like: Mercu – Venus – Ear – Mars – Jup, And then, Sat – U – Nep.

 

Pluto

There was once a concept that there were nine planets in our solar system after the discovery of ‘Pluto’. However, Pluto fails to meet the requirements defined by AIU having no “cleared the neighborhood” of its orbit. So, what does the term “cleared the neighborhood” mean? As planets form, they become the strongest gravitational body in their orbit in the Solar System. So with such strong gravity, they either consume other, smaller bodies, or sling them away when they interact with them. Pluto is only 0.07 times the other objects’ mass in its orbit. For this fact, Pluto has been taken out from the planet list and is then considered to be a dwarf planet.

Planet Nine

Astronomers are now on the hunt for another planet in our solar system beyond Neptune, a true-ninth planet, after evidence of its existence unveiled on Jan. 20, 2016. The “Planet Nine,” as scientists are calling it, is predicted 10 times the mass of Earth and 5,000 times the mass of Pluto. As of 2018, no observation of Planet Nine however had been announced. The evidence for Planet Nine was discovered by researchers Mike Brown and Konstantin Batygin at the California Institute of Technology in Pasadena, California. The scientists used mathematical modeling and computer simulations for the find, but have not actually observed the planet directly. The existence of the planet is inferred by the orbits of six other, smaller objects in the Kuiper Belt. The way those smaller objects orbit suggested the gravitational influence of another much more massive object, which Brown and Batygin named Planet Nine. 

Types of planets

There are many types of planets categorized by different characteristics. Common ways of categorizing them are:

  • by mass regime
  • by orbital regime
  • by composition
  • other

However, those in our solar system are of two such categories as terrestrial and jovian planets.

Earth-like or Terrestrial planets

Terrestrial planets are earth-like with hard surface built up with rock or metals. They also have a molten heavy-metal core, few moons and topological features such as valleys, volcanoes and craters. In our solar system, the four planets closest to the sun are terrestrial planets: Mercury, Venus, Earth and Mars. During the formation of the solar system, there seemed to be more terrestrial planetoids, but they might have merged with each other or been destroyed.

Jupiter-like or Jovian planets

Since the four planets close to the sun are terrestrial planets, the remaining other four _ Jupiter, Saturn, Uranus and Neptune – are grouped under the category of Jovian planets. Despite their variations in size, mass and composition, they all share certain characteristics that makes them significantly different from the terrestrial. Unlike the terrestrial planets, Jovian planets are largely composed of gases – mostly hydrogen and helium. Jupiter and Saturn are classified as “gas giants” while Uranus and Neptune are “ice giants” as they have higher concentrations of methane and heavier elements such as oxygen, carbon, nitrogen, and sulfur in their interior.

Exoplanets or Extrasolar planets

So far, we have done a brief study of the solar system and the planets in it. Then one might wonder what are there outside our solar system. Well, we have already learned that there are a lot more solar systems even in our galaxy, Milky Way. So it is not difficult to theorize that there should be other planets than those in our solar system as well. That’s scientifically true too. There definitely are planets outside our solar system. Those are called extrasolar planets or exoplanets. As of 23 March 2019, there are 4,016 planets confirmed in 2,999 systems, with 655 systems having more than one planet.

summary

Now we have a basic insight into our solar system and its content. So, let’s wind up a bit. Our solar system is the system of the sun and eight planets and everything else such as asteroids and comets, which orbit the sun in the Milky Way in the Universe. The formation of our solar system was about 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. There are eight confirmed planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune with Mercury being the closet to the Sun and Neptune, the furthest. The solar planets are classified into two main groups as Terrestrial planets and Jovian planets. Terrestrial planets are Earth-like planets with rocky surface whereas Jovian planets are Jupiter-like covering with gases and ice. The four planets – Mercury, Venus, Earth, Mars – closest to the Sun are terrestrial planets and the four outer planets_ Jupiter, Saturn, Uranus and Neptune – are Jovian. There are also many more planets outside our solar system and they are called Extrasolar planets or Exoplanets. Studying so far what we can say for sure is that the space science is full of wonders.