What is Planet? This seemingly simple question is not really that simple. You see, science is very dynamic and a single thing can have many iterations or variations when it comes to definitions. Planet is one such thing. The definition of a planet has changes many times and those changes were forced into acceptance when more and more planets were discovered.
One small example will be Pluto. It was once considered as a planet and then, one fine day, it was no longer a planet. The conventional definition has changed because of a number of new and startling discoveries that did not fit into the conventional definition. This brings one question… “What was the conventional definition of a planet?”
Let’s find out…
What is Planet: Old Definition of Planet
A planet is a celestial object that revolves around a star, reflects the light of that star for shining and is bigger than an asteroid.
This definition was there before the 1990s. It was not really precise. However, it was good enough to define the celestial objects we knew at that time. Everything however changed during the 1990s when the scientists made us startling discoveries.
What Discoveries Led to the Change in Definition of Planet?
Nice question! Here is what was found. Astronomers notices an area of icy objects beyond Neptune. That area has the shape of a donut and today, it goes by the name – Kuiper Belt. There and hundreds of thousands of icy objects in the Kuiper Belt. Many of them are pretty large.
That wasn’t the only discovery. Astronomers even studied stars other than our Sun and found many other planets. It turned out that many of these newly discovered planets didn’t have the orbit that we usually see for the planets in our very own Solar System such as the Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
In addition to that, astronomers have also found small celestial objects – brown in color – that were small enough (in astronomical terms) to called as dwarf. These objects blurred out the distinction between a star and a planet.
More perplexing was the discovery of planet-like objects which were drifting alone in the universe. They moved through the darkness of the interstellar space and didn’t seem to be attached to any star in any orbit.
What is Planet: New Definition of Planet
The aforementioned discoveries were so startling that they clearly made the previous/old definition of planet untenable. The need for a new definition was felt very badly and that’s why, the International Astronomical Union (abbreviated as IAU) came up with a completely new definition in August 2006.
The new definition of planet was this:
A planet is a celestial object that orbits a star or revolves around a star. That object just needs to be big enough to be able to achieve a round shape and also, the object is just big enough to be able to clear the area surrounding its orbit.
This new definition threw Pluto off its planetary status.
Did you know that this new definition was not unanimously accepted? It turned out that some astronomers were so against this new definition that they organized a petition in protest of the new definition and even refused to use the definition. These protesting astronomers actually said that the new definition was imprecise and arbitrary.
Is the New Definition Arbitrary?
No, not really, no! The new definition actually reflects the planets are objective class. The definition is not arbitrary. The new definition also reflects our Solar System’s architecture. It also reflects the architecture of other systems.
What is this architecture?
Scientists say that these systems (including our Solar System) were actually formed within rotating disks through a process called accretion. They say that within this rotating disks, small grains started clumping together, forming bigger bodies. These newly formed bigger ones then clumped with each other forming even bigger ones. The process continued until it happened that a few really massive objects were created. They became the planets.
The smaller ones became comets and asteroids and they represent celestial debris that were left behind after formation of planets. So, ‘Planet’ is not really just any arbitrary object. They were formed through a process. That makes them fit into an objective class.
Was Earth a Planet Right From the Beginning?
Why this question? It is important to answer the question – What is Planet?
This might come as a shock but the truth is that Earth was classified as a planet at a much later time. Here, we need to step into history and talk of the Greeks who said that there are exactly 7 different types of lights. They said that these lights moved across the sky against background of stars.
Those seven lights that the Greeks mentioned were:
The Greeks used to call these ‘7 lights’ as ‘planetes’. The exact meaning of ‘planetes’ is ‘wanderers’.
If you see carefully, you will not find our Earth in that list of 7 lights. The problem is that for most of the history of humanity, Earth was considered as the center of the universe or rather the very foundation of the universe.
It was only after it was accepted that it was the Sun that was right at the center and not the Earth that people redefined things.
What really happened then is that Sun and Moon were removed from the list of ‘7 lights’. Earth was included into it and then came the definition of Planet. This happened in 1543. It was then that it was said that objects revolving around the Sun or orbiting the Sun are called planets.
Here, let us take a look at some of the discoveries in a tabular format:
|Before 1543||7 Lights or Planetes or Wanderers. They were Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn.|
|1543||Earth included and Sun and Moon deleted. So the list became: Mercury, venus, Earth, Mars, Jupiter and Saturn.|
|1847||Hebe, Iris, Flora|
|1850||Parthenope, Victoria, Egeria|
|1852||Asteroids were deleted from the list|
|2006||Pluto was deleted from the list|
You will notice from the table above that in 1801, Ceres was discovered. Initially, every astronomer thought of it as the missing planet between Mars and Jupiter. In 1802 however, Pallas was found, which had a similar orbit to that of Ceres. This is where the confusion came in.
Planets discovered earlier looked like discs. Ceres and Pallas looked like small dots. By 1851, 15 of those small dots were discovered. It was unusual. All of them couldn’t possibly be planets. So, the name ‘Asteroids’ was used as suggested by William Herschel. In 1852, astronomers removed all asteroids from the list of planets (by this time, the number of planets on the list had increased to 23). In 1852 however, when all the asteroids were removed, the planet count came down to 9.
By now, the astronomers had started cataloging asteroids in the order they were discovered and not in terms of their distance from our Sun.
Interesting Fact: If asteroids were counted as planets (as till 1851), the number of planets now would have been 135,000 or more.
Story of Pluto:
Without sharing the story of Pluto, we cannot give a proper answer to the question – what is planet? Pluto was discovered in 1930 and it was considered as the Planet X. What was Planet X? It was a theoretical planet until 1930 which would have enough gravitational pull to create the peculiarities observed in Neptune’s orbit.
Scientists were however not happy with their discoveries and also, they wanted to learn more and more. Eventually scientists found out the following:
- Pluto was smaller than all 8 planets.
- Pluto was smaller than 7 different satellites of various planets.
- Pluto was smaller than Earth’s satellite – the Moon.
- There were actually no peculiarities in Neptune’s orbit.
Pluto remained a questionable object but continued to enjoy the status of a planet until scientists found the Kuiper Belt.
Just like Ceres’ discovery helped us understand the asteroids were completely different breed of celestial objects, discovery of Kuiper Belt helped us understand Pluto’s anomaly. Scientists figured out the Pluto was in reality an object belonging to Kuiper Belt. The objects in Kuiper Belt are called KBOs or Kuiper Belt Objects.
Well, people were pretty accustomed to calling Pluto as a planet. That’s how it was taught in schools and that’s how all of us learned about it. Unfortunately, scientists found Eris in 2005. Eris was known as Xena or UB313 back in 2003.
Eris was way bigger that Pluto and here came a problem:
“If Pluto is a planet, so should be Eris and many more larger-than-Pluto KBOs and if Pluto is not a planet, other KBOs should not be planets either.”
In year 2000, Harold Levison and Alan Stern actually suggested a criterion for considering an object as a planet. The two said that a planet is one which is less massive than a star but it is quite large so that its gravity can overcome the rigidity of that object’s shape and make it round.
The problem is that if this definition was accepted then:
- Pluto would have stayed as a planet.
- Many more KBOs would have made it into the list of planets.
- Ceres, the only asteroid known to be a round, should be considered as planet.
So, roundness was not really a good criterion. It is then that a new proposal came in. It was proposed that if the ratio of mass of a massive celestial body orbiting the Sun and sum of masses of all celestial bodies that are within the orbital zone of that massive celestial body is at least 100, that massive body is to be considered as a planet.
Confused? Okay, look at this equation:
If the above criterion is fulfilled, A is considered as a planet.
With this criteria in mind, we find that μ for Pluto is 0.07 while μ for Ceres is 0.33.
So clearly, Pluto was not a Planet by this definition and neither was Ceres. Ceres was already considered as an asteroid while Pluto was later considered as KBO.
What is Planet? The Definition and Formation of Solar System
The new definition has shed some interesting light on how our Solar System was formed. The first thing that came in our Solar System was the Sun – the primordial one. That Sun was surrounded by dust and gas orbiting the Sun in form of a flattened disc.
The amount of dust and gas present in this primordial disc was limited. Planetary bodies started forming from this disc. Some of the bodies grew large enough to self-sustain their growth. Because of their large size, they further swept in (or consumed) smaller bodies or deflected them out of the Solar System completed and some were consumed by the Sun.
What remained at last were very few large bodies that cleared up their orbital zone either by sweeping in smaller objects or flinging away smaller objects. What about the satellites? They are smaller that planets but they are either not swept in or deflected away.
Stern and Levison proposed something. According to them, the lesser bodies that were formed during the formative days of the Solar System, couldn’t grow as big as the planets. Hence, they failed to sweep in smaller bodies or deflect them away. These lesser bodies took up temporary and unstable orbits or they became stabilized in their orbits by larger guardian bodies (planets). Thus, the planets held them in stable orbits and they became satellites. Whatever was leftover became the debris that includes asteroids, KBOs and comets.
So, What is Planet? Bottom Line:
Planets in a Solar System (not just our Solar System) are basically secondary objects that form by dust grains clumping together from a disk of dust and gas orbiting a primary object (the Star and in our case, it is Sun). These planets experience runaway growth when they sweep in most of the material from the surrounding gas and dust disk. A planet will at one point reach a critical stage where it manages to pull a gas envelop called atmosphere.
Planets are dynamically dominant and hence, they will be a few in number and maintain their own orbiting zone. They will clear up their surroundings by either sweeping in the smaller bodies (like asteroids hitting a planet) or they will deflect away those smaller bodies from their orbital zone.