How to tell Planets from Large Stars

False-color view of Pluto, as seen by New Horizons in 2015. Image via NASA/JHUAPL/SwRI.

The age-old debate over our solar system is whether Pluto is considered a planet or not. Most people would argue that Pluto is a dwarf planet, and some say that due to its size it can not be considered a planet, especially when compared to Mercury or Venus.

Most research professionals in the field use a list of criterions to determine whether or not a body of mass truly can be deemed a planet or not. For example, in order for a mass to be a planet, it absolutely must orbit around the sun as the large planets do. Thus, the moon which is a mass which orbits the Earth alongside while the Earth orbits the sun, is not generally considered as important for distinguishing a planet, the moon is considered less important than Saturn or Mars in this case.

Another common question is whether or not the Sun is a planet given its size. The answer to this is quite astounding. The sun is actually a star, in fact, the largest star in our galaxy The sun could be a planet if it were a star, due to the fact that star temperatures are significantly higher than that of any planet temperature. Also, stars produce their own energy, whereas planets receive energy from the stars surrounding them in orbit. Due to the size of the sun, and its centralized location in our solar system, it helps to equalize the gravity levels in our solar system with its magnitude.

In order to be a planet, a mass needs to be, in simpler terms, rounded. The planet needs to have an almost perfect round shape to ensure it fulfills the hydrostatic equilibrium necessary for its size .

Another key factor is the mass of the planet must be large enough to set the equilibrium into place for the planet shape to form a balanced system. Many researchers use the benchmark size for optimal mass to reach the equilibrium to be 1000 km. The typical way to describe when the size is accurately large enough to be considered a planet means that the planet in question is large enough to dominate the other bodies around orbit through gravity, meaning that its influence can thus make other bodies more comparable in size enough to make the planet in question a basic size.

The planet, Pluto, for example, once studied for years on end, was determined no longer a planet due to the size of other masses in orbit around Pluto that were extremely larger comparable in size, thus making Pluto not quite large enough to be considered a planet. However, it is considered what is now known as a dwarf planet.

Dwarf planets do not have as clear of a path around the sun as large planets do, but rather, face more asteroids and other unevenly-shaped masses. There are nearly two hundred dwarf planets in the solar system currently, which is why the size is a commonly presenting issue.

Pluto, cannot seem to “clear its neighbourhood” around its orbit, as required to be a planet. Due to its size an a number of challenges that damage the suface quite often, it lacks the full ability to be considered a planet, unlike Neptune, which does not have such a problem.

Another example of masses in orbit that cannot clear their neighbourhood that cause problems for a mass such as Pluto include minor planets. Minor planets are most commonly known as asteroids. Obviously asteroids are not planets. However, minor planets are the cause of issues for dwarf planets most commonly.

So, what is the importance of all of this information?

You can not properly answer when asked how many planets are there. The correct answer for the number of full planets in our solar system is eight- the four giant planets, known as Jupiter, Uranus, Neptune, and Saturn, then the four terrestrial planets, Earth, Mars, Mercury, and Venus.

Not only are you now equipped with this knowledge, but you can also explain this to those around you who do not know why Pluto is not a planet, but rather a dwarf planet, and what the differences are.

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