When we think of planets, we tend to think about the eight planets in our Solar System: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. But what about dwarf planets, exoplanets, terrestrial planets, gas giants, ice giants, and jovian planets? Keep reading to find out!
Before we get into specificities, let’s talk about what a planet means. A planet is a celestial body that essentially wanders in outer space around a host star. According to NASA, a celestial body must meet these three requirements in order to be classified as a planet: “It must orbit a star (in our cosmic neighborhood, the Sun), it must be big enough to have enough gravity to force it into a spherical shape, and it must be big enough that its gravity cleared away any other objects of a similar size near its orbit around the Sun.” Earth is a planet because it orbits the Sun, it’s a sphere in shape, and there are no objects as big as Earth within our own orbit. According to Smithsonian National Air and Space Museum's website, the word, ‘planet’ means ‘the wanderer’ in greek. In the olden days, planets would be deemed as unruly, majorly due to the fact planets do not follow a set path. They do not have a perfect circular orbit, and sometimes they seem to backwards in retrograde motion, according to a video by Mechanical Universe.
Within the general classification of planets, we have subcategories. The first subcategory will be terrestrial planets. Terrestrial planets are planets that, “have a compact, rocky surface like Earth's terra firma.” (NASA) Essentially, terrestrial planets are planets that have a rocky composure and are dense. Examples of terrestrial planets would be Mercury, Venus, Earth, and Mars. All four of the inner planets are terrestrial planets, because they all are compact and have a rocky surface.
The second subcategory are gas giants. Gas giants, also known as Jovian planets, are planets that are mainly composed of gas and not rocks. These planets are formed with dense gasses that keep the planet together instead of falling apart or evaporating into thin air. Planets that are gas giants include Jupiter and Saturn. They are also known as Jovian planets, because they are large in size. Jupiter is the largest planet in the solar system, followed by Saturn.
Third are ice giants. Ice giants are essentially gas giants that are too far away from the Sun. These planets are so far away, they are freezing cold. Virtually no sunlight reaches these planets. Ice giants include Neptune and Uranus, two bodies that have been recently found. They also happen to be the farthest planets away from the central sun.
After Uranus lies a dwarf planet we all know and love. Pluto, demoted in 2006, is classified as a dwarf planet by NASA, because it does not meet the third requirement of a planet. Pluto cannot clear its own orbit of objects as big as itself. Any body that is not classified as an asteroid or a comet (within our solar system) is a dwarf planet. Dwarf planets are planets that essentially do not meet all three requirements. Examples include Pluto, Ceres, Eris, Makemake, and more.
Last but not least, we have exoplanets. Exoplanets are planets that lie outside of our solar system. They are excluded from us [pun intended :)]. Exoplanets are essential, especially to Astrobiologists who are trying to find extraterrestrial life. Some exoplanets are also known as rogue planets, which are free-wandering planets. Rogue planets do not orbit a host star, but they orbit a galactic center. Because of NASA’s Kepler Space Telescope, we now know there are more planets than stars in the galaxy, according to NASA. Exoplanets are detected using several methods, which includes but not limited to: the transit method, using the Doppler effect, and the ‘wobble effect’.
As a little surprise bonus, we thought we would talk a little about Planet X! Planet X is a hypothetical planet that is hypothesized based on mathematical solutions. The planet in question is similar in size to Neptune, and its orbit is highly elongated that lies well beyond Pluto’s orbit. Planet X, also known as Planet 9, could be 10 times bigger than Earth’s mass, and it could be 20 times (on average) farther than the Sun on average than Neptune. For Planet X, it could take about 10,000 to 20,000 Earth years to orbit around the Sun. Astronomers have yet to discover this theoretical planet to this day.
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Bibliography
A.I.R.A.N.D.S.P.A.C.E. (n.d.). Ancient Times & The Greeks | National Air and Space Museum. Airandspace.
Retrieved February 6, 2022, from https://airandspace.si.edu/exhibitions/exploring-the-planets/online/discovery/greeks.cfm
Basics of Space Flight - Solar System Exploration: NASA Science. (n.d.). NASA Solar System Exploration.
Retrieved February 1, 2022, from https://solarsystem.nasa.gov/basics/chapter1-2/
N.A.S.A. (n.d.-b). Planets. NASA Solar System Exploration. Retrieved February 1, 2022, from
Overview | What is an Exoplanet? –. (n.d.). Exoplanet Exploration: Planets Beyond Our Solar System. Retrieved
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The Mechanical Universe [caltech]. (2016, December 19). Episode 9: Moving In Circles - The Mechanical
Universe [Video]. YouTube. https://www.youtube.com/watch?v=MSUkQXA5_bU
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