All Planets Considered Trivia Quiz

Answer:

The International Astronomical Union was established in 1919 to "promote and safeguard the science of astronomy" worldwide in all ways. Today the group numbers over 10,000 members who meet every three years to discuss definitions and designations for astronomical objects and principles. After a new celestial body, and potentially a tenth planet, named Eris was discovered in 2005, the IAU decided to meet and formally define the term planet. Their decisions obviously sparked a lot of debate among other scientists, and the discussions will undoubtedly continue over time as other objects are discovered.

The word planet goes all the way back to the ancient Greek word "planēt", which means wanderer. By that definition the wanderers, as far as Greek astronomers were concerned, included Mercury, Venus, Mars, Jupiter, and Saturn, along with the Sun and the Moon. They noticed that while some lights in the sky remained fixed, the "planētēs" moved across the sky. Interestingly, they did not think that Earth was one of these planets; instead, it was the object around which the others moved.

Sometime around 450 BC a Greek astronomer named Anaxagoras hypothesized that the Sun was actually a star - that is, a sphere of plasma that was held together by self-gravity by today's definition. Then Aristarchus of Samos (c. 310-230 BC) suggested that the Sun was the center of the universe. The discussion, however, was not seriously broached again until the 1500s.

At that time Copernicus suggested a heliocentric model of the universe, with the planets revolving around the Sun. The telescope had to be invented, however, before Copernicus' idea was commonly accepted among astronomers, which meant that the Sun became viewed as being something other than a planet. Galileo's observations seemed to confirm this by showing that other celestial bodies, like the moons of Jupiter, orbit a central point. They also showed that the Moon was not a planet, as it revolves around Earth, but not the Sun.

The Scientific Revolution (1500-1700) had to further progress along with some serious thinking and observing, before the modern definition of a star would emerge. By the early 1900s, astronomers generally agreed that a star produces energy by the fusion reaction of helium turning into hydrogen. Classification systems were developed over time to classify stars based on their chemical composition and temperature. So - the ancients were mostly correct with classifying the celestial bodies they could see with what they had - and, by the 1930s, it was generally agreed that Pluto should join their planets, along with Earth, Uranus, and Neptune, in order to create a new concept regarding our solar system with nine planets.

Isaac Newton's famous study concerning gravity in the late 1600s brings us to the next parts of the definition. He stated that gravity was the force that caused the planets to remain in orbit. Later scientists found that a planet's gravitational pull acts equally in all directions, pulling all material toward its center, causing it to be a spherical shape. It stands to reason that larger planets have more gravity. This gravity can also affect the orbit of other celestial bodies by potentially kicking them out of a planet's orbit.

So - going back to 2005 - with the discovery of a celestial body named Eris, it became necessary to redefine exactly what the requirements were for a celestial body to be considered a planet. After all planets were considered, Pluto was demoted to a dwarf planet because it does not have enough gravity to eject other bodies from its path.

And, in order to prepare for more new discoveries, a satellite was defined as an object that orbits a planet, a dwarf planet, or other celestial body in the solar system that doesn't have enough mass to form a spherical shape through its own gravity. Let's look at Earth and its Moon as an example. Both the Earth and Moon have a common center of gravity, which is called a barycenter. Their barycenter is located inside the surface of Earth, making the Moon Earth's satellite. Asteroids, comets, and Kuiper Belt objects would all fall under the definition of Small Solar System Bodies.

So there you have it! How will these views change in the future? Will they ever change? Only time will tell!