What actually qualifies as a planet in our universe?
In short: We often think of planets as any large sphere circling a star, but the scientific definition is much more exclusive. To be a planet, an object must satisfy three specific criteria, one of which explains why Pluto was stripped of its status.
The distinction between a planet and a dwarf planet hinges on orbital dynamics. To be classified as a planet, an object must orbit a star, possess enough gravity to maintain a round shape, and have 'cleared its orbital path.' This third requirement is the primary reason Pluto is categorized as a dwarf planet; while it orbits the Sun and is spherical, it has not cleared the material from its path. Other smaller bodies, such as asteroids and comets, also orbit stars but lack the mass or orbital dominance to be considered planets.
The hierarchy of the cosmos scales from these individual bodies to massive structures. A solar system consists of a star and all orbiting material, while a galaxy is a 'great island' of millions to trillions of stars held together by gravity. These galaxies can group into clusters or even superclusters—gigantic regions where galaxies are packed more closely than in other parts of the universe. The universe itself is the sum total of all matter and energy, though humans can only perceive the 'observable universe,' which is likely just a tiny fraction of the whole.
Understanding these objects requires mastering the scales of space and motion. Astronomers use the Astronomical Unit (AU)—about 93 million miles—to measure distances within a solar system, and light-years (about 9.46 trillion km) for interstellar distances. Furthermore, we must distinguish between rotation, the spinning of an object like Earth around its axis daily, and revolution, the orbital motion of Earth around the Sun annually. Finally, there is the expansion of the universe, which describes the increasing average distance between galaxies over time.
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Sources
- Summary by Intellect · source: Astronomy Basics | Astronomy Masters Program Series
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