The universe, galaxy, and planet are distinct astronomical terms that refer to different cosmic entities. Here's a brief explanation of each:
Universe: The universe encompasses everything that exists, including all matter, energy, space, and time. It is the entirety of all physical reality, including galaxies, stars, planets, and all other celestial objects. The universe is vast and constantly expanding, and it contains an immense number of galaxies and other structures.
Galaxy: A galaxy is a large, gravitationally bound system consisting of stars, stellar remnants, interstellar gas, dust, and dark matter. Galaxies can contain billions to trillions of stars, along with various other celestial objects. The stars within a galaxy are held together by gravity, forming various structures like spiral arms, elliptical shapes, or irregular distributions. Our own Milky Way galaxy, for example, is a barred spiral galaxy.
Planet: A planet is a celestial body that orbits a star, is spherical in shape due to its self-gravity, and has cleared its orbit of other debris. Planets are much smaller than galaxies and are typically part of a larger solar system within a galaxy. They do not generate their own light but reflect the light of their host star. Examples of planets in our solar system include Earth, Mars, Jupiter, and Neptune.
To summarize, the universe refers to the entirety of all existence, galaxies are large systems of stars and other celestial objects, and planets are smaller bodies that orbit stars within galaxies.