The universe, galaxy, and planet are distinct terms used in astronomy to refer to different levels of cosmic organization:
Universe: The universe encompasses everything that exists, including all matter, energy, space, and time. It is vast and comprises all galaxies, stars, planets, and other celestial bodies. The universe is believed to have originated from the Big Bang around 13.8 billion years ago.
Galaxy: A galaxy is a large collection of stars, stellar remnants, interstellar gas, dust, and dark matter, bound together by gravitational forces. Galaxies come in various shapes and sizes, ranging from spiral galaxies like our Milky Way to elliptical galaxies and irregular galaxies. They can contain billions to trillions of stars. Galaxies are separated by vast distances in the universe.
Planet: A planet is a celestial body that orbits a star, is spherical in shape, and has cleared its orbit of other debris. Planets are smaller than stars and do not generate their own light but reflect the light from their host star. They are typically composed of rocks, gases, or a combination of both. Our solar system, for example, consists of eight planets, including Earth.
In summary, the universe is the largest and all-encompassing entity, containing countless galaxies. A galaxy is a large assembly of stars, gas, and dust, while a planet is a smaller celestial body that orbits a star and has a defined shape and cleared orbit.