I will choose (a) Star and planet, and (b) Asteroid and comet.
Stars and planets significantly differ in their characteristics and formation. Stars are massive celestial bodies composed mainly of hydrogen and helium that undergo nuclear fusion, producing energy and light. They are formed from clouds of gas and dust that collapse under gravity, leading to fusion in their cores. In contrast, planets are much smaller bodies that orbit stars, formed from the leftover material in a protoplanetary disk surrounding a young star. Planets do not produce their own light and can be rocky, like Earth, or gas giants, like Jupiter.
Asteroids and comets also have distinct characteristics based on their composition and location within the solar system. Asteroids are primarily composed of rock and metal, typically found in the asteroid belt between Mars and Jupiter, and they have irregular shapes and orbits. Conversely, comets are made up of ice, dust, and rocky material, often exhibiting tails that form when they approach the Sun, causing their ices to vaporize. This difference in composition reflects their formation in different parts of the solar system, with asteroids forming closer to the Sun, where it was too warm for ice to remain solid.