I will choose options (a) Star and planet, and (b) Asteroid and comet.
(a) A star and a planet are fundamentally different in their formation, composition, and characteristics. A star is a massive celestial body that generates energy through nuclear fusion in its core, which produces light and heat. Stars form from clouds of gas and dust in space, known as nebulae, and undergo a significant gravitational collapse that leads to their ignition as they reach high temperatures. In contrast, a planet is a smaller celestial body that orbits a star and does not produce its own light. Planets form from the leftover material in a protoplanetary disk around a young star, accumulating mass through collisions and gravity. While stars can exist independently, planets require a star to illuminate them and often have atmospheres, moons, and other characteristics that define them.
(b) An asteroid and a comet also differ in their composition and origins. Asteroids are rocky or metallic objects that are primarily found in the asteroid belt between Mars and Jupiter. They are remnants from the early solar system that never coalesced into planets. Asteroids are generally composed of materials such as metals and silicate rocks, making them relatively solid and stable. On the other hand, comets are made primarily of ice, dust, and rocky material. They originate from the outer regions of the solar system, specifically from the Kuiper Belt and the Oort Cloud. When a comet approaches the sun, the heat causes the ice to vaporize, creating a glowing coma and sometimes a spectacular tail. This gives comets a distinctive appearance that sets them apart from asteroids.
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