Red planet is a term commonly used to refer to the fourth planet from the sun, Mars. However, there are various synonyms that can be used to describe this planet. One of the synonyms is the "Rusty planet," which is derived from the red, iron-rich soil found on the planet's surface. Another synonym is the "Fire planet," which references the planet's red color, which is similar to the color of fire. Additionally, "the Crimson planet" and "the Mars planet" are commonly used synonyms for Mars. All these synonyms provide different ways to reference Mars and expand the vocabulary used to describe it.