Which of these is NOT a planet in our solar system?

Regulus is not a planet in our solar system; it is a star located in the constellation Leo. In contrast, Earth, Pluto, and Mars are all recognized celestial bodies classified as planets within our solar system. Earth is the third planet from the sun and the only one known to support life. Mars, known as the "Red Planet," is fourth in the solar system and has been a focus for exploration due to its potential for past life. Pluto, once classified as the ninth planet, is now considered a dwarf planet due to its size and the criteria established by the International Astronomical Union. This context about the correct answer helps highlight the distinction between planets and stars, clarifying the understanding of our solar system's makeup.