The planet that is closest to the Sun is Mercury. It is the smallest planet in our solar system, with a diameter of about 4,880 kilometers. Mercury orbits the Sun at an average distance of approximately 57.91 million kilometers (about 36 million miles), making it the innermost planet. Due to its proximity to the Sun, Mercury experiences extreme temperature fluctuations; daytime temperatures can soar to about 430°C (800°F), while nighttime temperatures can plunge to -180°C (-290°F). Mercury has a rocky surface and lacks a substantial atmosphere, which contributes to these temperature variations.