Stars are so distant that they appear as points of light even through powerful telescopes.
Reflective objects like asteroids and planets, on the other hand, display some size, even if it's not evident to the unaided eye. That little bit of dimension makes their images resistant to distortion from our churning atmosphere. Stars twinkle, then, but planets do not. A planet's reflectivity determines its appearance and gives instant clues to its surface composition. Reflectivity is described in terms of albedo, which is the percentage of light that's reflected.