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Why are planets much harder to observe directly than the stars they orbit?

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Habitable zone The habitable zones (green) for stars that are like the Sun (middle), hotter than the Sun (top), and cooler than the Sun (bottom). The red areas are those in which liquid surface water would be lost as a result of a runaway greenhouse effect, and the blue areas are those in which liquid surface water would be completely frozen.

Planets are significantly more difficult to observe directly than the stars they orbit primarily because of the extreme difference in brightness. A star is always much more luminous than its attendant planets; for example, Jupiter is only one-billionth as bright as the Sun.1

Because of this vast disparity in light, a planet typically appears so close to its parent star that it becomes undetectable using direct observation, even when viewing the nearest stars. Due to these challenges, astronomers often rely on indirect methods to detect extrasolar planets, such as measuring the periodic change in a star’s radial velocity caused by a planet’s gravitational pull or observing a transit, where a planet passes in front of its star and causes a dip in the star's brightness.1