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Space and Sky · entry 50 of 52
Planets do not produce visible light of their own; they shine by reflecting sunlight.
The longer version
Planets shine by reflected sunlight because they lack the mass needed to ignite fusion in their cores. Their surfaces and clouds bounce a portion of the incoming light back into space, and that is what telescopes detect. Jupiter would need far more mass to become a star.
The fraction reflected varies with composition: ice and thick cloud return much more light than dark rock. Planets also emit infrared radiation from their own warmth, which is how their temperatures are measured. In visible light, though, everything we see of a planet is borrowed sunlight. Brightness therefore depends on size, distance and surface type.
Where this entry sits
The night sky looks calm, yet almost everything up there is moving, burning, or slowly changing. These facts cover planets, stars, the Moon, gravity, black holes, comets, and the story of how people began exploring beyond Earth. The full list sits on the Space and Sky page, where every entry is listed in order.
More From Space and Sky
Eight more entriesEarth is the only planet known to hold liquid water across much of its surface.
Read 52The Moon has no atmosphere, so surface tracks there can remain for millions of years.
Read 1The Sun contains roughly ninety-nine percent of all the mass in the solar system.
Read 2Light from the Sun takes about eight minutes to reach our planet.
Read 3Ocean tides are caused mainly by the Moon pulling on Earth's water.
Read 4The Moon always shows Earth the same face because its rotation matches its orbit.
Read 5Venus is hotter than Mercury because its thick atmosphere traps heat efficiently.
Read 6Mars appears red because iron oxide dust covers much of the rocky surface.
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