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They're reflecting sunlight.
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Jupiter is about 5 astronomical units (AU) from the Sun. An astronomical unit is the distance from the Earth to the Sun. When Jupiter and the Earth are on the same side of the Sun, Jupiter is 5-1=4 AU from the Earth and when Jupiter and the Earth are on opposite sides of the Sun, Jupiter is 5+1=6 AU from the Earth.
A light year is 63,241.1 astronomical units.
The nearest star besides the Sun is Proxima Centauri and it is 4.2 light years from Earth. It is a dim star and is unable to be seen without optical aid. Its distance is 265,612.62 astronomical units, or about 53 thousand times as far away as Jupiter.
Rigel, the bright star on the opposite corner from Betelguese in the rectangular constellation Orion is about 85,000 times as bright as the Sun but it is about 800 light years away from Earth, or about 42 million times as far away as Jupiter.
The relationship of brightness and distance is the inverse square law. It means that if there are two items of the same actual brightness, and one is twice as far away as the other, the farther away one will look only one-fourth as bright as the nearer one. If there are two items of the same brightness, and one is 100 times as far away, it will look one ten-thousandths as bright as the nearer one. The point is that farther away, the much less bright something will look.
So, the main reason that Jupiter and Venus shine so brightly is because stars are tens of thousands or millions of times farther away than these planets.
Even though the sunlight is reflected from various colors of clouds over Jupiter, the planet is so much brighter than the blackness of space that the glare of it hides the colors. It's like if you painted a brown stripe on a car headlight, you would not see any brown stripe if you were a mile away.
A light year is 63,241.1 astronomical units.
The nearest star besides the Sun is Proxima Centauri and it is 4.2 light years from Earth. It is a dim star and is unable to be seen without optical aid. Its distance is 265,612.62 astronomical units, or about 53 thousand times as far away as Jupiter.
Rigel, the bright star on the opposite corner from Betelguese in the rectangular constellation Orion is about 85,000 times as bright as the Sun but it is about 800 light years away from Earth, or about 42 million times as far away as Jupiter.
The relationship of brightness and distance is the inverse square law. It means that if there are two items of the same actual brightness, and one is twice as far away as the other, the farther away one will look only one-fourth as bright as the nearer one. If there are two items of the same brightness, and one is 100 times as far away, it will look one ten-thousandths as bright as the nearer one. The point is that farther away, the much less bright something will look.
So, the main reason that Jupiter and Venus shine so brightly is because stars are tens of thousands or millions of times farther away than these planets.
Even though the sunlight is reflected from various colors of clouds over Jupiter, the planet is so much brighter than the blackness of space that the glare of it hides the colors. It's like if you painted a brown stripe on a car headlight, you would not see any brown stripe if you were a mile away.