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| Space on Mar 21, 2014

What makes Venus the brightest planet?

Venus is very bright. That’s partly because sunlight is easily reflected by acidic clouds that blanket the planet’s atmosphere.

Venus is so much brighter than any other planet viewed in the sky. Why is it so bright?

Venus and the moon by EarthSky Facebook friend Mike O’Neal. Beautiful Mike!

Astronomers use the term “albedo” to describe how bright a planet is. When light strikes a planet, some is absorbed by the planet’s surface or atmosphere – and some is reflected. Albedo is a comparison between how much light strikes an object – and how much is reflected.

Venus has the highest albedo of any planet in our solar system. Venus is so bright partly because it reflects over 70 percent of sunlight striking it. It owes its reflective ability to the fact that it’s blanketed with clouds. Sunlight bouncing from these clouds is what makes Venus so bright.

When the moon is close to full, it can look a lot brighter than Venus, but the moon reflects only about 10 percent of the light that hits it. The moon has a low albedo of around .1, meaning that it reflects about 10% of the light that it receives. Venus, on the other hand, is the most reflective object in our solar system, with an albedo of close to .7. The moon has a low albedo because it’s made of volcanic rock. It appears bright to us only because it’s so nearby.

Photo credit: James Jordan

The clouds in the atmosphere of Venus contain droplets of sulfuric acid – one of the eye-stinging ingredients in our urban smog – as well as acidic crystals suspended in a mixture of gases. Light bounces easily off the smooth surfaces of these spheres and crystals. That’s one reason the clouds of Venus are so good at reflecting light.