The planets of the solar system are varied in their appearance. Mercury is slate gray while Venus is pearly white, Earth a vibrant blue, and Mars a dusky red. Even the gas giants are different, Neptune and Uranus an opaque blue, while Jupiter and Saturn are mostly beige with brilliant red-brown belts. But why are these planets so different?

It starts at the beginning

As it turns out, stars and their planets form at the same time from a disk of gas and dust known as a solar nebula. Most of the gas — predominately hydrogen and helium — was swallowed by our young star; no surprise considering the Sun contains somewhere between 99.8 and 99.9 percent of the solar system’s total mass.

At the same time, debris mixed into the nebula collided over and over again, eventually accreting into planetesimals and then protoplanets. Jupiter, Saturn, and even Neptune and Uranus were able to pull in some of nebula’s hydrogen and helium to swaddle their cores, causing them to grow to truly massive sizes.

Closer to the Sun, the heat was so intense that it vaporized anything without high melting points; only rocks remained. Iron, sulfur, aluminum, nickel, and other metallic compounds circled the baby Sun round and round for millions of years, crashing into each other, eventually coalescing into the inner planets. But these young planets were unable to pull as much gas to themselves as their larger siblings. Whatever they did manage to pull in is unlikely to have lasted. Instead, the inner planets relied on liquids and gases gathered from impacts and volcanic outgassing to form the atmospheres we see today.

All this to say, the early building blocks of each planet contributes to our colorful planetary panoply. But what is it exactly about each planet in the solar system that determines its unique appearance?

A system of wild colors




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