Dawn Planets

Two solar-system opposites huddle quite close together in the dawn sky right now — the planets Mercury and Saturn. They’re low in the east-southeast about 45 minutes before sunrise.

Although they’re sister worlds, it’s hard to imagine two more different planets.

Mercury is the smallest of the Sun’s planets — a dense ball of rock and metal that’s less than half the diameter of Earth. Saturn, on the other hand, is close to 10 times Earth’s diameter — a huge ball of gas surrounding a rocky core.

The reason for the difference is their proximity to the Sun.

Both worlds formed as small chunks of rock and ice merged to form larger objects. Mercury is the closest planet to the Sun, so the ices in its “building blocks” quickly vaporized. But Saturn was born much farther from the Sun, so there was a lot more ice around. The extra material allowed Saturn to grow much bigger and heavier. That gave it a much stronger gravitational pull, allowing it to grow bigger still.

That region of the sola