Mars and Jupiter are siblings. They were born at the same time, from the same cloud of material that encircled the embryonic Sun. Yet there’s absolutely no family resemblance – they could hardly be more different.

Both planets appear near the Moon at dawn tomorrow. Mars looks like a bright orange star to the upper right of the Moon. Jupiter is even brighter, and crouches below the Moon, low above the horizon.

Mars is small – about half the diameter of Earth. It’s made of rock and metal. Jupiter, on the other hand, is the largest planet in the solar system – a giant ball of gas that’s 20 times wider than Mars, and about 3,000 times as massive. In fact, Jupiter is more massive than all the other planets and moons in the solar system combined.

Mars and Jupiter are the fourth and fifth planets out from the Sun, so you might expect them to be more alike. But Jupiter might have formed farther from the Sun than it is now. At that distance, conditions would have been quite cold. There would have been a lot of ice and gas for Jupiter to sweep up, allowing it to grow bigger and bigger.

Mars was born closer to the Sun. Solar heat and radiation cleared out most of the ice and gas in its region. That left mainly bits of rock and metal – the stuff that came together to form the planet Mars – a world quite different from its giant sibling.

We’ll have more about the Moon and Jupiter tomorrow.

Script by Damond Benningfield