NOVA: The Planets

The four planets closest to the sun, called the rocky planets, were born from the same material in the same era. But they couldn’t be more different: Tiny Mercury is the runt of the litter, almost like a moon. Venus is devilishly hot, and Mars is a frozen desert world. Only on Earth do we find the unique conditions for life as we know it. But why only here? Were Earth's neighbors always so extreme? And is there somewhere else in the solar system life might flourish?

The Red Planet was once a vibrant blue water-world, home to raging rivers, active volcanoes, and even an ocean. But as the young planet’s core cooled, its magnetic field and protective atmosphere faded, eventually exposing it to the wrath of the sun. With its volcanoes extinguished and its water lost to space, Mars became the frozen desert planet we know today. But if it once had many of the ingredients necessary to form life, how far along might that process have gotten?

Jupiter is not just the oldest planet orbiting the sun—it’s also the largest. So when the young gas giant went on a rampage through the inner solar system, it shaped the fate of everything in its path. Speeding towards the Sun, Jupiter’s massive gravitational force hurled debris into interstellar space, stunting the growth of would-be planets. Earth might have been doomed had Saturn not pulled Jupiter back. Today, Jupiter resides in the outer solar system, where its gravity bends the paths of asteroids and stokes volcanic activity on its moon Io. But it could one day wreak havoc again.