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.

• Konstantin Batygin
• David Grinspoon