Which planet's year lasts about 12 Earth years?

Jupiter, at 11.86 years - and that number has an unexpected consequence on Earth. It is close enough to twelve that Jupiter moves roughly one constellation of the zodiac per year, which is why Chinese and several other Asian calendars run on a twelve-year animal cycle: the planet was used as a year-counter, and the ancient Chinese name for it, Suixing, means year star.

Its size shapes everything around it. Jupiter has more than twice the mass of all the other planets combined, and its gravity governs the asteroid belt - the Kirkwood gaps are orbits swept clear by resonance with it - and deflects or captures comets. Whether that makes it a shield protecting Earth from impacts or a slingshot sending more our way is genuinely argued about; simulations have supported both.

It also does not orbit the Sun in the way the other planets do. Jupiter is massive enough that the centre of mass of the two lies just outside the Sun's surface, so strictly the Sun and Jupiter orbit a point in empty space between them. Every other planet's barycentre with the Sun sits inside the Sun.